DIYstompboxes.com

So I just built a "Korg MS-20" based OTA (13600) filter, something I found on a dutch forum whilst googling "MS-20 filters." I have lost the link so I'll have to refind it, although I'm hesitant to post it as there are some errors in the document.

Anyways, in spite of the errors (and the fact that I can't read Dutch) it sounds good. It is switchable between Lowpass and Highpass. Runs on 9V. Seems very similar to the Tim Escobedo Sallen Key circuit except with clipping diodes and highpass settings added.

Now, I have an original Korg MS-20 keyboard and the real magic of it is that you have both high pass and a lowpass filter - I'm not sure if they are in series or in parallel, but you can use the two together to create bizarre, vocal-like "formant" sounds, sort of a bandpass filter. Paul Perry's Frostwave "Resonator" circuit is the only pedal I know of to have an identical configuration. They are no longer made and I've never seen a schematic to know how devoted it is to the MS20's original design (I did have the luck to borrow one for many months a few years ago.) As a "pedal", having the two filters together is really, really much more interesting than having just lowpass or highpass because you can develop lots of interesting "extreme resonant stuck wah" sounds, or you can apply different CV's to each filter (say, two LFO's) for very complex modulations. Sounded great with basically everything I put through it from guitar to keys, drum machines etc.

If I wanted to build a "workalike" of the original filter could I put a highpass and a lowpass in series? ...Or would I want to have the signal split into each filter and then the two mixed back together?

Given that I know very little of circuit development, anyone know of a DIY MS-20 filter project that is devoted to the "dual highpass and lowpass" of the original synthesizer?

Could this be a forum project in the making? Anyone else interested in this? I have some MS-20 service doc's ( I think) but probably not the technical expertise required to adapt the real synth circuit to a 9V, DIY-friendly pedal format. Willing to help where I can if there's any interest

STRATEGY

Oooh, fun.  You probably have seen Jurgen Haible's MS-20 schematic?  It's hiding out in the schematics section with the JH-720 stuff.  

Another good one is the Rene Schmitz version http://www.uni-bonn.de/~uzs159/

Those seem to be the two general approaches, Jurgen used transistors and Rene used OTAs.  I've tried both of those and a couple of others floating around out there, they all sound kind of different, but all sound good to me and stick out as "MS-20ish".  Note that Rene's has a crazy amount of resonance available, way beyond oscillation, for crazy frequency locking stuff kind of like Ian Fritz's Threeler.  You can turn down the gain of the feedback amp if needed easily enough.

I personally like the Rene one, I have thought about doing a dual pedal kind of like you are talking about with it.  I think with a VCF with such wild resonance and such interesting behavior past self oscillation it would be cool to include a gate circuit, so when the resonance was cranked it would be muted while you were not playing.

Back to the main point, the MS-20 filters were in series for sure, but if I were making one I would include provisions for running them in parallel as well, both ways will sound cool and do things the other way can't.  I think a useful starting point for this kind of effect is the general layout and interface of the Mu-Tron Biphase, most of what you would want it to do is covered already.

There is a nifty way to have series or parallel options using switching jacks (without an extra patchcord), so you don't need a dedicated switch for it like the Biphase has, I can sketch it up if we get that far.  Some people like to mess with that switch mid performance though, so you may want to leave it.

Another cool mode is "difference" where you input the same signal to both filters and then sum the outputs but with one of them inverted, I've only done it with two low pass filters but there may be some cool stuff with HP and HP/LP combos.  Two LPFs set up this way sounds kind of like a band pass response, and kind of like a phaser.

I think an envelope follower is almost a must, in addition to at least one LFO, preferably two.  In a recent VCF pedal I did I used an envelope follower with an "attack" control which, when set to the fastest attack setting let a ton of ripple through, which sounds kind of like FMing your filter with the same signal going through it, except an octave higher.  I might consider something like that or just a provision for FMing the filter with the input signal.

For a project that is already going to be big I say just suck it up and do the bipolar power supply, it's barely more work than all the biasing and AC coupling you need to do otherwise, but that may not be the majority opinion.

I have a bunch more ideas for this type of pedal, and a bunch of it has actually been tested already, so I'm excited to hear someone else bring it up!

jordan

Is this at all based on the MS20 or is it just a marketing gimmick?
http://martoneaudio.com/BrainFreeze.htm

Quote from: rousejeremy on April 16, 2012, 08:14:48 PM
Is this at all based on the MS20 or is it just a marketing gimmick?
http://martoneaudio.com/BrainFreeze.htm

I bet it's very similar (or identical) to one of the well known variants out there, it's only a couple bucks in parts and the schemos are well known.  It probably sounds cool but if I was doing it I would definitely include some modulation options on board, kind of a pain to have to patch up something you are going to want 99% of the time.

cheers,

jordan

Ive been in touch with Martone, almost bought a Brain Freeze. Again, it's like the one I built: *either* LP or HP, but not both simultaneously, which is my desired pedal :-D

@ Jordan A. do you think the Rene Schmitz circuit is scalable down to 9V? I wonder if mods are required to make it work that way or you just get a lower overhead overall?

I'm pondering just building 2x this circuit I just made, hardwiring one for hp and one for lp and then boxing them up in series!

Strategy

Quote from: Jordan A. on April 16, 2012, 08:33:09 PM

Quote from: rousejeremy on April 16, 2012, 08:14:48 PM
Is this at all based on the MS20 or is it just a marketing gimmick?
http://martoneaudio.com/BrainFreeze.htm

I bet it's very similar (or identical) to one of the well known variants out there, it's only a couple bucks in parts and the schemos are well known.  It probably sounds cool but if I was doing it I would definitely include some modulation options on board, kind of a pain to have to patch up something you are going to want 99% of the time.

cheers,

jordan

Subscribed! We'll talk about this later sweetheart.  :-*

I'm not the only Dutchie on here, but holler if you need some translation services ;P

the layout and schematic you refer to are on www.synthforum.nl (a dutch synthesizer forum)

heres the thread http://www.synthforum.nl/forums/showthread.php?t=138512&highlight=ms+20+filter

hope you can see the attachments without an account

If you combine the two schematics, so you put a 10k or 50k potentiometer to fade between the low-pass and hi-pass INPUTS (low-pass is into the OTA, Hi is into the cap in the last stage), you will get the filter response of the MS-20 (more or less, the resonance is a bit different in the OTA version linked). This will create a notch or band-stop filter. Those can sound very vocal.

Good news, the Rene Schmitz version seems to work fine at +/- 9 volts, I just lowered the supply voltage on mine and at a first glance not much changed.  I'll spend a little more time with it to see if anything major needs fussing with but it seems promising.

Some changes I would make/general notes if using it for instrument level signals:

-the resonance control is set up for bonkers amounts of feedback, really cool for whacky sounds but if you want a more "normal" amount of feedback change the 1k8 resistor going from the inverting input of A3 to ground to something like 6k8 - 8k2 (or whatever you like, bigger resistor means less feedback).

-for instrument levels i would change the LEDs in the feedback loop of A3 to something with a lower voltage drop, 1N914 or whatever small signal diodes you have around work, or whatever you like.  With diodes with a larger voltage drop the feedback gets dangerously loud compared to your guitar signal.

-I used an LM13700 with no modifications, I sometimes see people tying the input to the darlington buffers to v- when they are not used, maybe someone who knows what they are talking about could weigh in on that?

-I used the first PNPs i found in the box, since we are not too worried about 1v/oct performance or drift you probably don't need to be too picky about these.

-it might want a high impedance buffer first if using it with guitars, a FET would be cheap and easy, or another opamp/whatever you like.  It does work fine without it and sounds good to me as is, so whatever is your preference.

-for high pass you just do the old trick of grounding the input resistor, lifting the second caps grounded leg and inputting your signal there, just like the schemo from the dutch forum.  You can do this with a switch or switching jacks, or just hardwire it, or mess around with faders like Skrogh says.

-it doesn't take up much room on a breadboard and it's not picky about exact parts or values, might as well do it up and get it working how you want first.

ok!

jordan

:edit for more notes:

yup, it sounds good with a good range for the cutoff frequency.  I measure between 8 and 13 milliamps drawn for the positive rail and 9 and 14 milliamps for the negative rail at +/- 9 volts, for just the circuit as drawn by Rene, LFOs/buffers and what not would add a bit.  Current drawn changes with cutoff frequency, higher frequency = more current.

I have a deadline so I won't be able to give this more juice for a couple of days but you all rock for the great tips/comments!

Thanks to all too, I am interested in building two filters in parallel, with different CV's, and am now 10 times wiser, appreciate the input.

I had forgotten about Tim Stinchcombe's study of the various MS20 VCFs, interesting reading for sure http://www.timstinchcombe.co.uk/index.php?pge=korg (http://www.timstinchcombe.co.uk/index.php?pge=korg)

It looks like Korg put the HP first, if it matters to anyone.

jordan

Quote from: Jordan A. on April 18, 2012, 11:08:47 AM
It looks like Korg put the HP first, if it matters to anyone.

I think you're right. There's a neat trick you can do with an MS-20 where you turn up the resonance on the HP, and set the cutoff knob pretty low. This creates a big bass hump, which you then filter with the LP. Mega bass!

Quote from: Jordan A. on April 18, 2012, 11:08:47 AM
It looks like Korg put the HP first, if it matters to anyone.

That's a good strategy, as any self noise/hiss from the HP filter circuit gets filtered out by the LP filter after it.  Cool thread!

OK, finally some time to reply.

So, the great ideas in the thread have presented a number of questions about how to proceed with a "true" HP/LP Korg MS-20 filter. In the meantime I have spent a few days with my "semi-true" Dutch-layout MS-20 filter - single sided power supply, etc. While it does sound awesome, I can say that the low headroom is a little bit noticeable.

I'm wondering how this could become a forum project with a proper layout and how I could sort of chip in and do my part given that I don't know how to create layouts. In any case I would be able to commit to organizing a group buy of PCB's, perhaps using the Dorkbot-PDX service (based in my town). Any of you layout savvy people up for participating?

So first, questions:
1 - What are peoples' objections to an onboard bipolar power supply, if any? 18v (or so) wallwart would be required. I know most of us favor 9V but the +/-15V psu could be incorporated into the circuit simply and give a LOT of the headroom. And I'm thinking back to my time with the Frostwave Resonator, which sounded killer, and used the full power supply but without making the project extra big or bulky. It just means you need that particular wall wart and can't use batteries or your 1-spot. In my opinion the sonic benefits outweigh the inconvenience or cost of having 1 extra wall wart.

2 - What are peoples' opinions about 13xxx chips vs. CA3080? there seem to be more producers/versions of the 13xxx chip and relatively good availability, although they are not cheap. CA3080's aren't the most expensive vintage chips in the world, BUT, I just sort of have a tendency to avoid obsolete chips on principle. Opinions? This would determine to some extent what circuit is used as a starting point, e.g., the Rene Schmitz version, one of the Korg versions, or a 13xxx version.

3 - How much modulation is enough? I am torn between the simplicity of the Frostwave Resonator (CV inputs, no onboard modulation - supply your own LFO's!) and the idea of having one LFO per filter and possible an envelope generator. I know there are plenty of compact versions of those that can plug in.

My idea for the "MS-20 Filter Pedal" project - bare bones version:
Classic configuration, serial with HP before LP. Plenty of resonance!
Mix/blend control
input gain/volume
CV input for HP
CV input for LP

Mega version:
HP into LP plenty of resonance
Mix/blend control
input gain/volume
CV input for HP
CV input for LP
CV input for resonance
LFO for HP
LFO for LP
Env. follower
The onboard CV's should be able to go to zero so that you can use just external CV's when you want or to use the filter in a static/manual way.

I propose to name this FROSTY THE FILTER or something else that acknowledges the legacy of erstwhile forum member PAUL PERRY who many of you will remember. My project seems to be clone-ish of his out of production Frostwave Resonator so it seems appropriate to pay respects where due :)

Opinions appreciated and layout-creators sought!

Please post your interest in a possible board run if this were to go forward!

STRATEGY

1) You can easily use the MC34063, UCC380x or a similar SMPS controller to build a 9V to +/-15V power supply. If you wanted to get real clever you could even find an off-the-shelf coupled inductor with two secondary windings to generate your +/-15V in a very small footprint. The first option requires you to run two separate chips (one as a boost and another as an inverter) but would have cheaper/easier to source magnetics. The flyback is the optimal solution for small, simple power (especially because we aren't talking more than 50-100mA current draw).

2) The 13700 has a larger input range and will give you more headroom before distortion. The CA3080 will act more non-linear which can be a very cool thing in a synth type filter (especially when the distortion is introduced IN the filters and/or feedback loop). With that said I've used the 13700 in two filter designs and had no real qualms about it. The filters were DC coupled wherever possible so the bass response was great (sounded HUGE with low frequency PWM'ed square waves).

3) This is a tough one but IMO the modulation options should be made as flexible as possible with as little circuitry as possible. You can use switching jacks to disable internal LFO's when an external CV is used, this is commonplace in the synth world. If you do separate LFO's then it's nice to be able to sync them and also for guitar purposes a flexible envelope follower is always nice. I think in the end it depends on what the end user wants to use the filters for.

@Cliff, I'm not familiar with those power options, will research!

I think I am also leaning toward 13700. The project I just built (link above) uses 13600, what's the relative advantages of that vs. 13700?

re: uses of the filter pedal - I think it should be a "use for everything" project, which is where the Resonator shines - it was very popular in spite of not having on board modulation, even as a 'static' filter you can dial such crazy overtones. I would probably not do switching-jacks to disable LFOs because it can be cool to mix LFO with your incoming CV signal, too.

Strategy

Quote from: Cliff Schecht on April 20, 2012, 06:52:07 PM
1) You can easily use the MC34063, UCC380x or a similar SMPS controller to build a 9V to +/-15V power supply. If you wanted to get real clever you could even find an off-the-shelf coupled inductor with two secondary windings to generate your +/-15V in a very small footprint. The first option requires you to run two separate chips (one as a boost and another as an inverter) but would have cheaper/easier to source magnetics. The flyback is the optimal solution for small, simple power (especially because we aren't talking more than 50-100mA current draw).

2) The 13700 has a larger input range and will give you more headroom before distortion. The CA3080 will act more non-linear which can be a very cool thing in a synth type filter (especially when the distortion is introduced IN the filters and/or feedback loop). With that said I've used the 13700 in two filter designs and had no real qualms about it. The filters were DC coupled wherever possible so the bass response was great (sounded HUGE with low frequency PWM'ed square waves).

3) This is a tough one but IMO the modulation options should be made as flexible as possible with as little circuitry as possible. You can use switching jacks to disable internal LFO's when an external CV is used, this is commonplace in the synth world. If you do separate LFO's then it's nice to be able to sync them and also for guitar purposes a flexible envelope follower is always nice. I think in the end it depends on what the end user wants to use the filters for.

In the 13600 the bias of the output buffer is dependent on the current flowing into the Iabc (current control) pin. This is not what you want for audio because that bias jumping all willy nilly can cause the buffer circuit to make popping sounds. The 13700 has a buffer whose bias is independent of the Iabc current which makes it much better for audio. With that said, for filtering applications I ignore the output buffer completely and use a TL07x or similar op amp to buffer the OTA/cap integrator circuit. Prevents the stages from loading each other and makes for a more accurate filter (assuming you match the caps decently) which IMO gives circuits a better sounding resonance.

I've layed out (single sided) PCBs for both 3080 and 13700 based VCFs, I've found that the 'mirror-image-ness' of the two OTAs/buffers of the 13700 can make for layout difficulties, requiring some complex routing and jumpers, and wasted space. Whereas 3080s you can just line 'em up - even adding FET buffers takes up little space in comparision with 13700. Of course, this would not be an issue if you go for double sided. Examples:

http://dl.dropbox.com/u/967492/121%20VCF/Roland%20121%20VCF%20Build%20Doc.pdf

http://dl.dropbox.com/u/967492/Neu%20Multimode%20PCB.gif
http://dl.dropbox.com/u/967492/Neu%20Multimode%20PnP.gif

Having said that, I'd always go for 13700 over 3080 if possible and practical, for price and availability.

Re the LFO(s), have you considered that the TAPLFO2 would make a very flexible addition to this design?

Ultimately to do a group PCB order double sided is probably required, I don't think the Dorkbot submittals can be single sided. So PCB 'real estate' and size are a secondary consideration, I think, behind choice of best parts for the sound.

I'm not familiar with the TAPLFO, I know of it but haven't used it in a project yet. A couple of questions about it:
- Is it likely to have longevity or will this project age out relatively quickly if the relevant parts go away?
- Would it be possible to develop a bare bones version of the project without modulation and do the modulation as an optional daughter board? Seems crazy doesn't it but I like the idea of a project which is expandable/customizable based on desired configuration. So there would be a filter core PCB and then a modulation PCB.

Last question: Rick any interest in being the vision-quest-leader for the layout aspects of the project? As I've never done a layout, this could really take a VERY long time unless I rope others in to help :)

STRATEGY

(subscribe)

TAPLFO2 is coding onto PIC16F684, so as long as PIC16F684 is available. The code is freely available:
http://www.electricdruid.net/index.php?page=projects.taplfo
...though I buy programmed PICs from Tom (support the designer!).

TAPLFO2 is used in a number of commercial devices:
http://steamsynth.com/e_ModulationOrgy.aspx
http://steamsynth.com/m_TTLFO.aspx
http://www.catalinbread.com/SemaphoreTremolo.html

It's the best LFO out there IMHO.

Being vision-quest-leader for the layout is tempting, but I don't lay out for double sided (yet). I've got a complete TAPLFO2 synth module project out there already though:
http://dl.dropbox.com/u/967492/ED%20VCLFO/Modulation%20Druid.pdf

Strikes me that this is actually a modular synth project anyway.  :)

Quote from: Strategy on April 23, 2012, 03:09:55 PM
I'm not familiar with the TAPLFO, I know of it but haven't used it in a project yet. A couple of questions about it:
- Is it likely to have longevity or will this project age out relatively quickly if the relevant parts go away?
- Would it be possible to develop a bare bones version of the project without modulation and do the modulation as an optional daughter board? Seems crazy doesn't it but I like the idea of a project which is expandable/customizable based on desired configuration. So there would be a filter core PCB and then a modulation PCB.

STRATEGY

It sounds like the Tom Wiltshire PIC LFO, from here http://www.electricdruid.net/index.php?page=projects.taplfo

It uses a PIC 16F684, which will eventually go away, I'm not familiar enough with the PIC series of chips to know if whatever replaces it will likely drop right in, maybe with a teeny bit of tweaking.  For a project like this I would not worry about it too much.  It helps that Tom W. is active in the synth diy community, and even posts here sometimes I think.

One nice thing about that one is that with an investment of about $40 or less you can get a pic burner and burn as many as you like, for backups or future projects.  I can barely check my own email and I managed to successfully burn a bunch of PIC projects with a pickit 3, it's definitely a doable thing if you have the inclination.  Alternately, you can buy preburned PICs from a couple of sources, the TAP trem project from here uses exactly this chip.

The other cool thing about this LFO is that it is very easy to build it your way, you can add or leave off as many features as you desire.  It is definitely a lot of LFO for the square footage it takes up on a board, but to get the most out of it you need to add a bunch of extra switches and knobs.  I'm personally kind of divided, on the one hand it's really cool to have an ultra bitchin' LFO, but on the other hand it's kind of nice to have a simple and uncluttered layout and only use stuff you probably already have in the parts bin.

Let the discussion continue...

jordan

:edit, posted at the same time as FC:

yeah, what he said!

Being vision-quest-leader for the layout is tempting, but I don't lay out for double sided (yet). I've got a complete TAPLFO2 synth module project out there already though:
http://dl.dropbox.com/u/967492/ED%20VCLFO/Modulation%20Druid.pdf

Strikes me that this is actually a modular synth project anyway.  :)
[/quote]

Well, for one thing I think developing the decided upon schem + PNP as a testing ground is probably a best first step before committing anythign to a board run. I have friends in the local Dorkbot unit who could probably help me with adaptation from there, and in fact that part is probably something I could endeavor to do on my own. I'm hesitant to really do the board run until there is a project with the known features in place and a few people have tried it and revisions made before going double sided

So if you'd be into helping develop vers. 1 that get's us very far into the process. Ponder it!

If we go with the bipolar power onboard, it could be optionally a modular synth or a "standalone" pedal project - I like ones that work that way, all of Juergen Haibles effects projects were developed as either one or the other.

MS20 schematics:

http://dl.dropbox.com/u/967492/Korg%20MS20/MS20%2001.gif
http://dl.dropbox.com/u/967492/Korg%20MS20/MS20%2002.gif

Ah yes with the 'Korg35' IC. I will see if I have the 13700 version on file later when I'm on my laptop!
Strategy

MS50 schematics, VCF on page 11. I always found Korg schematics so hard work to read...

http://dl.dropbox.com/u/967492/Korg%20MS20/ms50schemos.pdf

Yes - very hard work. It might be advantageous to crib from a modern remake that uses the desired chips.

For bipolar power I favor the Thomas Henry PSU configurations, will post if I can dig it up

Strategy

Quote from: frequencycentral on April 23, 2012, 06:44:39 PM
MS50 schematics, VCF on page 11. I always found Korg schematics so hard work to read...

http://dl.dropbox.com/u/967492/Korg%20MS20/ms50schemos.pdf

I keep going back to the simplicity of the Rene Schmitz version but with LM13700 switched out for the multiple CA3080s...I see many have done this to his circuit but I can't find the modified schem actually drawn anywhere (yet)

strategy

Quote from: Strategy on April 23, 2012, 06:47:29 PM
Yes - very hard work. It might be advantageous to crib from a modern remake that uses the desired chips.

For bipolar power I favor the Thomas Henry PSU configurations, will post if I can dig it up

Strategy

Quote from: frequencycentral on April 23, 2012, 06:44:39 PM
MS50 schematics, VCF on page 11. I always found Korg schematics so hard work to read...

http://dl.dropbox.com/u/967492/Korg%20MS20/ms50schemos.pdf

This is the 'Miss Ten' VCF/VCA from E.A.S's project on MW forum:

http://www.flickr.com/photos/23140551@N08/6051155716/lightbox/

As far as the OTA version goes, Tim Stinchcombe's schematic seems the easiest to read (page 6):

http://www.timstinchcombe.co.uk/synth/MS20_study.pdf

KLM-307 daughter board (2 x LM13600), from the MS-20 "Version 2":

http://machines.hyperreal.org/manufacturers/Korg/MS-synths/schematics/KLM-307.GIF

René Schmitz's 3080 (LPF) version:

http://www.uni-bonn.de/~uzs159/rs20.png

Still hunting for a version of the Schmitz with 13700's, otherwise I think we should go with the Stinchcombe page 6 version, sub the 13600s for 13700s and plug in the other aspects of the circuit...

Will post more tonite...

Quote from: frequencycentral on April 23, 2012, 07:30:59 PM
René Schmitz's 3080 (LPF) version:

http://www.uni-bonn.de/~uzs159/rs20.png

Quote from: Strategy on April 23, 2012, 07:39:30 PM
Still hunting for a version of the Schmitz with 13700's, otherwise I think we should go with the Stinchcombe page 6 version, sub the 13600s for 13700s and plug in the other aspects of the circuit...

Will post more tonite...

Quote from: frequencycentral on April 23, 2012, 07:30:59 PM
René Schmitz's 3080 (LPF) version:

http://www.uni-bonn.de/~uzs159/rs20.png

You don't need to change anything as far as the OTAs go, just use one half of a 13700 in place of one CA3080, mapping the pinouts and ignoring the diode bias and darlington buffers.  I hinted at it earlier, but since the darlington buffers are not used, I'm not sure if anything needs to be done with them, I should probably just check the datasheet, but not tonight.

glad to see this project has some legs!

jordan

...A little more time to think about this...

Here is how I would probably do it-

-Pick whichever version of the VCF itself suits the masses, they all sound cool.  Anyone curious could breadboard a couple of the variations and see if a consensus arises.

-Build up the two VCFs, with one hardwired as LP and the other with a switch for HP/LP.  I would build it with two inputs and three outputs, with switching jacks for series/parallel operation.  Input A for parallel, input B for series, output A, output B and sum output.  

   Each VCF gets a resonance and mix control, additionally there is a master frequency control and a "separation" control, which sweeps the frequency of VCF A up while sweeping VCF B down.

-Two simpleish LFOs.  LFO A switchable between triangle, slewed square and sample and hold, like Jurgen Haible shows in the Son of Storm Tide schematic, but no CV of frequency and built with opamps for lower parts count.  LFO B triangle only.  Each LFO gets a RATE and DEPTH control.

    A three position switch for LFO mode, with NORMAL (LFO A sweeps VCF A, LFO B sweeps VCF B) SYNC (LFO A sweeps both VCFs, LFO B disabled) and ADD (LFO A and LFO B are summed together and the summed control voltage sweeps both VCFs) modes.

    The CV input for VCF B has a switch to invert whatever is sweeping VCF B, for reverse sweeps in SYNC and ADD modes.

-An envelope follower with sensitivity and attack controls.  Two attenuverters send the envelope signal to VCF A and VCF B, for independent up and down sweep of each.

-An onboard wall wart power supply, using an AC wallwart + half wave rectifier + three terminal regulators for +/- 15 volts (or whatever, it will probably be happy with something lower as well).  This keeps people from messing with mains wiring and works well.  It does mean you end up with an AC wallwart which you need to be careful of/make sure you can't plug it into the wrong pedal.  

...anyways, that is an approximation of one way to go about it.  A setup like that offers a whole lot of control without getting bogged down too much by feature creep...

fun!

jordan

:edit: now that I think of it, I think you can get all of the output modes from just two output jacks with some clever switching jack use and an additional summer, I'll think about this some more if it is something people care about.  Stereo in-stereo out, mono in-stereo parallel out, mono in-mono out, mono in-series mono out, mono in-parallel mono out.

Something I noticed recently about AC wall warts was the plugs on them are 2.5mm barrels instead of 2.1mm. This means that AC adapters (usually) won't apply to a standard Boss type power adapter, unless you insert it crooked or bend the male pin over a little to make contact. I think this (wall wart, rectifiers, regulators) is the best power option as of now just for the sheer simplicity of two regulators and a handful of caps and diodes.

agree with the power option, will be simple and compact. Didn't get to find my favorite Thomas Henry super simple power schem yet.

@ Jordan A. that setup seems scaled nicely between features. still unsure of whether the simpler or more complex versions will have wider appeal on this forum (and others)

EDIT: I think I'd like to stay away from serial/parallel channels, it makes bypass switching a huge pain, is just not a typical direction for projects in the forum, would like to make this project do-able for "paint by numbers" builders (like myself! haha.) But I do like the syncable/invertable LFO's idea.

Strategy

Another thought occurred to me: if we're running this circuit with a lot of headroom does that make some kind of boost stage up front necessary? for all the guitarists in the house? that part is certainly not hard

I was messing around with DIY layout creator and have a very long way to go without help!  :icon_biggrin: :icon_eek: hint hint layout experts

will have to re-scan the Thomas Henry super simple rectifiers/regulators +/-15V power supply. Used it in my DIY Maestro Ring Modulator, did it on perf and can't find the document I worked from (was my first ever DIY project - what was I thinking??!? what a hard project!)

Strategy

If you can hand me a full schematic I can knock out a layout pretty quickly. Since I have a soft spot in my heart for synth filters of any kind I would enjoy doing the board. I usually don't offer to do boards for others anymore because I've had quite a few bad experiences with people getting pushy even though I'm doing work for FREE but again, I'm a sucker for any VCF topology I haven't played with. :icon_mrgreen:

I also think we should look at adapting the Steiner filter (diode/cap based VCF) for use with guitar. When I was at PAiA I actually adapted and modernized the design a bit and REALLY enjoyed playing with that filter. They never released it as a product but I have a bunch of working prototype boards that work off of +/-12V or +/-15V (part of the circuits biasing has to be modified depending on the supply voltage IIRC). If there is interest I can dig up my old project files and post them up.

Also FWIW I think it's worth the effort to try to redesign everything to run off of a single 9V neg center power supply. While this really means adding a boost SMPS for +12V/+15V and an inverting SMPS for -12V/-15V, neither of this circuits are hard to build or troublesome! The parts needed for these supplies rival the cost of a linear regulator solution and you can eliminate the need for extra wall warts (a big plus in my book). The flyback option is also there but I will have to look and see if a suitable off-the-shelf thru-hole coupled inductor is available for cheap.

that rules Cliff. thank you!!! WE HAVE A VOLUNTEER!  ;D

What about some kind of MAX1044 based power solution? +9V boosted up to 12 or 15 bipolar? The Gristleizer project and others use this but I can't remember it boosts power and produces the negative power as well. Smallbear and popular suppliers carry the chip and this forum is pretty comfortable with projects that use it.

Consider this work not for me, but for the forum community. If the test case goes well I'll do a run of PCBs, an organized project document, a build thread etc. and we can get some people going on a classic filter in an organized fashion.

I have heard similarly good things about the Steiner. The MS-20 takes the cake for me and it does have its place in FX/guitar/history, via Frostwave and others - it's a good filter for this particular group. We'll consider it the "gateway drug" to the forum's future synth filter addiction!

the Stinchcombe or Rene Schmitz schematics Rick posted seem to be the best starting points although the Schmitz one ought to have the CA3080's replaced with the LM13700.

the input stage (accommodate input levels) could be a typical LPB, correct? Or would this even be necessary?
seems like the modulation theme that keeps popping up is one LFO per filter but I'm still stumped on a circuit that allows syncing or inverting them. My feeling is, simple is good and MODDABLE is good.

Maybe for a layout we start with the most basic building block, the filter itself and do the modulation as a companion board once the elemental part of it is verified? or knit the two layouts together...

Strategy

Quote from: Cliff Schecht on April 26, 2012, 12:20:21 AM
If you can hand me a full schematic I can knock out a layout pretty quickly. Since I have a soft spot in my heart for synth filters of any kind I would enjoy doing the board. I usually don't offer to do boards for others anymore because I've had quite a few bad experiences with people getting pushy even though I'm doing work for FREE but again, I'm a sucker for any VCF topology I haven't played with. :icon_mrgreen:

I also think we should look at adapting the Steiner filter (diode/cap based VCF) for use with guitar. When I was at PAiA I actually adapted and modernized the design a bit and REALLY enjoyed playing with that filter. They never released it as a product but I have a bunch of working prototype boards that work off of +/-12V or +/-15V (part of the circuits biasing has to be modified depending on the supply voltage IIRC). If there is interest I can dig up my old project files and post them up.

Also FWIW I think it's worth the effort to try to redesign everything to run off of a single 9V neg center power supply. While this really means adding a boost SMPS for +12V/+15V and an inverting SMPS for -12V/-15V, neither of this circuits are hard to build or troublesome! The parts needed for these supplies rival the cost of a linear regulator solution and you can eliminate the need for extra wall warts (a big plus in my book). The flyback option is also there but I will have to look and see if a suitable off-the-shelf thru-hole coupled inductor is available for cheap.

Quote from: Strategy on April 26, 2012, 01:01:44 AM
that rules Cliff. thank you!!! WE HAVE A VOLUNTEER!  ;D

What about some kind of MAX1044 based power solution? +9V boosted up to 12 or 15 bipolar? The Gristleizer project and others use this but I can't remember it boosts power and produces the negative power as well. Smallbear and popular suppliers carry the chip and this forum is pretty comfortable with projects that use it.

Consider this work not for me, but for the forum community. If the test case goes well I'll do a run of PCBs, an organized project document, a build thread etc. and we can get some people going on a classic filter in an organized fashion.

I have heard similarly good things about the Steiner. The MS-20 takes the cake for me and it does have its place in FX/guitar/history, via Frostwave and others - it's a good filter for this particular group. We'll consider it the "gateway drug" to the forum's future synth filter addiction!

the Stinchcombe or Rene Schmitz schematics Rick posted seem to be the best starting points although the Schmitz one ought to have the CA3080's replaced with the LM13700.

the input stage (accommodate input levels) could be a typical LPB, correct? Or would this even be necessary?
seems like the modulation theme that keeps popping up is one LFO per filter but I'm still stumped on a circuit that allows syncing or inverting them. My feeling is, simple is good and MODDABLE is good.

Maybe for a layout we start with the most basic building block, the filter itself and do the modulation as a companion board once the elemental part of it is verified? or knit the two layouts together...

Strategy

Quote from: Cliff Schecht on April 26, 2012, 12:20:21 AM
If you can hand me a full schematic I can knock out a layout pretty quickly. Since I have a soft spot in my heart for synth filters of any kind I would enjoy doing the board. I usually don't offer to do boards for others anymore because I've had quite a few bad experiences with people getting pushy even though I'm doing work for FREE but again, I'm a sucker for any VCF topology I haven't played with. :icon_mrgreen:

I also think we should look at adapting the Steiner filter (diode/cap based VCF) for use with guitar. When I was at PAiA I actually adapted and modernized the design a bit and REALLY enjoyed playing with that filter. They never released it as a product but I have a bunch of working prototype boards that work off of +/-12V or +/-15V (part of the circuits biasing has to be modified depending on the supply voltage IIRC). If there is interest I can dig up my old project files and post them up.

Also FWIW I think it's worth the effort to try to redesign everything to run off of a single 9V neg center power supply. While this really means adding a boost SMPS for +12V/+15V and an inverting SMPS for -12V/-15V, neither of this circuits are hard to build or troublesome! The parts needed for these supplies rival the cost of a linear regulator solution and you can eliminate the need for extra wall warts (a big plus in my book). The flyback option is also there but I will have to look and see if a suitable off-the-shelf thru-hole coupled inductor is available for cheap.

I'm just not a big fan of charge pumps. They are quick and dirty power solutions meant for low current applications. They don't provide regulated voltage either, the output voltage is a function of the input voltage and load resistance. With an SMPS solution the output voltage is stable regardless of input voltage and load resistance (assuming you stay within the specs). For designs where there are bias points based on the supply voltage (like the linear-to-expo converter circuit) you want the supply to be pretty steady. Since the OTA's supply current varies with the bias pin current you will be causing the supply voltage to droop a bit as you go towards the higher octaves. This is mainly why I'd like to avoid charge pumps, especially when an SMPS solution is really just as cheap and simple.. People should not be intimidated by switching power supplies, in just about any situation we encounter they are easy to design and use, plus there are many people on this forum who can help others figure out any issues they are having..

Going from the 3080 to the 13700 isn't a big deal as long as you adjust the bias circuit accordingly to properly feed current into the Iabc pin (I have to look into this). The rest of the circuit has essentially the same drive requirements. FWIW I think it's worthwhile to use the linearizing diodes in the 13700, I use them in all of my filter designs to get a bit more headroom before distortion. Also since you have a larger input range you can get a better signal to noise ratio (i.e. a quieter circuit in operation).

I don't see why a BJT-based input (LPB) would be needed when an op amp circuit is simple and just as effective. We aren't driving long capacitive cables, we are driving the inputs to an OTA (relatively high impedance) which an op amp is fine and dandy for. The veritable TL07x op amps are great in filter circuits IME as they have super high input impedance, decent drive capability and decent DC performance (important for DC coupled circuits mostly).

I'll look at the design tomorrow but will leave it up to you to choose the LFO options you want. It's really the OTA linear-to-expo converter that is the tricky part, and I have a simple yet effective circuit already designed to do this with the 13700's.

Cliff, it's great to see you involved in this one, I have always done linear power supplies but it would be fun to mess with whatever you come up with.  I'm pretty useless for PCB layouts but I can breadboard and test various options as they come up, as well as building a hard version on perfboard when it comes to proof of concept time.

Strategy, is it correct that for now you want to focus on a hardwired HP into LP series setup?  I appreciate you reigning things back into sanity territory (but I do think we should make it easy to access the parallel and stereo modes, they come for free when you build it!) and I agree that getting the filter itself built up first is a good idea, and afterwards modulation options can be worked out.

I think I'll breadboard a second VCF and put it in series with the one I have and see how it sounds, and then spend some time modulating them.  It would be cool if someone else did this as well so we could compare findings.

What do you guys think about the frequency control setup I proposed (one master frequency knob and one "separation" knob)?  It's either that or two independent frequency knobs, but that way you need two hands to do a manual frequency sweep.  The "separation" control does add one opamp and four resistors, but I like it better in practice.  The separation control could also be considered a bandwidth control.

As far as levels, it seems like there are two ways to go- boost the guitar signal up so that it is not swamped by the resonance when you crank it up, or make the clipping diodes smaller so that the resonance signal doesn't get so big in the first place.  If you use an opamp first and add a volume control at the end you can set the input gain to be whatever you want and then keep the volume level reasonable on the way out, kind of a boost it up and then knock it back down approach.  This is another thing that I would recommend breadbaording before deciding one way or another, since I have one built and working already let me know if you want me to try anything.

ok!

jordan

@ Jordan, its probably a little bit dogmatic of me but I'm definitely inclined to follow the original MS-20 layout as the essential building block, so two separate frequency controls is ideal. The greatest thing about the filter I think is that you can control the two separately and dial in much more complex voicings.

The reason I keep leaning towards "simple" is maybe personal and practical in nature. I just find that with the more complex projects, it can take me literally years to finish them. Super fast projects, on the other hand, can be a little limiting -- and the 'cut corners' can be apparent (like in the Dutch MS-20 filter I just built.) There's kind of a medium zone that has a) easy to find parts b) moderate cost c) not-too-crazy wiring that makes a project "just right". Where you can sit down, do it in a couple of days and go straight to music making.

On the other hand I think all the mod ideas are crucial, so the project/layout/pcb has to leave "doors open" to do those - extra ground pads, maybe even a grid of extra pads somewhere? An objective would be to develop specifically docuemented expansions and mods here in the forum. Given that it's a synth module with pretty easy CV connections, I can think that someone with pretty basic knowledge could customize the circuit in a lot of different ways. I would rather open the doors to that than develop a project where the "optional" configurations are prescribed for the builder and might scare off people with less experience (people like me!)

Strategy

Quote from: Jordan A. on April 26, 2012, 10:15:04 AM
Cliff, it's great to see you involved in this one, I have always done linear power supplies but it would be fun to mess with whatever you come up with.  I'm pretty useless for PCB layouts but I can breadboard and test various options as they come up, as well as building a hard version on perfboard when it comes to proof of concept time.

Strategy, is it correct that for now you want to focus on a hardwired HP into LP series setup?  I appreciate you reigning things back into sanity territory (but I do think we should make it easy to access the parallel and stereo modes, they come for free when you build it!) and I agree that getting the filter itself built up first is a good idea, and afterwards modulation options can be worked out.

I think I'll breadboard a second VCF and put it in series with the one I have and see how it sounds, and then spend some time modulating them.  It would be cool if someone else did this as well so we could compare findings.

What do you guys think about the frequency control setup I proposed (one master frequency knob and one "separation" knob)?  It's either that or two independent frequency knobs, but that way you need two hands to do a manual frequency sweep.  The "separation" control does add one opamp and four resistors, but I like it better in practice.  The separation control could also be considered a bandwidth control.

As far as levels, it seems like there are two ways to go- boost the guitar signal up so that it is not swamped by the resonance when you crank it up, or make the clipping diodes smaller so that the resonance signal doesn't get so big in the first place.  If you use an opamp first and add a volume control at the end you can set the input gain to be whatever you want and then keep the volume level reasonable on the way out, kind of a boost it up and then knock it back down approach.  This is another thing that I would recommend breadbaording before deciding one way or another, since I have one built and working already let me know if you want me to try anything.

ok!

jordan

Quote from: Cliff Schecht on April 26, 2012, 01:47:36 AM
I'm just not a big fan of charge pumps. They are quick and dirty power solutions meant for low current applications. They don't provide regulated voltage either, the output voltage is a function of the input voltage and load resistance. With an SMPS solution the output voltage is stable regardless of input voltage and load resistance (assuming you stay within the specs). For designs where there are bias points based on the supply voltage (like the linear-to-expo converter circuit) you want the supply to be pretty steady. Since the OTA's supply current varies with the bias pin current you will be causing the supply voltage to droop a bit as you go towards the higher octaves. This is mainly why I'd like to avoid charge pumps, especially when an SMPS solution is really just as cheap and simple.. People should not be intimidated by switching power supplies, in just about any situation we encounter they are easy to design and use, plus there are many people on this forum who can help others figure out any issues they are having..

Going from the 3080 to the 13700 isn't a big deal as long as you adjust the bias circuit accordingly to properly feed current into the Iabc pin (I have to look into this). The rest of the circuit has essentially the same drive requirements. FWIW I think it's worthwhile to use the linearizing diodes in the 13700, I use them in all of my filter designs to get a bit more headroom before distortion. Also since you have a larger input range you can get a better signal to noise ratio (i.e. a quieter circuit in operation).

I don't see why a BJT-based input (LPB) would be needed when an op amp circuit is simple and just as effective. We aren't driving long capacitive cables, we are driving the inputs to an OTA (relatively high impedance) which an op amp is fine and dandy for. The veritable TL07x op amps are great in filter circuits IME as they have super high input impedance, decent drive capability and decent DC performance (important for DC coupled circuits mostly).

I'll look at the design tomorrow but will leave it up to you to choose the LFO options you want. It's really the OTA linear-to-expo converter that is the tricky part, and I have a simple yet effective circuit already designed to do this with the 13700's.

@ Cliff, I did not know that about the charge pump option, so that is really helpful information. And the input circuit as an op amp also sounds good. In the meantime I have been looking in the Gallery section to see what people have done for LFO's. The TAPLFO is extremely tempting but I keep thinking "what do you do when it's gone?"

Strategy

@Strategy, I appreciate the dogmatic approach, and think the goal of staying close to the original is laudable.  With that in mind...

With the original MS20, for each VCF you have frequency and peak controls, plus a level control for "modulation generator" and level control for "envelope generator 2".  So eight knobs total.  It seems like a pretty nice setup.

The modulation generator looks to be a triangle/square LFO with a "skew" control to tilt the triangle into ramps and alter the pulse width, we can whip up something like that easily with a dual opamp.  Two knobs here if you want to keep the skew function the way they did it, plus a switch if we want to switch between triangle and square.  

If we want to invert the LFO going to VCF B we could add a switch for reverse or normal sweep, or make the VCF B LFO level control an attenuverter.  Either way takes one opamp, the attenuverter option has one less switch but it's a little weird to have one knob go from 0 to 5 to 10 and the other go from -10 to 0 to +10.  I should add that a reverse sweep mode makes more sense to me in parallel mode, which sounds like it may be abandoned anyways, so this may be a non issue.

Instead of "envelope generator 2" I imagine an envelope follower will be easier and more flexible.  I personally get by with just sensitivity and attack controls, everybody feel free to suggest EF circuits if you have favorites.  I think attenuverters for the EF level controls make sense, it offers maximum flexibility with minimum panel controls.  I imagine the whole EF circuit plus attenuverters would be between 3 and 5 opamps.

So doing it pretty much like korg, I come up with 12 knobs, kind of a lot.  If you feel like that is getting out of hand the knob count could be thinned out some.  You may only need one control for the LFO level, for instance, and one control for the EF level.  Scraping the LFO skew function saves a knob, losing the EF attack control saves one.  The EF sensitivity control could also serve as the EF level control.  Get rid of all those and you are down to 7 knobs.  

I think I'm starting to see why you don't see to many commercial versions of this with modulation on board, it gets knob heavy really quick...

For those of us not scared of panel wiring, or those of us skilled at making layouts utilizing board mounted pots, it's still definitely worth while.

jordan

:edit: oh yeah, strategy, with the master frequency + separation control setup I was suggesting, you can still get the entire range of sounds.  You detune the filters with the separation/bandwidth control and sweep them both together with the frequency control, all variations are available.  That said, I'm happy to keep things towards the authentic side if you like, but for my own I know I prefer the one where you don't need both hands to sweep it.

cheers!

So this is where my dogmatic nature is quirky/unpredictable...I really want to remain true to the MS-20 filter core but I'm not as attached to the MS-20's modulation section. For one thing the native LFO does not go slow enough for my taste (maybe my original MS-20 needs a calibration or some mods!) :)

So we could have 2x LFOs and an EF but should consider using circuits other than or smaller than MS-20's. 12 knobs is pretty intense. I've done my fair share of mega projects, but none that big yet. (anyone following the String Ringer thread?? holy crap - I've been working on one for about 2 years!)

I am more and more starting to feel like we could do the modulation section as a daughter board. Would be inexpensive and give some options for people. There is a side benefit that people could build standalone 9V modulation generators for use with projects that accept CV's (like Ring Stinger, some Tim Escobedo project versions, etc.) or commercial pedals that are expandable with CV's (like Moogerfoogers.) The filter core should have a 9V out for chaining sub boards and add on's. There are a number of minimal and low-ripple EF's in this forum perhaps like the ones used in Clari Not and others. It might be fun to see if the Meatball's Vactrol EF is applicable in some manner but vactrol's ROHS certification is twilighting and the manufacturer has been bought out. I don't believe it's discontinued line, but, they have gone up in price at least 150%.

I found a couple LFO layouts in the Gallery, but they  are untested by me. Will post links.

@Cliff: perhaps the filter core layout can proceed without any modulation at all?

@Jordan: aside from TAPLFO and the MS-20's native modulation, are there any "minimal" LFO circuits that jump out as good options?

Strategy

Quote from: Jordan A. on April 26, 2012, 05:23:33 PM
@Strategy, I appreciate the dogmatic approach, and think the goal of staying close to the original is laudable.  With that in mind...

With the original MS20, for each VCF you have frequency and peak controls, plus a level control for "modulation generator" and level control for "envelope generator 2".  So eight knobs total.  It seems like a pretty nice setup.

The modulation generator looks to be a triangle/square LFO with a "skew" control to tilt the triangle into ramps and alter the pulse width, we can whip up something like that easily with a dual opamp.  Two knobs here if you want to keep the skew function the way they did it, plus a switch if we want to switch between triangle and square.  

If we want to invert the LFO going to VCF B we could add a switch for reverse or normal sweep, or make the VCF B LFO level control an attenuverter.  Either way takes one opamp, the attenuverter option has one less switch but it's a little weird to have one knob go from 0 to 5 to 10 and the other go from -10 to 0 to +10.  I should add that a reverse sweep mode makes more sense to me in parallel mode, which sounds like it may be abandoned anyways, so this may be a non issue.

Instead of "envelope generator 2" I imagine an envelope follower will be easier and more flexible.  I personally get by with just sensitivity and attack controls, everybody feel free to suggest EF circuits if you have favorites.  I think attenuverters for the EF level controls make sense, it offers maximum flexibility with minimum panel controls.  I imagine the whole EF circuit plus attenuverters would be between 3 and 5 opamps.

So doing it pretty much like korg, I come up with 12 knobs, kind of a lot.  If you feel like that is getting out of hand the knob count could be thinned out some.  You may only need one control for the LFO level, for instance, and one control for the EF level.  Scraping the LFO skew function saves a knob, losing the EF attack control saves one.  The EF sensitivity control could also serve as the EF level control.  Get rid of all those and you are down to 7 knobs.  

I think I'm starting to see why you don't see to many commercial versions of this with modulation on board, it gets knob heavy really quick...

For those of us not scared of panel wiring, or those of us skilled at making layouts utilizing board mounted pots, it's still definitely worth while.

jordan

:edit: oh yeah, strategy, with the master frequency + separation control setup I was suggesting, you can still get the entire range of sounds.  You detune the filters with the separation/bandwidth control and sweep them both together with the frequency control, all variations are available.  That said, I'm happy to keep things towards the authentic side if you like, but for my own I know I prefer the one where you don't need both hands to sweep it.

cheers!

Frostwave Resonator panel layout so you can get a sense of the elegant simplicity!

EDIT: I forgot that it had VC for resonance! I never found a completely familiar musical use for that but it is a popular bonus feature for filters in the modular synth universe

(http://3.bp.blogspot.com/_rScBRKlTdoE/TEh2X5bxF6I/AAAAAAABTJc/2ZVJCcpMf80/s1600/4817589389_36145f66d7.jpg)

VC resonance: just another OTA, see P5 rev2 VCF schematic.

This control is especially easy to implement because the V-to-I converter for the OTA is linear instead of exponential (typically). I like the option for either linear or expo for some controls but haven't played around much even with the voltage controlled resonance I've implemented in designs.

Not much time this morning, but yes, a classic integrator/comparator LFO is super easy to build and only uses two opamps.  It's very easy to set those up for whatever speed you like, and it's also easy to set them up with a very large range of speeds available.  If you have time check out the LFO in Jurgens SOST flanger, it's super cool- triangle, slewed square and sample + hold, it can be built with six opamps (no OTAs) and one single pole on-off-on switch.  I think a very nice setup is one LFO like that + one simple triangle only LFO, for eight opamps total you get a LOT of LFO action.

I don't personally see VC resonance being all that useful, it's not too hard to implement but it does add parts.

cheers,

jordan

Spent some time with two filters in series today, big fun.  An reverse sync option for the LFO will be quite useful I think, lots of "yoy" in there with the two cutoff frequencies dialed in.

My opinion after playing with it, if this is going to be a strictly series affair, is that one LFO will do.  It's still definitely cool with two, but if we are counting features and trying to keep it trim I think one LFO (with the option to reverse the sweep to one of the VCFs) will suffice.  I'm a fan of wild modulation and I could change my mind about that at any time, but after playing with it I still think separate LFOs make more sense when the VCFs are in parallel.  You can always add jacks for CV input.

Another issue is the feedback, Rene's version as drawn will self oscillate over a large portion of the resonance control, and the oscillation signal gets up to around 15 volts peak to peak at certain frequencies with the control maxed, you probably don't want to send that to a guitar amp.  I think the easiest and possible best solution will be to simply change the clipping diodes in the feedback amp, I'll mess with this some more.  I also added a 5k trimmer wired as a variable resistor in between the 1k8 feedback resistor and ground, so I could tailor the feel of the feedback control.  Since the whole point of an MS20 VCF is wild and nasty resonance we don't want to do anything to kill off it's character, just ensure that we don't blow up our speakers.

One of you guys has got to breadboard one so we can compare notes!  C'mon, do it!

jordan

You can always design less gain into the resonance circuit as well. IME a poorly set range on any resonance pots can really make some ears bleed. Maybe some sort of simple compressor would be of use to prevent volume spikes from overwhelming an amp. This could also help counteract the relatively large amplitude variations between the highest point of the passband and the lower portions (especially when Q is set high).

Quote from: Cliff Schecht on May 02, 2012, 08:17:57 PM
You can always design less gain into the resonance circuit as well. IME a poorly set range on any resonance pots can really make some ears bleed. Maybe some sort of simple compressor would be of use to prevent volume spikes from overwhelming an amp. This could also help counteract the relatively large amplitude variations between the highest point of the passband and the lower portions (especially when Q is set high).

Yes, that's what i did with the trimmer in series with the lower leg of the feedback divider.  The thing is the out of control way beyond oscillation stuff is sort of what makes this VCF special in the first place, so too much "improvement" might end up neutering it.  I absolutely like your idea of a compressor at the end, when set up for a narrow band pass sound the volume is obviously reduced, a compressor would help with that as well.

There are really two distinct parts to this i guess,

-the gain of the feedback amp as set by the resistors, this largely controls how the resonance control feels and how soon oscillation sets in

-the clipping threshold as set by the diodes, which largely controls how large the oscillation signal is after the turn diodes on

(I know you already know all this stuff Cliff, I'm figuring it out as i go)

I think I like the idea of a trimmer for resonance gain and smaller clipping diodes, for general guitar use.  Maybe a zener clipper at the end would serve as a super simple limiter, it might allow the resonance do it's wild thing while preventing speakers from being shredded?  I'll try some more stuff out...

jordan

I don't like trimmers. To me they are a necessary evil and circuits that rely too heavily on them annoy me (I'm trying to refrain from being too snobbish about circuit design on a DIY forum :)). With that said, if you are intending to increase the flexibility then why not just have a resonance pot that you can tweak externally? This is one of my favorite knobs on a synth usually..

That's exactly the reason I recommend the compressor. The voltage controlled resonance circuit (if this is implemented, I LOVE this on a synth fwiw) would only use one side of a 13700 and so the second side could be used to implement a simple one or two knob compressor. I like the idea of a compressor more than a limiter because as you said filters can vary all over the place amplitude-wise. This might help make the effect more usable in a much wider variety of places. This is the reason most synth filters don't make great guitar filters, they tend to get much higher Q than anything you typically find in effects pedals.

Quote from: Cliff Schecht on May 03, 2012, 03:42:26 AM
I don't like trimmers. To me they are a necessary evil and circuits that rely too heavily on them annoy me (I'm trying to refrain from being too snobbish about circuit design on a DIY forum :)). With that said, if you are intending to increase the flexibility then why not just have a resonance pot that you can tweak externally? This is one of my favorite knobs on a synth usually..

That's exactly the reason I recommend the compressor. The voltage controlled resonance circuit (if this is implemented, I LOVE this on a synth fwiw) would only use one side of a 13700 and so the second side could be used to implement a simple one or two knob compressor. I like the idea of a compressor more than a limiter because as you said filters can vary all over the place amplitude-wise. This might help make the effect more usable in a much wider variety of places. This is the reason most synth filters don't make great guitar filters, they tend to get much higher Q than anything you typically find in effects pedals.

Sure, I just stuck a trimmer in so that I could easily change the gain of the feedback amp while playing it.  I do have an external resonance control (actually two, one for each VCF).  I think this is a parameter that people may want to tailor to their own needs, ideally a few other people will breadboard it and we can decide on what to do with it.  I posted earlier about a value change for one resistor that will make the resonance control feel more "normal", alternately you can stick a trimmer in and set it to your liking, or you could switch between two resistors with a "tame" and "wild" switch.  I'm quite happy deciding what to do for myself, but my preference may not be to everyone's liking.  I'm just trying to feel out what you guys, and specifically Strategy, will like.  I agree that in the final product a trimmer here is probably unnecessary, in the meantime it seems useful, until the gain of the feedback amp is decided on.

I did a stereo series/parallel filter box a few months ago (very similar to what we are talking about here, but with two 4 pole LPFs), and I did include two NE570 compressors, with just a simple blend control to dial in how much compression was wanted.  I stuck it on a dual pot so one knob controlled both sides.  I also included a simple FET distortion circuit x2 before the VCFs, with one dual pot controlling both channels.  Both of those additions are super handy.  This is just to say that I'm a fan of big bloated circuits, but I've been trying to reign in my own tendency towards wanton excess in order to keep this project accessible.  I think an almost more handy feature for this specific filter would be a noise gate, to mute any oscillation when you are not playing...

On another note, changing the clipping diodes to 1N914 seems to be just about right for my guitar (with P90s).  The resonance when oscillating doesn't get speaker rippingly loud compared to the guitar signal and the general character seems intact.

And I know this isn't where you wanted to go with it, but Strategy, when you get one built up you must try them in parallel.  If you are a fan of complex formanty sounds parallel offers a lot of variation you don't get with series operation, as you can have the HP up higher than the LP without the sound going away, extra yoy.

cheers,

jordan

I'm back after a few days gone...

resonance control for both HP and LP will be important. that makes two minimal compressors. One BIG advantage to including those is that when you raise the Q there is the appearance of overall volume drop in many filters. Which is why so many synths need a compressor after them so that when you modulate your sound from an low-resonance, fairly unfiltered sound to a very filtered one you get relatively even volume across the whole sound.

I did not realize that a compressor could be done with part of an LM13700. What a flexible chip! And I just dug up a ring modulator circuit that also uses this same chip! What CAN'T it be used for?  :)

Strategy

going back to the power supply- I think that it would a smart move to make it 9v powered. not doing this might turn a lot of people away, and compared to the rest of the circuit, this would not be that difficult to add. is the hesitance in the power supply being unstable?

Quote from: Strategy on April 26, 2012, 08:02:22 PM
Frostwave Resonator panel layout so you can get a sense of the elegant simplicity!

EDIT: I forgot that it had VC for resonance! I never found a completely familiar musical use for that but it is a popular bonus feature for filters in the modular synth universe

(http://3.bp.blogspot.com/_rScBRKlTdoE/TEh2X5bxF6I/AAAAAAABTJc/2ZVJCcpMf80/s1600/4817589389_36145f66d7.jpg)

Aww man!  Now you made me go and start missing Paul Perry all over again!!

Why'd you have to go do that, and where the dickens is Paul??

Quote from: teej212 on May 03, 2012, 11:18:00 AM
going back to the power supply- I think that it would a smart move to make it 9v powered. not doing this might turn a lot of people away, and compared to the rest of the circuit, this would not be that difficult to add. is the hesitance in the power supply being unstable?

Stability would not be an issue. There are two options here WRT powering off of 9V. You can either bring in 9V and run it through some sort of switching supply(ies) to convert it to +/-12V or +/-15V. Or you can redesign the circuit to work off of a lower voltage and single supply. IME the second one can neuter a filter, you need a certain amount of headroom for the resonance circuit to work right especially. Also reducing the supply can in general change how the effect sounds because all of the currents being sourced to the OTA's are dependent on the supply voltage as well (meaning changing the supply voltage will change the cutoff and resonance). I'd still like some switchers onboard but I looked into what is available off the shelf and a flyback converter would probably require custom magnetics (not cheap, unless someone buys a bunch in bulk).

what about charge pumps?  Im not too educated in the area but if you got a voltage boosted from 9v to 30v you could make it a +15/-15 v from 9v, no?

Talking psu and charge pumps, here's the converter from the lt1054 datasheet on stripboard, with zener regulators, which works well, but *maybe* doesn't have enough current for a 2 filters?

(http://i48.tinypic.com/15fi0es.png)


Andy

Not quite. What you need is a charge pump that goes from 9V to 15V and another one that either goes -9V to -15V or you can run a simple inverting charge pump to change +15V to -15V. But you can't (easily) make +/- voltage from a 30V supply, or better stated you wouldn't want to.

As I stated earlier the problem with charge pumps is they are unregulated voltage. The output voltage is a function of the output current, meaning the output voltage varies with the amount of current the circuit is drawing. For OTA based filter circuits I think what we will find is that the filter starts to drift worse and worse as you push the cutoff frequency higher and higher. As you go higher in cutoff frequency more current has to be driven into the Iabc pin of the OTA which means that the biasing circuit will also have to draw this additional current. As the supply voltage drifts, the overall bandwidth of the filter will change as well, not something I want to unintentionally add into any filter circuit.

I still hold that the best solution is two switching power supplies or even a single flyback supply. I can design them with easy to obtain parts and off-the-shelf magnetics that would make these no more difficult to build than your run of the mill charge pump circuit. To be honest it annoys me that people are so adamant about NOT using switching power supplies when they usually are the most viable solution to the problem (especially in this case where we want clean, regulated power!). I want DIY'ers to get over their SMPS-phobias and start applying these circuits instead of bulky, unregulated charge pumps. I can understand people not liking SMD parts but most any SMPS that we would need can be made from one of a handful of general purpose controllers (MC34063 or UCC3804 for example) and a small range of inductors (anywhere from like 10uH to maybe 1mH) depending on application and current draw).

Sorry but that charge pump circuit looks pretty massive. The power solution should not take up as much real estate as the circuit itself IMHO...

With that said, another usable option for us would be a 9-15V boost power supply and a charge pump to derive the negative supply rail. The negative supply in these circuits usually draws much less than the positive one and if you run an inverting charge pump off of the +15V, you will at least have a bipolar power solution where the plus and minus voltage track each other decently well.

Cliff, what you've said is exactly what happens with the above charge pump circuit. I use 18v zeners and the voltage seems to stabilize at about 16v, but this is running just 1 filter. I'm keen to see a design using the 34063 which would be more stable. I've tried the double chip boost and invert but unless I made an error somewhere seems to have the same problem of voltage drop with surrent draw, but worse.

(http://i46.tinypic.com/mvj8kz.png)

In the LT1054 datasheet see application 30 and 31 for both a regulated positive and negative voltage doubler.

With the V L of the LT1054 and diodes you should be able to get regulated +/-15v from a 9v supply.

There are also Push Pull PWM IC's like the SG3524. There's an app in the datasheet for a relatively simply flyback +/-15v supply.

I'm going to reverse myself a little, with the clipping diodes I'm liking two 1N914s in series (in both directions) better than just one 1N914.  An LED with a similar voltage drop would probably give similar results.  This is really an area where people can pick whatever is their preference, just reporting my findings.

Cliff, or anyone who has done work with this, I would be exited to try out any power supply option you cook up if you can knock out a quick schemo, as long as I can find whatever I need at mouser/digikey.

jordan

Yeah more diodes in the negative feedback amplifier circuit means more negated amplitude is sent back to the filter which will give you less resonance overall (if I understand what you are doing correctly).

I have figured out a couple of viable power solutions for this using off the shelf components. I'm thinking either a flyback controller where we have users wind their own coupled inductors from purchased/scavenged toroids (this is a DIY forum, right? :P) or use a coupled inductor on an inverting regulator to generate both +/- voltage. I'm thinking the second option both because it offers a very compact solution and it uses only off the shelf components. Everything for that can be bought from Mouser for ~$5.

Regarding clipping diodes for the resonance loop, I've tried alot of combos, and keep coming back to 2 zeners, but in standard clipping configuration, so about .5v forward voltage. 2 leds with a 1n34a on each end also sounds really good, but makes the self oscillation too loud. With the 2 zeners, or possibly any silicon diode, the res only takes over when the input level is backed off, and is about the same volume as the input signal was, which makes the filter much more usable imho.

here's a link to Thomas Henry's ±15v power supply (http://www.nutsvolts.com/uploads/magazine_downloads/11/January%201998%20Thomas%20Henry%20-%20Power%20Supplies%20for%20Electronic%20Music.pdf), if anyone is still interested.

That schematic is the most common method for deriving split supply rails from a single wall supply. We've discussed that circuit earlier and decided that we'd like to power this pedal from the same supply as any other 9V negative center pedal would take. I'd still like to at least do a version with a proper switching power supply (or set of them). If you sync the oscillators between the two chips then you can eliminate the possibility of them crosstalking and sending either supply off the deep end. Also with a well designed filter(s) the switching will be nearly invisible to the audio circuit, at least in the sense that it will never be heard by the human ear. The real trick here is to try guarantee stability and at least decent regulation under all operating conditions while making the supply easy to implement with inexpensive off-the-shelf through hole components.

TBH I've set this project aside for now just because this thread has been pretty dead. I have been toying with the power options and have a few I still need to try out, but nothing that will take that long I don't think. If Strategy says that he's ready to move forward I will start working up the schematics and boards. I think we should make a couple of versions to start with with different power supplies and LFO's to see what set of features we really want for guitar.

i'd love to start breadboarding, sorry i can't help with much

Quote from: garcho on May 17, 2012, 11:01:17 AM
i'd love to start breadboarding, sorry i can't help with much

Sorry folks work has kept me away from the forum. The Dorkbot-pdx circuit board orders are twice monthly so as soon as we have layouts to test I'm happy to do a test run of 5-10 boards and distribute them to the group! As for the power options I like the idea of being able to get people powered up from typical 9V like Cliff is suggesting.

I realize I need to weigh in on the many modulation options. I'm going to go back through the thread and respond definitively. The number of options are a little overwhelming to me.

Strategy

As for LFOs, I'm thinking that if we give one LFO per filter that is a LOT of modulation by comparison to other pedals, which usually give one LFO with numerous waveshapes. Separate modulation of the two filters will allow some of the formant/"yoy" effects mentioned. As for waveshapes, do we want to have switchable between sine/square/tri? Gristleizer and some other circuits offer ramps but with two filters being modulated I think the nuances of ramp will be lost amongst all the "mangling".

What I like about this is that it does something that an unpatched MS-20 can't do: more than one LFO, one for each filter. On the original MS-20 you have one LFO for everything, oscillator, filter, etc., which is one of the real limitations of the synth. So in that way, a 2-LFO pedal, even if limited in terms of number of waveshapes, is offering up some possibilities beyond the original synthesizer.

This seems like a good balance between the minimal and maximal proposed modulation options. I still don't understand how sync/unsync would be implemented or whether its desirable. I don't have any devices with synced LFOs for comparison.

Paul

One thing I'd like to suggest is that the CV inputs for filter cutoff freq and resonance should be in parallel with the onboard LFOs and should not deactivate the LFO's via switching. That way you can have the LFO's running AND pipe in additional CV's, for very complex modulations.

Strategy

Quote from: Strategy on May 17, 2012, 12:46:25 PM
As for LFOs, I'm thinking that if we give one LFO per filter that is a LOT of modulation by comparison to other pedals, which usually give one LFO with numerous waveshapes. Separate modulation of the two filters will allow some of the formant/"yoy" effects mentioned. As for waveshapes, do we want to have switchable between sine/square/tri? Gristleizer and some other circuits offer ramps but with two filters being modulated I think the nuances of ramp will be lost amongst all the "mangling".

What I like about this is that it does something that an unpatched MS-20 can't do: more than one LFO, one for each filter. On the original MS-20 you have one LFO for everything, oscillator, filter, etc., which is one of the real limitations of the synth. So in that way, a 2-LFO pedal, even if limited in terms of number of waveshapes, is offering up some possibilities beyond the original synthesizer.

This seems like a good balance between the minimal and maximal proposed modulation options. I still don't understand how sync/unsync would be implemented or whether its desirable. I don't have any devices with synced LFOs for comparison.

Paul

QuoteThat way you can have the LFO's running AND pipe in additional CV's, for very complex modulations.

i like that.
i also much prefer 9V, just didn't read the thread close enough. i'm super excited to start playing guitar through something like this...

I feel slightly responsible for cluttering up the discussion with too many options too fast, I like that we're getting back to practical territory. 

Any of you guy's can start breadboarding these things, if you don't have access to a bipolar power supply you can always just use two 9 volt batteries, it works at +/- 9 volts, definitely well enough to give you an idea of it's capabilities.

If you need ideas for LFO circuits I can try to do a quick pen and paper sketch (if I can manage to figure out how to post pictures here), pretty much any LFO that swings a few volts and has a comfortable frequency range will work well.  I think a good old two opamp integrator/comparator LFO is the ticket.  This is an (approximately) 1 volt per octave filter, so an LFO that is 10 volts PP (for example) will modulate the VCF through 10 octaves when the modulation depth control is all the way up, which is more than enough range, IMO.

I'll try to draw up a proposal for an LFO mode switch, that I think is very nice.  The option to sync (and reverse) the sweep of the VCFs is very important, I think.  When you get it breadboarded up I think you will see what I mean, with two free running LFOs the modulation is a bit "wandering", for sure a cool effect but it's nice to be able to get a more "focused" sound when you want it.

ok!

jordan

Any schematic anyone recommends more than any other? To start I'll use a simple LFO and 2 9v batteries.

I'm excited to eventually use the TAPLFO with this project.

If you have the time you could try both of these -

http://www.jhaible.de/jh_720_vcf.gif

http://www.uni-bonn.de/~uzs159/

I've been mostly messing with the Rene one, but i do remember breadboarding the Jurgen Haible one a while ago.  I'm not sure how Jurgen's will do at +/- 9 volts, but I know the Rene one will work.

Jurgen's has the benefit of being really tiny and cheap to make, and it will probably work with just about any NPNs, PNPs and opamps you have.

The tapLFO would absolutely work well.

jordan

Thanks Jordan, I will try this out next week or the one after. After that I won't have any time until August or so, but I'll keep checking in.
I recently used the LFO from the Tremulous Lune to control an active filter, sounded great. Lots of shaping possibilities. Getting excited!

Started following this thread as it was dying down and I'm now starting to breadboard the Schmitz filter to compare the 3080s to the 13700.

I'd like to find a solution for +/-15v and I'm looking at this MAX743 DC-DC converter: http://datasheets.maxim-ic.com/en/ds/MAX743.pdf (http://datasheets.maxim-ic.com/en/ds/MAX743.pdf)

Does this look like a good way to go?  I'm pretty new to electronics DIY so any help is greatly appreciated!

Been moving real slow on this one...

I'm looking at Rene's schematic. My plan is to swap the CA3080 out for a LM13700. Anything I should know about that? I plan on leaving the 13700's diode biases floating, the buffer outs floating, and the buffer ins grounded. Any other concerns I should have?

CA3080
p2 - inverted in = p4 on 13700
p3 - non-invert in = p3 on 13700
p5 - amp bias = p1 on 13700
p6 - output = p5 on 13700

I was thinking of using the MFOS wall wart power supply for ±12V. Any more thoughts on power options?
Will swap the BC558C for 2N3906, yeah?

Ok, one more post, sorry about all this, I should've just waited.

Found these (http://www.synthforum.nl/forums/attachment.php?attachmentid=67521&stc=1&d=1297613272) MS-20 schematics in this (http://www.diystompboxes.com/smfforum/index.php?topic=73021.160) post. 

Yeah, thoughts on power options.  I should have put this in my previous post, but the MAX743 dual output switch-mode regulator takes 5v and outputs +/-15v or +/-12v. Also it's not SMD so it looks good to me.

I'm not breadboard enabled and have been in overtime at work but look forward to hearing the testing results so we can settle on the desired configuration for a pcb layout! I suspect the "rene schmitz but configured for LM13700" like garcho is doing will be the chosen solution

Will see if I can drum up an etchable version for the sake of testing since i am not breadboardy...(I know, I know I should be...)

Strategy

Subscribed. Interesting thread

ive said it before, but the circuit not being 9v will be a big turn off for many people. and if not a turn off, having 9v is undeniably a big plus, and the max743 circuit is quite simple, so why not?

i suppose while breadboarding its fine to use the +12/-12 supply thats from MFOS.  and tap lfo would be awesome in this circuit!

EDITS:

So 9V into a 78L05 into the MAX743? And then into two regulators?

How about current draw, charge pumps and this circuit - something to worry about? The 743 datasheet says at 15V it can deal with 100mA. That's gotta be plenty, right?

I thought I'd use the MFOS style ±12V for the breadboarding simply because those parts are laying around my 'lab', not because I think it's specifically an elegant solution for the PS.

I erased a few things because I just reread the thread, should have done that earlier, sorry for clutter

Strategy, when is your birthday? I'll send you a breadboard and jumper cables.  ;)

The MAX743 is not necessarily a bad solution but the implementation of a circuit using this is relatively bulky and I'll bet the part itself isn't cheap. I don't dislike Maxim parts specifically but I tend to only use their stuff if it meets a specific niche that nobody else does (which is IMO what they do especially well!).

Charge pumps aren't a bad solution for low current, non-critical applications but as I said before, you really want regulated voltage when you are running filters. You will hear the filter change cutoff frequency if the supply voltage drifts enough to cause the control circuits (for frequency and Q) to drift. I played around with a neat solution that uses a single coupled inductor to generate +/- power from a single 9V input but still haven't gotten around to building a real prototype version and trying it in an audio circuit.

Interesting problems with the power supply; good for the build threads.  :)

Sorry to unnecessarily clutter this thread with a thousand posts, but I have some questions.
Looking at Rene Schmitz's schematic:
What does the 'pitch' node get connected to? I don't know anything about synth circuits (i.e. gate vs pitch). I'm planning on only using the 'In' node, for instance, with my guitar.
What does the pot (47KB between +15V and -15V) directly below the 'pitch' node alter? Is that the frequency?

Quote...for high pass you just do the old trick of grounding the input resistor, lifting the second caps grounded leg and inputting your signal there...

So the "real" input for 'FROSTY the FILTER' is through the grounding cap (1N) of the second OTA stage (O2)? Keep everything else the same, except for grounding the 10k 'In' resistor?
Then the output of the HP goes straight into what's listed on the schematic as 'In' (of the LP stage) and the 'Out' (again, of the LP stage) goes to amp?
Is the eventual goal: audio in -> buffer -> HP w/frequency and resonance pots, LFO/CV -> LP w/frequency and resonance pots, LFO/CV -> compressor -audio out?
2 separate LFOs?

This thread has more info and a stripboard layout for Rene's OTA version

http://electro-music.com/forum/topic-43085.html

garcho - that 47kb is the for filter frequency

The filter can be switched from LP - HP using both inputs too but can't do both at the same time, you need 2 filters.

@Garcho- The "pitch" input is for use in a modular synth where you have a calibrated 1 volt per octave control signal, usually from a keyboard controller.  In guitar world we don't have to worry about it.

You can see in the upper left of the schematic a junction of four 100k R's, an 1k8 R and the base of a PNP transistor, this is a summing node for control voltages, basically you feed a voltage to this node through a resistor and that voltage will sweep the frequency of the filter.  We can add any number of inputs to this node just by adding more 100k resistors, one is for manual control like you suspected and the others could be used for LFOs, envelope followers or whatever.  Those 47k pots could be anywhere from 10k to 100k without noticing any difference.

Your description of the signal path sounds good for the classic HP into LP setup.  I don't know if an integrated compressor will be mandatory, it's more of a luxury.

The power supply does seem to be a bit of a sticking point.  I suggested an AC wallwart/half wave rectifier bipolar supply earlier (like the MFOS "wall wart power supply"), and I still think it's a pretty good solution.  It keeps people away from mains wiring and works well enough, and it's barely more work/real estate than a charge pump.  I guess the main drawback is the special wall wart you need to carry around.

It's possible we could get adequate performance using a charge pump to get +/- 9 volts just like RG shows in the neutron documentation, but with two VCFs and two LFOs the current draw will be higher than the neutron for sure.  I don't have much experience with those, but that seems to be the most pedalboard friendly setup.

jordan

:edit: as for subbing a 13700 for the 3080s, just drop it right in and ignore the diode bias pins and darlington buffers.  PNP transistors can probably be whatever you have, 2N3906 will definitely work.  cheers!

 :icon_eek:



This is the first time I've ever really wished I could curse on this forum. I just breadboarded Rene Schmitz's schematic and it sounds ____ amazing! I used two 9V batteries, a LM13700, two 2N3906s, two random GE diodes instead of the LEDs, 2K resistors instead of 1K8, and fed the CV1 in signal from the simplest dual opamp function generator I could find.
Because I didn't do the math on the LFO, my initial test started out with a triangle wave oscillating at like, 250Hz, which, of course, sounded like a ring modulator! With the resonance all the way up there was oscillation squeal but with the resonance turned down about 50% or so, the oscillation shrinks so far down it's hard to hear unless you turn your amp up way loud, and then it's still a fairly small signal. It sounds incredible, I'll record samples soon. With the LFO slowed down into tremelo territory, depending on how the resonance, frequency and CV in mix pots are adjusted, the filter sounded like a strange flanger or something. I was having so much fun playing through it I ran out of tinkering time! I played through a MOSFET boost before the filter, definitely needed extra gain. I haven't converted it to HP yet (waiting for more parts before I can breadboard both filters simultaneously  :(  ). I can't wait to try it out with both, and with LFO CV more interesting than my square and triangle generator (which will work perfectly for ring mod/trem-ish sounds). Time to start looking for the simplest ADSR generator schematic I can find. I'll report more later, hopefully I'll have more time next week, otherwise it might be a month or two. Thanks for turning me on to this y'all, it's becoming my favorite project! Big thanks for the detailed help Jordan!

EDIT: Just realized ADSR doesn't apply here, but some sort of CV shaping outside of basic wave forms would be nice. Maybe a CV expression pedal...

Can't respond in great length but independent CV inputs for LP cutoff, HP cutoff, LP resonance and HP resonance will be crucial to the final version of the project so user's can insert whatever they want in parallel to the LFO's. In the example of the Frostwave Resonator I borrowed, I used the Frostwave FAT Controller sequencer (compact 2-channel CV/gate sequencer) to rhythmically modulate the cutoffs in a loopy fashion...it is endlessly musical and addictive

ADSR's from a modular synth, or anything would be good

As for foot control, it should be easy for most DIY'ers to use switching jacks to add expression pedal input to the desired controls? I have done this with many of my builds

Strategy

Quote from: garcho on June 11, 2012, 06:48:14 PM
:icon_eek:



This is the first time I've ever really wished I could curse on this forum. I just breadboarded Rene Schmitz's schematic and it sounds ____ amazing! I used two 9V batteries, a LM13700, two 2N3906s, two random GE diodes instead of the LEDs, 2K resistors instead of 1K8, and fed the CV1 in signal from the simplest dual opamp function generator I could find.
Because I didn't do the math on the LFO, my initial test started out with a triangle wave oscillating at like, 250Hz, which, of course, sounded like a ring modulator! With the resonance all the way up there was oscillation squeal but with the resonance turned down about 50% or so, the oscillation shrinks so far down it's hard to hear unless you turn your amp up way loud, and then it's still a fairly small signal. It sounds incredible, I'll record samples soon. With the LFO slowed down into tremelo territory, depending on how the resonance, frequency and CV in mix pots are adjusted, the filter sounded like a strange flanger or something. I was having so much fun playing through it I ran out of tinkering time! I played through a MOSFET boost before the filter, definitely needed extra gain. I haven't converted it to HP yet (waiting for more parts before I can breadboard both filters simultaneously  :(  ). I can't wait to try it out with both, and with LFO CV more interesting than my square and triangle generator (which will work perfectly for ring mod/trem-ish sounds). Time to start looking for the simplest ADSR generator schematic I can find. I'll report more later, hopefully I'll have more time next week, otherwise it might be a month or two. Thanks for turning me on to this y'all, it's becoming my favorite project! Big thanks for the detailed help Jordan!

EDIT: Just realized ADSR doesn't apply here, but some sort of CV shaping outside of basic wave forms would be nice. Maybe a CV expression pedal...

Now that I think of it adventurous users could plug in simple CMOS sequencer like in Vanishing Point, Seek Wah, and others

HexInverter has a compact easy sequencer project right now called SympleSeq (PCBs) that should run off +/-9V

I really think of this project as a core that can then be added on to, I still wonder if a version without any LFOs or any modulation is desirable so it's just a core! Somewhat modular for a pedal I guess but even without modulation it is a deep tone shaper

Strategy

Here's how I'm feeling at the moment. I'm gonna start small and work up to the pro version. It is definitely doable with Electric Druid's TAPLFO (http://www.electricdruid.net/index.php?page=projects.taplfo). Wouldn't even be that huge of a PCB. Off board wiring headache though - 16 knobs, 8 jacks, 6 toggles, and 2 foot switches! Also, at $10 each, the TAPLFO is probably a bit of an extravagant chip to double-up, but then again, a 16 knobber is extravagant no matter how you cut it. Unsure of how to work the parallel/series switching out without a patch bay; new territory for me. Regarding the LFO sync switch, I assume that one LFOs can sync the other through the external clock pin of the second TAPLFO, maybe that's not the case. The tap tempo would control both LFO rates. Also, still unsure of how to rectify the power supply situation with our +9V guitar pedal universe.
If I understand the conversion to HP filter correctly, it seems like switching between HP and LP would be a simple matter of wiring a 3PDT toggle switch for each filter:
(http://farm8.staticflickr.com/7094/7364366562_f700d9893b.jpg)
Does that look right to anybody?
There would be 4 filter combinations that way (HP->LP  HP->HP  LP->HP  LP->LP). If I understand the circuit correctly and that switching works, I'll try those combos out and make clips of them eventually, and decide if it's worth it to add to the core version, or just leave it HP into LP.
What am I forgetting?

frosty the filter:
buffer -> HP -> LP -> out
8 knobs (2 freq, 2 res, 4 CV in mix)
4 CV ins
possible HP/LP switching
power?

frosty the pro (with TAPLFO):
buffer -> series or parallel -> out
16 knobs (2 freq, 2 res, 4 CV in mix, 2 LFO shape, 2 LFO speed, 2 LFO mix)
6 CV ins (2 wave distort CV ins for the LFOs)
series/parallel switch
2 HP or LP switches
2 native LFOs
2 LFO switches (freq or res control)
2 LFO external sync ins
LFO sync switch (so both LFOs are in sync together)
tap tempo foot switch for the LFOs
power?

Nice work Garcho!  I think we can do HP/LP switching with two poles and two throws and use a cheaper switch, let's see if I can get a picture to show up-

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/IMG_0202.JPG.html

Here is an idea for series/parallel/stereo operation using switching jacks and an additional opamp, hopefully it makes sense.  Unmarked resistors are all the same value, I used 100k.

-If we plug into "input A" and come out of "output B" we get VCF A cascaded into VCF B for series mode.

-If we plug into "input B" and come out of "output A" we get the output of VCF A added to the output of VCF B for parallel mode.

-If we plug into "input B" and run "output A" to one amp and run "output B" to another amp we get mono in/stereo out.

-If we plug one signal in to VCF A and another signal in to VCF B and use both outputs we get stereo in/stereo out.

-We can use either/or VCF independently.

-One quirk is that if we plug in to "input A" and come out of "output A" the output of VCF B will be added to the output, even though there is no input signal for VCF B.  This might be weird or confusing if VCF B is oscillating, but if the resonance control of VCF B is turned down it shouldn't be a problem.

-Another quirk is that it will make bypassing hard/weird, I have not given it much thought yet.

-Any improvements or suggestions are welcomed.

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/IMG_0200.JPG.html

:edited for an output mode I forgot:

Bravo Jordan!

DPDT is way better, nicely done. I like the series/parallel ideas, too, thanks for drawing those out.
Do you have any desire for native LFO?
Any ideas on PS?
Would anyone be into having a miniature Bantam patchbay on their pedal? At the very least it would look hilarious...

This might help if you just want simple series/parallel switching:

(https://dl.dropbox.com/u/967492/Gemini/hidded/Gemini%20Project%20Publish/Series%20Parallel.JPG)

Yes, if you want to mess with series/parallel on the fly a switch like that is cool, no stereo though...

Here is an idea for a switching scheme for two LFOs, this gets you a great many modulation options with two switches.  It's a little weird to look at so I'll go through it.

http://www.aronnelson.com/gallery/main.php/v/diyuser/IMG_0203.JPG.html

-The "sweep mode" switch is shown in the "normal" position, LFO A sweeps VCF A and LFO B sweeps VCF B, each with its own depth control (the pots wired as voltage dividers).

-In the middle position we have a "sync to LFO A" mode, LFO A sweeps both VCF A and B, the depth can be set separately for each VCF.

-In the bottom position we have an "add" mode, the sum of the two LFOs modulates both VCF A and B.  In this mode the depth control for LFO A controls how much of LFO A is fed to both VCF A and B, and the depth control for LFO B controls how much of LFO B is fed to VCF A and B in the same way.

-The other switch chooses an inverted or normal LFO signal to drive VCF B, this is handy in the "sync to LFO A" and "add" modes.

I think it's a pretty fun setup, I've used this exact scheme in a similar project and remain happy with it.  Again, any suggestions or improvements are welcome.  I used 100k linear pots and 100k for all unmarked resistors.

As for the LFOs themselves, it becomes a preference thing really.  The two tapLFOs would be super duper deluxe, one simple triangle only LFO with just a "reverse B" switch would still be fun for days.  My vote is for one "fun" LFO and one simple triangle only LFO .  The "fun" one could be the tapLFO, my vote is for an LFO I stole from Jurgen Haible, from his "son of storm tide flanger" that gives you triangle, square or sample and hold waveforms.  If you don't need voltage control of LFO frequency (I don't think it's all that important for this kind of project) an LFO that does that can be made with six opamps, so ten opamps total for the LFOs and inverters for VCF B.

jordan

@FC>UK
thanks for the scheme, that looks perfect. personally, i'd take that simplicity over the stereo option. Deluxe w/stereo for sure.

@Jordan
love those LFO thoughts, will look at those Monday, exciting!

Quote from: garcho on June 11, 2012, 06:48:14 PM
:icon_eek:



This is the first time I've ever really wished I could curse on this forum. I just breadboarded Rene Schmitz's schematic and it sounds ____ amazing! I used two 9V batteries, a LM13700, two 2N3906s, two random GE diodes instead of the LEDs, 2K resistors instead of 1K8, and fed the CV1 in signal from the simplest dual opamp function generator I could find.
Because I didn't do the math on the LFO, my initial test started out with a triangle wave oscillating at like, 250Hz, which, of course, sounded like a ring modulator! With the resonance all the way up there was oscillation squeal but with the resonance turned down about 50% or so, the oscillation shrinks so far down it's hard to hear unless you turn your amp up way loud, and then it's still a fairly small signal. It sounds incredible, I'll record samples soon. With the LFO slowed down into tremelo territory, depending on how the resonance, frequency and CV in mix pots are adjusted, the filter sounded like a strange flanger or something. I was having so much fun playing through it I ran out of tinkering time! I played through a MOSFET boost before the filter, definitely needed extra gain. I haven't converted it to HP yet (waiting for more parts before I can breadboard both filters simultaneously  :(  ). I can't wait to try it out with both, and with LFO CV more interesting than my square and triangle generator (which will work perfectly for ring mod/trem-ish sounds). Time to start looking for the simplest ADSR generator schematic I can find. I'll report more later, hopefully I'll have more time next week, otherwise it might be a month or two. Thanks for turning me on to this y'all, it's becoming my favorite project! Big thanks for the detailed help Jordan!

EDIT: Just realized ADSR doesn't apply here, but some sort of CV shaping outside of basic wave forms would be nice. Maybe a CV expression pedal...

What was the supply voltage of the LFO? When you use the filter with a +15V -15V supply do your LFOs voltage swing have to match that?

9V, because I used two batteries instead of a 'real' power supply.
There is a pot that bleeds the LFO's signal to ground.

Quote from: garcho on June 15, 2012, 04:36:16 PM
9V, because I used two batteries instead of a 'real' power supply.
There is a pot that bleeds the LFO's signal to ground.

Wouldn't you get 18V by using two batteries? BTW how did you connected the two batteries? 9V/0V/-9V or 18V/9V/0V?

-9 0 9

Quote from: garcho on June 16, 2012, 08:22:22 PM
-9 0 9

I've made an LFO based on a now discontinued MoltenVoltage IC but it can put out only positive control voltages. (from 0V to whatever depending on the supply voltage of the analog side)

Could i supply this filter with a voltage something like +24V +12V 0V so the LFO's control voltage could better match the filter's voltages. (i developed a fear from trying this without asking since i killed two other filters messing around with their suppply voltages like this wasting some vintage russian ICs, handmade pcbs and lots of hours)

So, I'm using this project to learn to use Eagle.  I've got the LP and the HP on a board and want to implement the series/parallel switching next. 

Looking at FC's schematic I'm assuming that in addition to the DPDT, we'll need the opamp for the parallel situation to sum the two outputs (sorry, absolute beginner here).

I'm not sure what the Vref input going to the + pin of the opamp is, though.  Some help please?

Quote from: ml on June 22, 2012, 09:31:03 PM
So, I'm using this project to learn to use Eagle.  I've got the LP and the HP on a board and want to implement the series/parallel switching next. 

Looking at FC's schematic I'm assuming that in addition to the DPDT, we'll need the opamp for the parallel situation to sum the two outputs (sorry, absolute beginner here).

I'm not sure what the Vref input going to the + pin of the opamp is, though.  Some help please?

VREF is always half of the supply voltage. (it is 4.5V when you use a 9V battery to power the pedal)

btw i made that René MS10 in addition to Escobedo's, sadly i had to take the LM13700 out of the Escobedo to get the René work.
(caution the vero in that René thread is full of mistakes, there is an extra, non-needed cut around the IC, then you should put cuts under the CV resistors (the 100k ones) and the transistor orientation is probably wrong, i used 2n3906s)

Quotewe'll need the opamp for the parallel situation to sum the two outputs

Have you breadboarded this yet? You'll want that opamp in the front too, boosting the signal, at least for guitar.

Noob question: Is the opamp at the end of the circuit more important for buffering the output than summing the parallel filters?

Excited to hear how your PCB-ing goes.

Well, I breadboarded until the point when I realized it wouldn't fit on my half breadboard. Then I decided to try to make a pcb, since I've just gotten a laser printer. 

I've got one filter laid out on a board and am trying to fit the other filter on it (the free version of Eagle does maximum 10 x 8cm).

Once a +/-15v power supply configuration is decided upon, I plan to put that and possibly two TAPLFOs on a daughter board. 

I can post the eagle files if it might be useful to anyone.  The only thing is that I'm using the 3080.  Easy to swap it in the schematic, though. 

Please share your eagle files when the time comes, i'd be into some experimenting with that!

If people have success or need PCB's I can coordinate a limited run of pcbs through this group-buy thing:

http://oshpark.com/ (http://oshpark.com/)

Strategy

Quote from: ml on June 26, 2012, 05:30:21 PM
Well, I breadboarded until the point when I realized it wouldn't fit on my half breadboard. Then I decided to try to make a pcb, since I've just gotten a laser printer. 

I've got one filter laid out on a board and am trying to fit the other filter on it (the free version of Eagle does maximum 10 x 8cm).

Once a +/-15v power supply configuration is decided upon, I plan to put that and possibly two TAPLFOs on a daughter board. 

I can post the eagle files if it might be useful to anyone.  The only thing is that I'm using the 3080.  Easy to swap it in the schematic, though. 

I'll definitely post the files once I have a working board!

In the meantime, for those who might find it useful, here's a video of a lecture in which the Rene Schmitz filter is discussed. The cap before the output is bigger in the video, but aside from that, it's the schematic we're working with.

http://blip.tv/abovenyquist/ems-2010-session-24-sallen-key-filters-part-2-3614551

He starts talking about the MS-20 filter at around 8 minutes.

I've tried to watch that guys lectures before but he just doesn't understand electronics well enough to make me want to really listen to him. He needs to be more careful about how he words things, some of the stuff he says can be either misleading or wrong.

The circuit itself is actually pretty simple. The way the circuit is inverting in the first stage and not in the second stage is a way to get a signal for the negative feedback without an additional inverting op amp. Instead they have the resonance pot which feeds into a high impedance "+" input. While this circuit is a fun way to save an op amp, I think for our purposes it would be smart to change the input stage to a non-inverting stage and use an inverting amplifier for the feedback stage (you could use a quad op amp and buffer the resonance pot before it sees the low input impedance inverting op amp stage). For me this fixes the problem of inverting polarity (which they obviously didn't care about in the original circuit) but we still have the problem of a very low input impedance to that OTA stage. Some older effects would definitely not enjoy a low input impedance very much.

Getting even more clever with the design you could you could again change that input stage to non-inverting (moving the first 10k to the "+" input) and buffer the input with an op amp. Then setup the resonance circuit as an inverting circuit (which then keeps the correct negative feedback relationship with that first stage changed to non-inverting). Keep the resistors in the feedback network large to keep input impedance high (SNR won't be a problem here in a feedback path). The last two op amps go where they already are in the filter schematic.

Ideally the second change would give you functionally the same circuit with a couple of the modern "niceties" that I expect from modern pedal designs. You guys can still go crazy with whatever LFO and power solutions you want, but at least make the heart of the overall filter circuit non-inverting! If you were to setup any sort of wet/dry knob this would cause problems for instance...

I can also redesign it to be a single supply circuit most likely without complicating the design too much, but I haven't really looked into this much. If there are enough requests to either change the power rails (to say +/-12V instead of +/-15V) or to try to go to a single supply design then I can figure out how to do this.. We could probably even get away with a single 9V supply although you lose some noise performance (lower SNR).

QuoteWe could probably even get away with a single 9V supply although you lose some noise performance (lower SNR).

For a project on this particular forum, I think a single 9V supply justifies lower SNR. For the super mega deluxe pro platinum version maybe we should go all out and have 12 or 15 bipolar.

So people have tested the Rene schematic with a 13700 and it didn't work properly? I'd much prefer this part over a 3080 for the OTA's. If it doesn't work I can run some simulations to figure out what needs to be changed. I suspect the 13700 has *slightly* different requirements for the ibias pin but it should not be a big deal to switch to the superior (and cheaper!) chip. I'll look into this as well as a single supply, the latter will be trickier to implement methinks.

I got the circuit working pretty quickly in simulation with a single supply and the small changes I suggested. I think I will need to do go back to pen/pad to figure out what some of the resistor changes need to be to work correctly with the 13700 but I've certainly got a good starting point to do so. Also I didn't muck with the OTA bias circuit yet but this shouldn't be too bad to swap over to single supply either.

I'll post schematics and layouts as I finish them..

Cliff thanks for jumping in on behalf of the 13700 that is my preference as well!

Quote from: Cliff Schecht on June 27, 2012, 03:05:48 PM
I got the circuit working pretty quickly in simulation with a single supply and the small changes I suggested. I think I will need to do go back to pen/pad to figure out what some of the resistor changes need to be to work correctly with the 13700 but I've certainly got a good starting point to do so. Also I didn't muck with the OTA bias circuit yet but this shouldn't be too bad to swap over to single supply either.

I'll post schematics and layouts as I finish them..

Quote from: Cliff Schecht on June 27, 2012, 01:03:27 PM
So people have tested the Rene schematic with a 13700 and it didn't work properly? I'd much prefer this part over a 3080 for the OTA's. If it doesn't work I can run some simulations to figure out what needs to be changed. I suspect the 13700 has *slightly* different requirements for the ibias pin but it should not be a big deal to switch to the superior (and cheaper!) chip. I'll look into this as well as a single supply, the latter will be trickier to implement methinks.

No, I've built it several times using 13700s.  Seems to work just fine, definitely sounds good, nice range of frequency control, etc.

Also I've lowered the power rails to +/- 9 volts without noticing any big change in normal use, but I have not messed around with a single rail supply, I would love to see what you come up with!

The weather here is too nice for me to be dorkin' around online, but it's fun to see people messing with this one,

jordan

The single supply circuit I am running seems to work alright. I still have to figure out the expo current source and resonance circuit but the heart of the filter works off a single supply (on the bench at least :D). So far I'm running the circuit off of 15V (what my powered breadboard has available) but surely 9V won't be an issue either. Next I need to build a proper current source and the resonance section to complete the system, but I don't anticipate either of these giving me problems. I'll keep you guys updated and try to post some schematics once I get a bit further.

Isn't the Korg Monotron already contains an MS10/20 filter which is single supply/low voltage? Its schem was published by korg.

I think in the league of simple 12db low pass filters the king is the Polivoks. It's a shame that according to the forums there are not too many of us who could make it work (including me, it didn't work on the breadboard nor on a homemade etched PCB for me)
I think i gonna soon order a PCB for it and gonna continue messing with it, (it would be nice to turn it into 9v/single supply) it's sound would be pretty cool for filtering distorted guitar signals.

I just want to second Jordan and say that there are no issues I noticed when using LM13700 or 9V bipolar. I haven't tried this circuit with the 3080 or 12-15v so I can't compare the two. Put that iron down it's summer! (for some of us  ;))

After spending the entire day messing with various expo current source circuits, I found an NPN variant that works well with a single supply. It's functionally equivalent to what the MS-20 uses (or good enough at least). Since the NPN's only sink current in this circuit, I mirror the expo current up to a PNP current mirror and source current to the Iabc pin from the second PNP. Works in simulation and should work fine on the bench.

Here is what I have so far (verified in simulation and parts on bench, may need values tweaked!): http://www.aronnelson.com/DIYFiles/up/ms20_9vsupply.pdf

Also worth note is the expo circuit currently is backwards from standard convention, that being a 0V input to the cutoff pin is the highest cutoff frequency and a 10V input is fully off. For guys just running an LFO this is no issue (and not fixing the polarity saves us an op amp), if you were to run a CV voltage from a knob then the cutoff knob would work backwards (if the pot is wired correctly!).

Awesome, thanks Cliff

Great, thanks Cliff.  I just built it up, using BC550 and BC560 for the BJTs, 1N914 for the diodes, and 10k for R3 and R7.  Sounds good!  I'll have to put the bipolar one and single supply version side by side, but the single supply circuit from you definitely sounds good, in that nasty kind of way.  When fed with a wave with sharp transitions (pulse/saw) and high resonance you get the really interesting frequency locking stuff as you sweep the frequency this VCF is known for.

nice work,

jordan

Sweet! I was wondering if anyone was actually going to build this circuit, that pesky Iabc bias circuit took me quite a bit of brain thinkings to get working correctly and I didn't want it to go to waste. The PNP version (meaning no NPN transistors in the bias source) would rail out the op amp before the BJT could actually bias itself on so I switched to the NPN version that mirrors up to PNPs. Seemed to work fine in sim (and the bipolar version has worked for me on the bench in the past). Either way glad it works for someone else!

Is the bass response good on it? I always use a large input cap on filters so I can get those low down dirty PWM'ed square waves to punch hard (they sound weak and buzzy if the input cap is too small). Any other complaints or tweaks? I think 10k was the right choice for those missing resistor values but you could try tweaking them if you don't like the range of the filter (smaller means more range). You can also tweak the 1.8k resistor but then you lose the 1V/octave effect which is why this circuit is used in the first place.

I think this effect would also benefit from some voltage controlled resonance. It could be eassily implemented with a second 13700. I'd like to have both cutoff and resonance controlled by pick attack to form a super funky envelope follower.

Quote from: WhenBoredomPeaks on June 23, 2012, 01:49:51 AM
VREF is always half of the supply voltage. (it is 4.5V when you use a 9V battery to power the pedal)

Great, thanks! I did a bit of reading and am under the impression that the Vref is only needed in the case of a single supply. 
If using +/-15V, then instead of Vref, it should just go to ground.  Is that right?

Lots of interesting stuff going on here. I'm interested to see how Cliff's single supply version sounds!

Quote from: ml on July 06, 2012, 04:26:16 AM

Quote from: WhenBoredomPeaks on June 23, 2012, 01:49:51 AM
VREF is always half of the supply voltage. (it is 4.5V when you use a 9V battery to power the pedal)

If using +/-15V, then instead of Vref, it should just go to ground.  Is that right?

yeah

will jump in the wagon, since i love this filter and this particular chip  :P

noiw, what has me really interested is the feedback path, i tried an opamp solution last time i tinkered with it, but to no avail, will see if this time it works, cause feedback path and clipping ala Escobedo's 9V MS20 sounds nasty

So, I've tried breadboarding Cliff's schematic a few times, and keep getting the same problems. Jordan said it sounded fine when he did it, so I'm sure I'm missing something and f-ing it up. I used 10k resisters for R3 and R7, tried Si and Ge diodes, as well as a variety of LEDs, and I wired the wiper from a 50k pot to the point marked 'cutoff', with terminals 1 and 3 going to +9V and 0V. I get constant oscillation, which can vary in tone while I move the cutoff pot from 0 to 9. The resonance doesn't change much in the sound, but will sometimes cause the sound to go silent temporarily. When I add a LFO via another 50k pot and 100k resistor in series, the oscillation and resonant wackiness increase. The signal from my guitar sounds fizzy and crappy. I've tried swapping out the ICs a few times with no luck. Here are some voltages I took:

voltage is 13.36 and VCC/2 is 8.05
when cutoff is 0V and the guitar signal is about 0.65V (going through a booster):
LM13700:
1 - 1.29
2 - 0.15
3 - 4.65
4 - 4.72
5 - 1.88
6 - 0.00
7 - -0.04
8 - 0.10
9 - 0.10
10 - -0.04
11 - 13.36
12 - 4.65 to 4.85
13 - 4.67
14 - 4.85
15 - 0.15
16 - 1.29

TL074:
1 - 5.95
2 - 5.95 - 1.90
4 - 13.36
5 - 4.71 to 4.81
6 - 4.45
7 - 4.40
8 - 2.60
9 - 4.72
10 - 4.76
11 - 0.00
12 - 5.00
13 - 5.43
14 - 4.62

Q1
c - 0.90
b - 12.75
e - 13.23
Q2
c - 13.23
b - 12.75
e - 12.80
Q3
c - 12.80
b - 8.23
e - 7.75
Q4
c - 7.75
b - 8.20
e - 8.20

when the cutoff voltage is at 50% (6.5V) the pins' voltages are significantly lower. I didn't test it with cutoff at 13V. I can post the other voltages if it helps.

any ideas?

EDITS: added transistor voltages, remeasured VCC/2

Something is wrong with your reference circuit I think. The OTA inputs should both sit at 1/2 VCC which they clearly are not. I think you should start here. Even the half supply point you measure isn't actually 1/2 VCC or really even close to what it should be (~6.7V). I would start here and figure out why the 1/2 supply part isn't working right.

Thanks Cliff, will try and hammer this out tomorrow

I forgot to mention that when I built the single supply version I buffered the bias voltage with an opamp, it was a while ago but I think I remember the bias voltage acting whacky until it was buffered. 

This is pretty much the first (solid state) thing I've built with a single supply, so opamp biasing using a resistive divider is new to me, but maybe buffering VCC/2 is something to try, Garcho?

jordan

Where is the source of VCC/2? From the base of T3? From pin 10 of the TL074?
To buffer the VCC/2, I take the bias from the voltage divider and send it to the non-inverting input of an opamp, then tie the inverting input to the output, and the output is the buffered VCC/2? So where in Cliff's schematic is the voltage divider? What point to I connect to the non-inverting input?
Am I way off? Sorry, I need someone to hold my hand.

You are spot on. Use something like a 100k/100k voltage divider (both resistors equal value is all that is really important, use a larger value to prevent excessive current draw) and buffer this with an op amp unity gain follower. I didn't show this in my schematic because I wasn't sure if a buffered 1/2VCC point was needed, although just looking at how much it gets used I think it's a pretty obvious point for doing so.

What I might do is add a dual op amp to the design to buffer the low-impedance input as well. The BJT's in the OTA have an input impedance that varies with the bias current which could cause loading issues to some older effects pedals if not buffered (plus this lets us add an input gain/attenuation knob to set the levels for different guit-fiddles).

Well, I've got the VCC/2 at buffered 6.7V, but both inputs on both OTAs are at 6.4 or so. The sound is better than it was, but still super f-ed up, with squealing, noise, fuzz and crap-outs, and no filter effect at all. I'll keep triple checking everything, but any ideas how that could be?

Anyone else looking at this? Still can't get the single supply version worked out...

Are you sure the 13700's are good? IME those are quite easy to blow up during the breadboarding process!

Maybe post your voltages again, everything will be different now that your VCC/2 is correct. You could also try converting your breadboard build to the original dual supply version and see if it works that way.

You said the thing squeals and such, sounds like the resonance circuit is out of whack. Try removing the output of the resonance circuit that feeds into the bottom of the cap after the OTA current output. Instead jump the bottom of this cap to VCC/2 (like you do in the second stage). Doing this removes the resonance circuit and lets you debug just the OTA filters.

I'll give it another go, thanks for the tip about the filter debugging Cliff!

I tried what you suggested Cliff, with grounding the capacitor, but I still got no filter effect, just weirdness. So I put the dual battery back on the breadboard to check the OTAs; they're all fine. I ran out of time, so I'll poke away at it more later this week. Thanks for the help.

That point isn't grounded, it goes to VCC/2 (as does the second cap). The circuit won't work correctly if the capacitors are tied to ground instead of VCC/2.

Quote from: Cliff Schecht on June 29, 2012, 02:59:04 AM
After spending the entire day messing with various expo current source circuits, I found an NPN variant that works well with a single supply. It's functionally equivalent to what the MS-20 uses (or good enough at least). Since the NPN's only sink current in this circuit, I mirror the expo current up to a PNP current mirror and source current to the Iabc pin from the second PNP. Works in simulation and should work fine on the bench.

Here is what I have so far (verified in simulation and parts on bench, may need values tweaked!): http://www.aronnelson.com/DIYFiles/up/ms20_9vsupply.pdf

Hey Cliff, I was hoping to breadboard/prototype this, but all of the values aren't there... Is there a more updated version of this schemo that I'm just overlooking? A 9v version of this filter that I could turn into a stompbox is like a dream come true.

Quote from: woolybeard on August 14, 2012, 01:46:38 AM

Quote from: Cliff Schecht on June 29, 2012, 02:59:04 AM
After spending the entire day messing with various expo current source circuits, I found an NPN variant that works well with a single supply. It's functionally equivalent to what the MS-20 uses (or good enough at least). Since the NPN's only sink current in this circuit, I mirror the expo current up to a PNP current mirror and source current to the Iabc pin from the second PNP. Works in simulation and should work fine on the bench.

Here is what I have so far (verified in simulation and parts on bench, may need values tweaked!): http://www.aronnelson.com/DIYFiles/up/ms20_9vsupply.pdf

Hey Cliff, I was hoping to breadboard/prototype this, but all of the values aren't there... Is there a more updated version of this schemo that I'm just overlooking? A 9v version of this filter that I could turn into a stompbox is like a dream come true.

do a quick google search for the escobedo 9v ms20 filter

btw in my opinion building the filter is not even half of the battle, after having built a few filters (escobedo ms20, "real ms20", minimoog, steiner, meatsphere etc. /btw the ms20 is the last one i would use from this list for guitar/) i think the CV/controlling element is more important. a voltage follower would be enough for guitar use but it is not as flexible as a real synth ADSR. (btw if someone knows a voltage follower with attack and decay/release controls please tell me, the one in the meatsphere is not really good) so i made a real ADSR and a circuit which makes gate signals from my guitar signals but it have it's own problems again because it needs complete silence for a few millisecs to end the gate signal and you actually have to concentrate pretty hard to put a bit of silence between your notes because that is not how you play guitar)

Quote from: woolybeard on August 14, 2012, 01:46:38 AM

Quote from: Cliff Schecht on June 29, 2012, 02:59:04 AM
After spending the entire day messing with various expo current source circuits, I found an NPN variant that works well with a single supply. It's functionally equivalent to what the MS-20 uses (or good enough at least). Since the NPN's only sink current in this circuit, I mirror the expo current up to a PNP current mirror and source current to the Iabc pin from the second PNP. Works in simulation and should work fine on the bench.

Here is what I have so far (verified in simulation and parts on bench, may need values tweaked!): http://www.aronnelson.com/DIYFiles/up/ms20_9vsupply.pdf

Hey Cliff, I was hoping to breadboard/prototype this, but all of the values aren't there... Is there a more updated version of this schemo that I'm just overlooking? A 9v version of this filter that I could turn into a stompbox is like a dream come true.

Ask and ye shall receive! I updated the schematic (link below) with the changes I mentioned before. Essentially I added an input buffer with some gain (which can be omitted if not needed), a buffered VREF (probably not optional) and some decoupling caps sprinkled around to keep crap out of the power supply with high dv/dt control voltages.

http://www.aronnelson.com/DIYFiles/up/ms20_9vsupply.pdf

Yeah Cliff, it sounds great! Tried out yer latest schematic and it sounds awesome. At first, it started making some fizzy noises and there was no filtering effect (sound familiar?). Then, I realized I was grounding the base of T4 via R15 (1k8) instead of connecting it to VCC/2. I bet that's what I was doing with my previous attempts. Stupid!!!  :icon_redface:

I'm using TL074s and 72s and blue LEDs. A 2k resister instead of 1k8, and fudging a little on some cap values. But it seems to work fine. So far I've only used it as a LP filter, not LP into HP or dual or anything yet. I'll take some voltages down in case other folks are having problems. Thanks for the PS help and schematics Cliff. Excited to get this together!

Inspired by this project, I've started putting together something similar with 2 BP filters in parallel. I'll post some clips of that when I get it up and running a little more polished. Right now I've only used it with R.G.'s max1044 bipolar 9V schematic because that's all I know how to do, but it works well.

EDIT: Oh yeah, anyone else having fun with LFOs for this project? I'm trying a few different things for some simple on-board LFOs.

Glad to hear it's alive! It is a funky little filter ain't it. 8)

Sure is! I'm gonna try out Ken Stone's Psycho LFO with it, see if I can hear that pseudo randomness.

Uh... don't know if anyone has mentioned it, if so disregard this.

Has anyone seen the Korg Monotron? Is this thing really running the MS-20 filter? If so, for 50 bux, is it worth building a filter, or can one just gut a Monotron and go from there?

EDIT* - Just found the schematic; http://www.korg-datastorage.jp/Manual/monotron_sch.pdf (http://www.korg-datastorage.jp/Manual/monotron_sch.pdf)

and a pic of the guts;
(http://www.timstinchcombe.co.uk/synth/monotron_ref_des.jpg)

nice, thanks for posting that Dino! but still gonna build one  :D

I've slowly been working on a single sided PCB for this, I'm sure it will be 2-3 times the size of the entire monotron. :P  It's Cliff's schematic mixed with Rick's switching and 2 ultra-basic LFOs, CV ins for the cutoff, and a clean blend pot.

I've played with one of those monotrons, and they're a lot of fun. Never plugged it into an amp, though. I've made some small 'travel synth' toys, basically audible Schmitt triggers a la Nicolas Collins and Ray at MFOS, and man, when you plug those in, they can rattle the windows! Next time you're on the road and you want to quell irritating companions, build yourself something like the monotron, just don't let anyone else get their hands on it!

that monotron schematic is pretty interesting, mostly op amps, Schmitt triggers and NPNs. sounds like a good project to start breadboarding!

I'm just REALLY curious what a guitar going through a fuzz box, then into one of these suckers would sound like. :icon_twisted:

EDIT* - Think I just found my answer... to some degree. I'm REALLY tempted now.

Quote from: digi2t on October 06, 2012, 07:14:15 PM
I'm just REALLY curious what a guitar going through a fuzz box, then into one of these suckers would sound like. :icon_twisted:

EDIT* - Think I just found my answer... to some degree. I'm REALLY tempted now.

fuzz into minimoog vcf: (and some other fx)

http://soundcloud.com/dptp2/primitive-filter-patch

pretty bad recording but it sounds kinda cool

The monotron filter is just LP, right? As opposed to LP->HP, or parallel?

So happy this thread is alive again

This is a nice 9v OTA VC-LPF too:

http://ericarcher.net/devices/diy-lpf/

Btw, all his stuff is pretty cool

wow, killer link, thanks Arnoud!

been thinking about this again, and what would make for a good guitar pedal version of this lovely filter.

9 pots: cutoff x 2, res x 2, LFO rate x 2, LFO/CV depth x 2, clean/wet blend

3 toggles: parallel/series a la Rick Holt's suggestion, LFO ramp up/off/down x 2

4 jacks: in/out, CV x 2 (for cutoff, seems more important than res for pared-down version)

2 OTAs, 12 op amps* (in whatever package configuration is most convenient for the Byzantine layout this will end up being), 8 BJTs, yay 50 resistors, 25 caps.
*8 for the 2 filter sections, 4 buffers (VCC/2, input, parallel/series output, blend)

I'm thinking HP/LP switching isn't so important for a MS-20 clone, and any complication of the LFO is doubled. It seems important to have some kind of on-board LFO if we're gonna make this a guitar pedal project, since most guitarist won't have too many modules or synthy things laying around. I think an op amp with rate, depth and ramping is good enough.

Pipe dream? I want to make this an etchable project, but maybe it's too much.  :P

Guitarists might not have CV-outputting "synthy things" but the (cut-off frequency) CV input can easily be contolled with an expression pedal for a superb wah sound - I've breadboarded the Eric archer 9-volt LPF linked above, and it sounds great as a quasi-wah. Vero layout to follow when it's verified.

this is still on my daydream list but I had to step away from DIY for some months - this summer and fall I had two albums come out, both my bands, local shows all summer, a couple of singles, and tours.

I've stopped all shows for the rest of the year and just fired up my soldering iron the other day - to add a CV input vactrol kludge to the delay time pot of my EHX deluxe memory man! It works!

Your list below sounds amazing! etchable project would be fantastic but if someone did 2-sided boards my offer still stands to coordinate a proto run and eventual group buy...

Strategy

Quote from: garcho on November 17, 2012, 11:53:05 AM
been thinking about this again, and what would make for a good guitar pedal version of this lovely filter.

9 pots: cutoff x 2, res x 2, LFO rate x 2, LFO/CV depth x 2, clean/wet blend

3 toggles: parallel/series a la Rick Holt's suggestion, LFO ramp up/off/down x 2

4 jacks: in/out, CV x 2 (for cutoff, seems more important than res for pared-down version)

2 OTAs, 12 op amps* (in whatever package configuration is most convenient for the Byzantine layout this will end up being), 8 BJTs, yay 50 resistors, 25 caps.
*8 for the 2 filter sections, 4 buffers (VCC/2, input, parallel/series output, blend)

I'm thinking HP/LP switching isn't so important for a MS-20 clone, and any complication of the LFO is doubled. It seems important to have some kind of on-board LFO if we're gonna make this a guitar pedal project, since most guitarist won't have too many modules or synthy things laying around. I think an op amp with rate, depth and ramping is good enough.

Pipe dream? I want to make this an etchable project, but maybe it's too much.  :P

Bravo! glad to hear it's been a productive season for you. Guess I'll have to buy a few new records now   8)

Designing a double sided PCB worth fabbing is way beyond my "skills". I'd love to buy a few once we get there, though. I'll put together a big schematic with all the elements. After a few more eyes and ears noodle with it and we get it finalized, maybe we can find a volunteer. Calling Mr. Livingston...

I still cast my vote for "Frosty" as the project name in honor of the venerable Mr. Paul Perry, erstwhile forum member and designer of the Frostwave Resonator that is the inspiration for this to a large extent.

Strategy

Quote from: garcho on November 18, 2012, 05:52:33 AM
Bravo! glad to hear it's been a productive season for you. Guess I'll have to buy a few new records now   8)

Designing a double sided PCB worth fabbing is way beyond my "skills". I'd love to buy a few once we get there, though. I'll put together a big schematic with all the elements. After a few more eyes and ears noodle with it and we get it finalized, maybe we can find a volunteer. Calling Mr. Livingston...

Agreed!

If you guys pick an LFO circuit you like (or envelope, whatever) I can probably work up both an etchable layout and a two-sided production board over Thanksgiving. Did anything get finalized for this project?

Cliff that's friggin' awesome! Unfortunately, I'm on the road until December, but will be breadboarding as soon as i get home. Any suggestions on the ideal LFO out voltages? I've made a number of LFOs, function generators, etc., but don't know how to make them ideal for the need, I just noodle by ear until they sound good. Anything seem obvious to you so far?

Questions for the gurus:

For this circuit at 9V, ideal full amplitude modulation for the LFO would be ±4.5V, biased from VCC/2 (don't know if I'm saying that correctly)? If it went beyond, would the excess peak cause any problems? The signal would just be clipped and a little squared-off, right?

This is a little premature, but when I eventually get around to the desktop version of this, I was hoping to use Electric Druid's TAPLFO for the on-board LFO. Being a PIC, I am assuming (?) it puts out ±2.5V. Is there a way to amplify the signal without distorting the wave shape?

I have a Frostwave on the bench right now. I've done a video to demo the controls. One I finish fine tuning it, I'll be getting on with tracing.



I'll post pictures and a schematic as soon as I can.

Quote from: digi2t on November 29, 2013, 10:49:55 PM
I have a Frostwave on the bench right now. I've done a video to demo the controls. One I finish fine tuning it, I'll be getting on with tracing.

I'll post pictures and a schematic as soon as I can.

Superb. I've got a second looper pedal on indefinite loan, and quite fancy a static filter with HP and LPF to noodle with.

That's great!

I put together the Rene Schmitz bipolar version back when this thread was last kicking and I'm about to tear it apart and put it in a modular format.

The frostwave in your video sounds very similar to the Schmitz design and the resonance response is as you describe it--very touchy.

I'm curious to take a look at the frostwave schematic since I'm still on the lookout for the "real" ms-20 filter!

I haven't had much time for DIY this year but this is likely to bring me back into orbit. Having an MS-20 and having spent a couple of years with a borrowed Resonator I can tell you that they're very, very close in sound - should have bought a Resonator while Frostwave was still around!

Nice demo, Dino.  :icon_biggrin:  I can see why people have so much fun with that thing, and with the Korg original.

The pitch-to-voltage (external input, KLM-129) section of the MS-20 has a similar sort of filter, with variable 2-pole highpass (what they call "low-cut") and 4-pole lowpass.  It is intended to zero in on the pitch fundamental range of whatever it is one wishes to do P-2-V on.  There doesn't appear to be any means to adjust resonance/Q, however.

yeah cool demo dino ,  you might want to check out nick.d's filter...very , very cool...

http://www.diystompboxes.com/smfforum/index.php?topic=104178.msg937481#msg937481

schemo at the top.. 8)

Nice D'Astro, did you get that hammered out? That one is really similar to the Schmitz and Schecht versions, which seem to be fairly close to the app sheet 4 pole with added exp current source and buffers. Only so many ways to skin an OTA, right? But it's super funky. Glacially piecing mine together; life keeps derailing me. Knowing Dino is making a vero version will light a fire under my ass to get the single sided etch version done. I'll be holed up in the mountains for the next month with my laptop so it might actually happen. Can't let the strip board guys win!  ;D    

breadboarded the Schecht version with TAPLFO and noodled with an envelope follower, i never got far; i just ended up playing my guitar through it for whatever time i had available to work on it because it sounded so rad  :P  multiple LFOs are really nice with this filter.

I figure that the LP>HP board will be big enough, so i plan on keeping the modulation on baby boards. Can't wait until Taylor finally makes a musicPCB board for this. *Ahem*

Speaking of musicPCB, the site is down, anyone know if Taylor's ok? Hopefully just the usual ups and downs of the internet.

^ thats not me, that's 'nick .d's..

he says he's getting it sorted soon... an 8 pot jobby.. :icon_eek: :icon_cool:

in my gallery you'll find a vero board layout for a MS20 VCF filter work-a-like. the layout is verified:

http://www.aronnelson.com/gallery/main.php/v/Gila_Crisis/ms20_filter.png.html

(http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=50498&g2_serialNumber=2)

Sorry folks, I forgot to link the Frostwave info here.

http://www.diystompboxes.com/smfforum/index.php?topic=105312.0 (http://www.diystompboxes.com/smfforum/index.php?topic=105312.0)

Everything you wanted to know, but were afraid to ask. Includes tuning guide.

Thanks for the updates!!!

I've fotgotten to write that I found the schematic at this link:
http://www.aleph.co.jp/~takeda/radio/MS20clone.html

there you find also a pcb layout and some infos as well if can read japanese...
Andgo and take a look on this website, is full of really interesting stuff!

Andy is working on transferring my schematic to PCB form. I'll be doing a survey soon to gauge interest. If interest is anything like it was for the GIII Dual Fuzz or PLL, production should be a non-issue. By the complexity of the circuit, I imagine board price might sit somewhere between the PLL and the Ludwig, we'll see.

I'll take one Dino.

I was thinking, it would be really easy to make a board that could be LP or HP, depending how you wire it. You could also make the buffers bypassable with wiring. Then those who want the real MS-20 LP->HP can buy two and connect them in series or parallel, and those who only want the LP or HP can buy one.

Any news Andy or Dino on a pcb for the frostwave/ms-20?

Quote from: agoldoor on August 03, 2014, 03:49:09 PM
Any news Andy or Dino on a pcb for the frostwave/ms-20?

Something special is coming. We've taken the Frostwave, and put it on steroids. It's still in the Skunkworks stage, but lets just say, it should blow some minds. I'm getting the prototype board soon. As soon as I do the proof build, and verify the operation, we'll release the hounds.

Very excited about this... Tell me there's potential for a 10k pot expression pedal for the LPF cut-off frequency!?
I got a korg monotron very cheap a couple of days ago, and it's got me hankering for a super squelchy standalone ms20 filter for guitar.

The korg is awesome btw, and well suited to hacking for use with something like the ehx 8 step program or diy sequencer, e.g baby10.

Quote from: nocentelli on August 03, 2014, 05:06:20 PM
Tell me there's potential for a 10k pot expression pedal for the LPF cut-off frequency!?

All I'm saying, is that the expression pedal angle, is the tip of the ice berg. If Andy wants to spill the beans, I'm cool with that, but you better have your tuque tied on with bailing wire, 'cuz it's gonna blow your mind.

I'm going back to my cave now.

:icon_mrgreen:

I'm so excited about this. Also digi2t, another hen's tooth filter pedal has crossed my path, gonna do a thread about it momentarily
Strategy

dino you teaser... ;)

looking forward to hearing this beast... 8)

Quote from: deadastronaut on August 04, 2014, 06:14:20 AM
dino you teaser... ;)

looking forward to hearing this beast... 8)

Yeah, well, I normally have no qualms about sharing, you guys should know me well enough by now. But, in this case, I had one of those "What if...." extreme psycho moments, and Andy decided to get on my crazy train. He brought it to reality, so I'll let him make the announcement when he feels ready about it.

I'll say this though, it's an honor working with this dude. He worked his tail feathers off to come up with the PCB that would fit my idea, and there was a lot of emails to and fro to get to this point.

Shit Dino, I'm not precious.
Happy to let you guys know what's going one.

First, I have to say that this is the result of a very, very sick mind (not mine!!).
Dino rightly says, there have been many emails back and forth.  It took me a while to realise that he was serious, that's when I realised what a sick puppy he was.

"So I want to be able to control any of the CV voltages with a wah pedal"
'Ooh, that'll be cool'
"And then I want to be able to choose if I'm negative toe down or positive toe down"
'Hmm, OK'
"And I want it to fit a cry baby shell"
'No s**t Sherlock'
"Oh, and I want to have as many of the CV inputs going at the same time and at any polarity"
' you are f***king mad!!!'

So that was the dialogue (sort of) and that was my brief.
And that's what we did.

This will not be for the faint hearted.  it's a tad more complicated and lots of flying wire.
There are four pots to mount somewhere on the pedal (it will NOT be me doing the prototype build).
There is a board to be mounted on the front of the pedal with four triple position slide switches (negative or positive toe down or off).
There are four bi-colour LED's that indicate positive, negative or off for each CV input.
There is a dual gang pot to be fitted.

Anything else...?
No

Gentlemen, good luck.

Cheers

The result of Andy listening to me.....

(http://i214.photobucket.com/albums/cc196/digi2t/Resonator/WP_20140801_001_zps3dbae938.jpg) (http://s214.photobucket.com/user/digi2t/media/Resonator/WP_20140801_001_zps3dbae938.jpg.html)

Like I said, he's a real gem.  ;D

^ can't wait to play with that thing!!!!!

That's awesome! Presume it can be done with a non treadle configuration as well? This is very much dedicated to the spirit of the Frostwave products which allowed robust I/O options, CV control, "all options open" kind of thing

Well done!!!!!
Strategy

Holy crap Dino and Andy!!! I was just checking in, not realizing how interesting (and crazy cool) you guys are making this! It sounds and looks so f'in cool!!! Can't wait til pcbs are available!