Here's a selection of Moog transistor ladder filter schematics gleaned by using the search function:
http://dropmix.xs4all.nl/rick/Emusic/Moog/moogvcf_schematic.gif
http://files.muziq.be/schematics/moog_904a.gif
http://www.elby-designs.com/asm-2/vcf2/vcf2-filter-asm2-cct.pdf
http://mwpc1.die.unifi.it/~maurri/synthdoc/VCFLPSCH.JPG
http://www.fantasyjackpalance.com/fjp/sound/synth/synthdata/16-minimoog/001/911-filt-print-circ-schem.gif
http://www.freeinfosociety.com/electronics/schemview.php?id=952
So why haven't we got a fabby-dabby Moog transistor ladder envelope filter project on the forum? Jeez, do I have to do-it-myself?
It'll need a charge pump for +/-9v power.............
(http://www.synthgear.com/wp-content/uploads/2009/05/tattoo_moog_ladder_filter_schematic.jpg)
Apparently.... It's pretty popuar... :icon_lol:
Maybe you should team up with that guy for a handy reference.
Why not do it yourself? You could adapt it to single supply.
Quote from: JKowalski on July 11, 2010, 01:18:50 PM
Why not do it yourself? You could adapt it to single supply.
Hey that's MY arm!
From what I've read, 9v isn't going to cut the mustard in this case
Quote from: frequencycentral on July 11, 2010, 01:28:48 PM
Quote from: JKowalski on July 11, 2010, 01:18:50 PM
Why not do it yourself? You could adapt it to single supply.
Hey that's MY arm!
From what I've read, 9v isn't going to cut the mustard in this case
Well, I didn't mean 9V specifically. 18V...? Buffered op amp Vref?
That's a weird circuit, now that I look at it again. It's refreshing to see something new and original in circuit design.
Here's an old Japanese 4-pole diode-ladder effect from the late 70's. I have the layout for it as well, but it won't fit in a 1590BB.
(http://img.photobucket.com/albums/v474/mhammer/JapaneseAutowah.png)
There is also this schematic:
http://yusynth.net/Modular/EN/MOOGVCF/index.html
The pcb is quite small, but it runs on +/- 15V regulated PSU of course.
It is mandatory to match the trannies in the ladder, so it requires a very precise multimeter.
I have this module fitted in a "work in progress" modular and it sounds very nice.
Then it will just need a CV pedal or and enveloppe follower to be used with the guitar (I have made a small circuit from the Formant Modular Synth to do that job).
I messed with it quite a bit for a while. It's voltage hungry and can have lots of hiss if you're not careful.
A four-pole active lowpass implemented another way works almost as well. For instance, using two LM13700s to do four single pole lowpasses is nice, if also a bit hissy, but it does work from ground and +9V well enough.
State variable filters set up as lowpass with a high Q work well too.
Quote from: R.G. on July 11, 2010, 04:45:50 PM
I messed with it quite a bit for a while. It's voltage hungry and can have lots of hiss if you're not careful.
A four-pole active lowpass implemented another way works almost as well. For instance, using two LM13700s to do four single pole lowpasses is nice, if also a bit hissy, but it does work from ground and +9V well enough.
State variable filters set up as lowpass with a high Q work well too.
I suppose that's a lot of diode drops to get over! It does seem like modern methods (what, this was mid 60's?) would render this obsolete, but then again it is said to have "characteristic distortion"... :icon_rolleyes:
Mark, what effect is that? Do you have any information on the completed unit? Looks like... envelope controlled... Hmm
Maybe the best route to go in this project is a transistor array chip...? Well matched, less space hungry?
Quote from: JKowalski on July 11, 2010, 06:40:40 PM
Maybe the best route to go in this project is a transistor array chip...? Well matched, less space hungry?
LM394 for a matched pair at least?
So why haven't we got a fabby-dabby Moog transistor ladder envelope filter project on the forum? Jeez, do I have to do-it-myself?
I'd be more inclined to foster interest if I knew anything about it.
It'll need a charge pump for +/-9v power
..I have a couple floating linear 9v supplies available.
RG seems to think there are other ways, to...
What's this thing actually do? [besides be an intersting choice for a temporary tatoo ?
Quote from: petemoore on July 11, 2010, 07:26:53 PM
So why haven't we got a fabby-dabby Moog transistor ladder envelope filter project on the forum? Jeez, do I have to do-it-myself?
I'd be more inclined to foster interest if I knew anything about it.
It'll need a charge pump for +/-9v power
..I have a couple floating linear 9v supplies available.
RG seems to think there are other ways, to...
What's this thing actually do? [besides be an intersting choice for a temporary tatoo ?
Looking it up, it was first developed as an adjustable low pass filter for a moog synth. Kind of an odd sound, not so sure I would like to use it that much, but it's interesting and may pique someones interest.
Quote from: caress on July 11, 2010, 07:20:57 PM
Quote from: JKowalski on July 11, 2010, 06:40:40 PM
Maybe the best route to go in this project is a transistor array chip...? Well matched, less space hungry?
LM394 for a matched pair at least?
Don't they make matched quad arrays?
Quote from: petemoore on July 11, 2010, 07:26:53 PM
What's this thing actually do? [besides be an intersting choice for a temporary tatoo ?
It's a lowpass filter, but one which has four time constants all at the same frequency, and both gain and feedback. That means the following in terms of frequency response:
- A single R-C lowpass has a rolloff slope which declines at -6db per octave; that is, after the critical frequency, output halves for each doubling in frequency. This is not a terribly fast rolloff.
- Each additional R-C lowpass section you add increases the ultimate rolloff slope by another 6db per octave. But they load each other down and interact. You can do better by buffering each section from the others to eliminate the loading. This gets to being lots of circuitry.
- A lowpass with resonance (the gain and feedback give you this) can produce a peak right at the rolloff frequency which then dives for minus infinity after the peak. This sounds rather like a bandpass with a low pass added to it. Conceptually it is the same.
A lowpass withe peaking is kind of an ideal wah filter, as it lets through the fundamentals of the notes, but gives you the bandpass "wah" resonance on the harmonics in the human vocal range. The Vox wah is in fact a two pole lowpass filter with peaking.
A four pole lets you have sharper peaks, and the ladder setup lets you have an exponential control characteristic, which is important for musical sounding controls in a synth context.
It never hit me til now, but for effects use, I bet you get 90% of the advantage of the ladder by using only two poles, not four. That reduces the voltage requirements a bit and might make a decent 9V only circuit for guitar pedals. Hmmm. Have to work on that.
There's a simpler version using diodes instead of transistors and some other good stuff in this thread http://www.diystompboxes.com/smfforum/index.php?topic=60127.0 (http://www.diystompboxes.com/smfforum/index.php?topic=60127.0)
The Moog filter is a bit of a holy grail in the synth world, very much desired and highly thought of. I've got one in my DIY modular synth, built using TBA331 transistor arrays and BC550's for the ladder - works very well. Certainly has a characteristic sound different to OTA 4 pole filters. I have no envelope follower in my modular system to 'simulate' a Moog style envelope filter, so I guess I'll have to breadboard one and hook it up.
Diode ladder filters are inferior in my book, I have one in my Synthi A clone, possible the weakest filter I have. I'm pretty blessed with synth filters. Curiously, while doing a bit of reseach I came across transistor ladder and diode ladder phasers, of which I was not previously aware: http://www.musicsynthesizer.com/Diode%20Phaser/Diode%20Phaser.html I'm now intrigued with the idea of a diode ladder phaser - more fodder for the breadboard!
Well, I guess the question that must be asked is whether the "weakness" of diode-ladder filters, in comparison to transistor ladder, bumps them below more pedestrian fare like 2-pole op-amp or OTA-based, or whether it positions them as more desirable than 2-pole, but just not as tasty as transistor-ladder.
A filter league table? :D That's difficult. And ironically my favourite filter is the Oberheim SEM multimode which is only 2 pole but bursting with character. Mine has outputs for each mode, panning the lowpass to the left and the highpass to the right makes for some monster filter sweeps. I also have two Roland 4 pole OTA filters which I sometimes use in series for 8 pole, or in combination to make highpass and bandpass. But as far as FAT goes, the Moog somehow rules the airwaves.
> for effects use, I bet you get 90% of the advantage of the ladder by using only two poles, not four.
The early synths could make some AWFUL RACKET with harmonics all up the audio spectrum. Sharp-corner squares, narrow pulse-widths.
Real music isn't like that.
And "normal" musical instruments can't make those sharp corners. Body-wood, pipe diameter, reed inertia, string mass and damping, pickup.... there's at-least 1 or 2 poles of high-cut already.
You didn't always need a 4-pole on an Arp or eMu (never touched the Moog) modular synth, but when you did you DID.
Agree that if a peak then 2-pole drop (~~14dB down the first octave) isn't enough for "normal" music instrument sources, then you are using the wrong instrument for the wrong sound. (Anyway for the CD you can always fix it in ProTools.)
P.S.: I know the Arp did not use Moog's ladder, did not have OTA, did have 4-pole, but it has been a while and it is much too hot to remember how they did it. I may be remembering two 2-pole filters in one module, easily slaved together?
Far better to use 2 batteries, as I always say. They always work, but then size of box is of no consequence to me. EH used a different VCF in the Crashpad, and that used a very tasty sweep generator if you'd care to look!
http://www.4shared.com/photo/1DEj4onP/EHcrashpad1.html
Just a little progress in the reseach to report. Yves Usson put me onto this variation:
http://www5b.biglobe.ne.jp/~houshu/synth/moritaVCF76.gif
......which implements highpass in addition to lowpass, without the 'bad behaviour' which results in mixing the unfiltered and filtered signals together, out of phase with each other, into a mixing opamp. By mixing the unfiltered and filtered signals prior to the resonance loop any naughtyness is avoided. I had noticed some inconsistencies with the Moog filter in my DIY modular when configured as a HP using a mixer and inverter - constantly had to retune the mix to achieve highpass (do we call that null offset?).
So now to decide upon the envelope part, this link was in a recent thread here, which seems interesting:
http://sound.westhost.com/appnotes/an001.htm
Quote from: StephenGiles on July 14, 2010, 09:19:26 AM
EH used a different VCF in the Crashpad, and that used a very tasty sweep generator if you'd care to look!
http://www.4shared.com/photo/1DEj4onP/EHcrashpad1.html
Stephen, why haven't you mentioned this before? ;D
Quote from: frequencycentral on July 19, 2010, 01:52:29 PM
Stephen, why haven't you mentioned this before? ;D
I believe that's an implementation of the "Steiner" filter; certainly the diffamp driving diodes as a central cell is.
The Korg-35 filter might be a good candidate for stompbox use:
http://www.aleph.co.jp/~takeda/radio/MS20clone2E.html
I know that a variant of this runs off of a battery in the Gakken SX-150:
http://matrixsynth.blogspot.com/2008/08/gakken-sx-150-schematic.html
It is a 2-pole filter, but it has a nice nonlinear behavior (I haven't tried it in person, but the sound examples are RAUNCHY). Most of the desirable synth filters have nonlinearities within a feedback loop, which results in various cool effects when the resonance is turned up or the input gain is increased. The Moog filter and the 4-pole OTA filter are identical from a linear perspective (4 1-pole filters of identical cutoff with negative feedback) - it is the different nonlinearities that result in different sounds.
A few other ideas for filters:
- A 4th order OTA-based phase shifter design is a good candidate for conversion to a 4-pole lowpass filter.
- By staggering the capacitor values of the different 1-pole stages to unequal values, a higher amount of feedback is needed to achieve the same perceived resonance as a 4-pole filter with equal capacitor values. The diode ladder filters require a lot more feedback to get their sound compared to the Moog ladder, as there is no buffering between the diode filter stages.
- You can insert nonlinearities as needed. The SH101 filter, for example, has back-to-back diodes going to ground in the feedback path. It sounds way more aggressive than the same OTA chip in the Juno-60, which has a fairly linear feedback path.
- The Minimoog and Moog Modular filters used AC coupling for the feedback differential amplifier. This results in highpass filtering of the feedback signal. My theory is that this results in less cancellation of the low frequencies when a lot of feedback is applied, hence the good reputation of these synths for bass.
Sean Costello
The Korg-35 looks interesting, never saw that one before. Some very good observations in your post Sean, almost tempts me to take a bunch of diodes to my Juno-60.
Quote from: frequencycentral on July 19, 2010, 06:52:54 PM
The Korg-35 looks interesting, never saw that one before.
I know that there is a 9V version of the OTA-powered MS-20 filter out there. The schematic I have seen leaves out the frequency to voltage converter, which would be useful when summing several control voltages together. I doubt that the Korg-35 has a particularly accurate sweep range, but if you are doing envelope filtering and/or a CV pedal, this doesn't really matter.
I am wondering which version of the MS-20 filter will be in the Korg Monotron. My guess is that it is a surface mount device. The Korg-35 filter would require a few surface mount transistors and diodes, and would probably be dirt cheap to manufacture in quantity.
Quote
Some very good observations in your post Sean, almost tempts me to take a bunch of diodes to my Juno-60.
My long-departed Juno-60 was a great synth, but the filter sound was always weaker than my SH-101. Same IR3109 used in both. It would be interesting to see what the addition of a few silicon diodes (12) would do to the sound of the Juno-60. For that matter, adding a bunch of germanium diodes might be useful as well.
Sean Costello
Hmmm, I have a Juno 60 and a 106...I've been meaning to sell the 106, but it sounds just a little bit more interesting than the 60. Maybe I could bring myself to part with the 106 if I did this mode to the 60.
Thanks!
Slightly OT:
Why hasn't anyone come up with a pitch tracking filter for guitar yet?
One of my favorite things about synthesizers
Quote from: spectraljulian on July 19, 2010, 09:51:09 PM
Slightly OT:
Why hasn't anyone come up with a pitch tracking filter for guitar yet?
One of my favorite things about synthesizers
I think EH did in their Rackmount Guitar Synth.
Ah yes, designed by David %^&*erell. I want one. And a VSC-3. And everything else he designed that I don't have. Well I guess not the akai stuff, I'm not into that.
I've looked at that P2V converter on Mark Hammer's site, and man. . . that thing scares me. It looks like a breadboarding nightmare.
The pitch tracking is also possible to do with an arduino, but the math right now is scaring me. It's been far too long since I've sat in a calculus class.
I've been pondering some form of synth filter adapted for pedal use for a while, but haven't gotten it off the ground yet, partially because I stumbled onto a used EH microsynth. I'm not sure if I like the standard ADSR implementation of the filter envelope for guitar playing versus the microsynth's method. I'd almost want the cutoff to be controlled by an expression pedal, but that quickly will become a pedalboard hog!
I don't know if it helps, but the soundlab runs on 9V, and there's a guitar trigger circuit on the site as well. The filter is different, but perhaps there is some information that can be gleaned from there.
Speaking of filters, have any of you ever used a Polivoks? (http://m.bareille.free.fr/modular1/vcf_polivoks/vcf_polivoks.htm)
http://isp.lynx.net/~jc/pedalsOSH.html (http://isp.lynx.net/~jc/pedalsOSH.html)
after all this (not to mention some fun control circuit tricks) I realized there's one filter topology that rises above the rest,
one that doesn't spike the ear (variable-state) or isn't lame sounding (ladder) when it comes to gtr
it's not an easy hunt - keep looking! ... ;)
Quote from: space_ryerson on July 20, 2010, 02:51:08 PM
I've been pondering some form of synth filter adapted for pedal use for a while, but haven't gotten it off the ground yet, partially because I stumbled onto a used EH microsynth. I'm not sure if I like the standard ADSR implementation of the filter envelope for guitar playing versus the microsynth's method. I'd almost want the cutoff to be controlled by an expression pedal, but that quickly will become a pedalboard hog!
I don't know if it helps, but the soundlab runs on 9V, and there's a guitar trigger circuit on the site as well. The filter is different, but perhaps there is some information that can be gleaned from there.
Speaking of filters, have any of you ever used a Polivoks? (http://m.bareille.free.fr/modular1/vcf_polivoks/vcf_polivoks.htm)
The EH Microsynth sweep generator is OK as far as it goes, but I once again suggest that a considerable improvement would be if the initial start frequency was controlled by picking strength. One of these days I'll get round to designing this. The sweep generator as is always sweeps to the point set by the start frequency pot when energised by a note played, no matter how hard the note is played, so the filter has no note dynamics imparted on the sweep. The decay is fine - no ripple as far as I can hear, so leave that alone. So effectively, the the start frequency point needs to be voltage controlled - there may already be elements within the control circuitry which would largely do the job, with the addition of a sample and hold.
Any takers??
Something like this perhaps, AX would be a sample & hold - just a sketch :icon_eek:
http://www.4shared.com/photo/tk3Z2yJi/EH_sweep_mod.html
Stephen...
that EH schematic looks an awful lot like the Stiener VCF, which is a two pole (basically) Sallen Key
filter. There is lots of info on this one at Synth-diy (check the archives). I wonder where EH got the
schematic... oh yeah ! it was published as a design idea, or idea for design in EDN or similar by its
daddy, Neil Stiener
H^)
Quote from: HarryBBD on July 22, 2010, 10:56:38 AM
Stephen...
that EH schematic looks an awful lot like the Stiener VCF, which is a two pole (basically) Sallen Key
filter. There is lots of info on this one at Synth-diy (check the archives). I wonder where EH got the
schematic... oh yeah ! it was published as a design idea, or idea for design in EDN or similar by its
daddy, Neil Stiener
H^)
That must have been a long time ago, I should think EH changed to this sweep generator in the mid 80s.
Quote from: StephenGiles on July 22, 2010, 10:18:02 AM
Something like this perhaps, AX would be a sample & hold - just a sketch :icon_eek:
http://www.4shared.com/photo/tk3Z2yJi/EH_sweep_mod.html
That looks like it should work to me. I've got the other version of the Microsynth, but basically the same thing should work for that as well. It would be easy enough to try out, for a quick test in only needs one wire to the start slider disconnecting.
OOps newbie mistake. I'm used to top posting so I was referring to the original schematic for the filter,
which has been identified as the Steiner by others already. I need to get up to date here
H^)
You know, Harry, ever since I had the pleasure of trying out your guitar synth in my basement, some years back (and the envy has not subsided), I have lauded it constantly. At the same time, I think I'm just abut the only person among the 18,303 members today, other than yourself of course, who has heard it. Any possibility for a soundclip that might give people a sense of how it tracks?
Apologies to Rick for swinging off at a tangent :)
Quote from: StephenGiles on July 22, 2010, 10:18:02 AM
AX would be a sample & hold - just a sketch :icon_eek:
Thinking out loud here, but do you even need the sample and hold? Once the trigger from the "attack detector" has kicked off the filter sweep does it matter if the start voltage changes?
It looks like the Korg Monotron has a Korg-35 filter variant, running on +5 volts:
http://www.timstinchcombe.co.uk/synth/korg.html
No schematic yet, but I'm sure one will emerge in the next few months.
Sean Costello
Quote from: slacker on July 23, 2010, 01:02:41 PM
Apologies to Rick for swinging off at a tangent :)
Quote from: StephenGiles on July 22, 2010, 10:18:02 AM
AX would be a sample & hold - just a sketch :icon_eek:
Thinking out loud here, but do you even need the sample and hold? Once the trigger from the "attack detector" has kicked off the filter sweep does it matter if the start voltage changes?
I don't know, I just whan the start voltage to be dependent upon the note picking level, which would then acheive a more or less perfect sweep generator as the sweep decay has no stutter at all.
I don't have a Microsynth so I would need to breadboard most of the circuit to try this out at some point.
It occurred to me this morning when throwing a ball for Bonzo, our 3 legged dog, that if the Microsynth start/stop frequency CV buffer output could be clamped to the peak voltage equivalent to the strength of notes played, it would do away with the need for a sample & hold .
anyone willing to give up at least the basics of how this works?
Is this using the trannies/diodes as variable resistors? I'm basically trying to figure out how you don't get extreme volume cut at the lower end of the sweep.
if it hasn't already been posted,
see this: http://www.timstinchcombe.co.uk/synth/Moog_ladder_tf.pdf
Dude, that's sweet. Thanks! I love articles like this.
For 9V operation it might be worth substituting MOSFETs in for the BJTs, although that would definitely have some effect on the sound it makes.
> MOSFETs in for the BJTs
1) MOSFETs usually need 2V-5V "grid cathode" bias, and a similar amount "plate cathode". The BJT biases 0.6V at the input and below 0.1V across the output. In a stack like that, at 9V, several layers of the BJT is tight, the MOSFET is one-stage at best.
2) The emitter impedance of a BJT is VERY predictable and linear over MANY octaves of current (nA to mA). You don't even have to know what part-number. 28 ohms at 1mA, 28,000 ohms at 1 uA, etc.
The same impedance of a MOSFET is not known even roughly until you pick a specific part number, not known closely until you measure a part in hand, and the impedance varies along three different laws (linear at ultra low current, square-law at low current, asymptotic to a parasitic resistance at high current). Even if you found two parts the same at 1mA, they are awful sure to differ at other currents.
"Much of the usefulness of the filter is the ability to apply voltage-control to the cut-off frequency..." - Timothy E. Stinchcombe
While you could vary the frequency with an FET, there's no real predictable relation between voltage (or current) and impedance (frequency). With the BJT the R/I (thus V/F) error over 1000:1 range may be near 1%.... your filters TRACK your oscillators (better than a Moog's oscillators track each other...). Stinchcombe's plots (for a SPICE-faked sim) show 2% error over the middle of the audio band. Some of this is my squint and his plot interval; I can't read the curve to 1% accuracy.
3) Bob Moog didn't have good MOSFETs. Even JFETs were rare.
> I love articles like this.
Good where it goes. I don't see where he says HOW the silly thing filters. The use of Ebbers-Moll seems clunky: I see it by using Shockley's Relation (remembering that SR un-simplifies when signals are large). The assumptions of balance and base-current are not essential. And the filter action does NOT happen in the "long tail pair". I see he work "my" way in section 2.2.... maybe I am less attached to reality than Timothy is.
> how you don't get extreme volume cut at the lower end of the sweep.
More correctly: why the gain/volume isn't varying over the ENTIRE sweep. You know that, usually, reducing device current while all else stays the same usually reduces gain.
In Stinchcombe's Fig 3: the input is across Q1 Q2 and the output is at the _emitters_ of Q11 Q12. The emitters are not amplifier stages, they are "passive" resistors.
The gain of a simple common-emitter amp is the load resistor over the emitter resistance. With resistor load, lower current is higher emitter resistance and lower gain.
With the "emitter resistance load", the "load resistor" VARIES with current. And because Q1 Q2 current is equal to Q11 Q12 current(*), the gain is always unity (until the caps shunt the signal).
And some -consistent- gain is vital when we wrap the feedback "boost" loop around, We will want gains like 0.980 which are STEADY across the frequency band, so the dB bump stays the same size (at least does not collapse or go into howl).
(*) Actually the base currents add-up going down. If hFE is 100 and Q11 Ie is at 1.000mA, then Q9 Ie is 1.010mA, Q7 is 1.02mA, Q5 1.03, Q3 1.04, Q1 is 1.05mA. So Q1 runs 5% richer than Q11 and the gain is 5% higher than unity. Which is moot because internal signal levels are MUCH lower than we want at the jacks, so the ladder is always followed by amplification which can trim-out any 5% fudge.
Thanks a bunch, Paul. I was skimming that article and decided to pop back on the boards for a moment. You pretty much answered most of my inquiry.
One question (if I don't figure it out before), does having more 'steps' in the ladder have a similar effect to adding stages to a phase shift circuit (in the most basic sense of 'sound', I guess)? Could a good experimenting point to be to start out with 2 steps, or does a single step provide enough effect to get a good starting basis, is I guess why I'm asking.
This circuit actually reminds me of discrete UV meters.
> does having more 'steps' in the ladder have a similar effect to adding stages to a phase shift circuit
One step is equivalent to a simple tone control: 6dB/oct slope, highs are less, not cut-off sharp. However the corner frequency is varied, which is less often done in tone controls (usually the corner is fixed, highs fall then shelf-off a variable depth).
So you need Q1 Q2, a C, and Q11 Q12.
This will not "bump up" with feedback. You need at least two stages for "some bump", at least three stages to bring "infinite bump" into reach (you don't want infinity but not much happens until you get close).
Note that input impedance is low (as low as 1K) and input level must be kept down to 20mV-50mV or it is more a "fuzz". You got some in-out interfacing to do.
> This circuit actually reminds me of discrete UV meters.
I do not understand. What UV meter? Light sensors can use many types of amplifiers. Light covers a wide range of intensity and it is often convenient to express and measure in log form. Log-converters are often based on diff-pairs, but wired differently.
Quote from: PRR on August 31, 2010, 01:46:19 AM
I do not understand. What UV meter? Light sensors can use many types of amplifiers. Light covers a wide range of intensity and it is often convenient to express and measure in log form. Log-converters are often based on diff-pairs, but wired differently.
Visual unit meter, the "decibel" light meters on stereos. They work, however, by dividing voltage down a set of buffers driving LED's. I'm sure you know what I'm talking about now ;)
Now that I understand the filter response, this doesn't remind me as much of how the meters work.
Multi Stage Waveshaper (http://www.experimentalistsanonymous.com/diy/Schematics/Guitar%20Synth%20and%20Misc%20Signal%20Shapers/Multi%20Stage%20Waveshaper.jpg)
Just came across this tonight... this is pretty much what I was talking about. ;D (BTW, is this by the same Christian that came up with the Bazz Fuss?)
Edit to add: Oh yeah... getting ready to upload a little finding of mine that may help this push along...
Ok, so I was playing around with this (inverter pair?) and wondered if the sound would change if I put a diode in between the bases. Yep... started to sound like a distorted hollow tube... kinda kewl sounding. Then I added a bunch more and got a REALLY kewl sound... WAIT! It's doing some kind of filtering... and this thread popped into my head, so I dropped caps from the junctions to ground, and VIOLA! It filters. I tried different combinations of caps, and all have their own characteristic sounds.
Notes:
It has a natural envelope to it, slow, looooong... but, that goes away if you buffer it, or have actives.
So, I decided I should be able to adjust the "Q" if I do like the second circuit. I used a 5k pot, but only about 2k of it was really active.
This seems like it could be a good base for stompbox synth filtering with 9V supply. I'm spent for the night, but figured I'd get this up for people to play with. I'm guessing that a buffered signal may improve the response (but not the sweep).
This needs some kind of sweep generation. ~2k range seems pretty sensitive, yet very controllable within the sweeps I was getting... should be easy to implement some kind of voltage control to one of these.
The "quack" I was getting with the 5k was really muffled, with a gradual build up to about 2k when the filter became easily apparent without sweep. After 2k, the thing literally quacked with pot rotation... like a wah... actually, more like a synth-wah.
Now I'm kinda wanting to do a Fet controlled by a LFO to ground with the feedback resistor as a "Q Range" control. Then can use envelope, the LFO... all sorts of things!
Anyways, here ya go.
(http://img121.imageshack.us/img121/2335/fauxladder.png)
ps: This thing pretty much makes the wave square, so I don't know that it's going to react much better if you insert a square wave like I've noticed some filters say. Plug your guitar straight in and you'll see what I mean. This is just a base circuit... if ya think I should, I could just start a development thread on this.
A similar filter was used in the EH Crashpad, designed in 1980!
Well that looks very cool and inspiring, such a low parts count too. There's clipping going on as well as some filtering? I'm gonna have to play with this when I find time. Nice work. I'd love to hear some soundclips if you can rustle some up.
Quote from: StephenGiles on September 18, 2010, 06:28:09 AM
A similar filter was used in the EH Crashpad, designed in 1980!
That's awesome! I wanna see the schem now, see how they implemented it... nothing that I can find online, though... bummer. New one to keep an eye out for.
Ok, I noticed basically, pot to the 3906 emitter, like I drew, gives a dampened Q, but if you drop it to ground, it works like you would expect it to. So, after breakfast I'm gonna try a couple things.
> Plug your guitar straight in
Input impedance is 10K clean (up to ~~20mV) down to under 100 ohms when clipping, with ~~0.1uFd in shunt and 0.2uFd in series.
This does "horrible" things to a guitar pickup signal. It's a messy filter, drooping bad above 200Hz. OTOH most guitar signal can slap those two collectors like my barn door in a windstorm, adding lots of distortion overtones and filling-in the treble that is lost at the input.
> goes away if you buffer it
For "consistency", you may want to buffer and then build-out a pickup-like input circuit. For a breadboard, you might just slap an old/tired pickup inside a steel box (for shielding) and use it series between buffer and "IN". Full "pickup emulation" needs another 200pFd-1,000pFd shunt, but your 10,000++pFd caps make that moot. (There's other ways to "fake" a pickup's inductance as a buffer detail {emitter followers tend to have a trace of inductance} but hard thinking is needed.)
Yar... I figured with the people watching this thread I didn't need to say much about 'why' it is getting a natural envelope. It's the same effect as what drives the Bronx Cheer to do what it does, essentially.
Whilst playing with this, I decided to put a parallel set of diodes to separate the caps and put a 3906 at the top of it. The emitter gets a diode from the output, and the base is driven from the output through a .1u cap. Been playing with parts around this, but gives ultra-high Q and much better envelope. Oh yeah, also driving it with a square wave makes a bunch of difference (big surprise there). Now if I could find that one control point, lol... I'm thinking the 3904 to ground was working better as far as having some kind of control, but wasn't producing as much of the sharp peaking.
The obvious control is the "+9V" or the current through the "10K" resistor.
With your posted plan, 3V turns it off and 15V doubles current (halves the impedances). You actually only need a couple Volts across a resistor there; how about 3.3K and 5V nominal varying from 9V or 10V or 12V down to 3V? Or use a PNP current source down from the rail; that could be controlled over many decades of current.