I had off work on Monday due to snow, and came up with the idea of tossing together a quick overdrive to pass the time. And because I'm not good at doing simple things like that, I instead ended up breadboarding and working on a new design all week. ::)
The Snow Day OD is a FET- and MOSFET-based amp simulation overdrive running on 18V that includes a switchable miniature compressor circuit and soft clipping in a "power tube" section. It goes from glassy "mostly" clean to either open or slightly compressed edge of breakup that feels very similar to one of my favorite amps, up to a medium gain compressed sound. It has lots of harmonics without sounding overly clipped, and a very wide dynamic range.
It uses some principles from several Runoff Groove circuits, Aquataur's mini compression circuit from his mods to the Umble, and Mark Hammer's Stupidly Wonderful Tone Control (which rolls off treble without changing the volume).
Although it's not directly based on any amp in particular and I wasn't going for a particular sound, I did have a couple amps in mind (both Deluxes -- a friend's 5E3 clone and a 1949 Deluxe at Invisible Sound Studios in Baltimore) while I was designing, aiming for a similar feel and gain level to them if not necessarily their tonality.
The pedal runs on 18V internally for a lot of drive but also a lot of headroom, making for plenty of volume dynamics even with the clipping.
Schematic(https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20mini%20layouts/Snow%20Day%20Overdrive/Snow%20Day%20Overdrive%20schematic.png)
EDIT 3/10/14: I will add the missing Va connection to Q5 ASAP.
Demo (using my Sheraton)
Audio-only demo (https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20mini%20layouts/Snow%20Day%20Overdrive/Snow%20Day%20Overdrive%20demo.mp3) (using the DQ caster; no EQ, no effects, direct into the recording interface, also includes stacking with a My Little Klony and a Fatpants).
How It WorksPower sectionThe power section is a standard voltage doubler. The additional voltage helps coax more gain out of the FETs. FETs always clip when the input signal exceeds a certain voltage, but there is a limit to the voltage gain they can provide at any given supply voltage. They also need to be biased in a particular way to ensure the correct harmonics to emulate tube breakup. (See Runoff Groove's FETzer valve article for more information on the harmonics generated.) The higher supply voltage means that we can use 2N5457s, which have a higher input voltage threshold before clipping, while getting closer to the voltage gain of a J201 (which clips much, much sooner). This helps increase the total dynamic range of the pedal.
D2 is out polarity protection. Z1 is optional as overvoltage protection, but sometimes zeners can go bad when used with charge pumps. Otherwise the standard power filtering is there.
"Preamp"The "preamp" section is primarily three FETzer valves in a row. Each 2N5457 is biased to 2/3 supply voltage (ended up being about 11V in mine). The bias point helps generate the correct harmonics to get the FETs sounding closer to tubes.
The gain control is right after Q1. R4 provides some minimum resistance (I didn't have room for this on my perfboard layout, but I added it to the schematic). The lowest setting will be clean even with humbuckers. Noon has a really nice edge of breakup sound -- extremely dynamic and picking attack responsive. Since the gain is simply a volume control after the first stage, rolling down your guitar's volume is pretty much identical, so the effect also cleans up nicely and smoothly.
Different FETs could be used with different results. For instance, a J201 in Q2 (with appropriate re-biasing) would be much gainier (and will also lose some clean settings). I got some good results with a 2SK170 in Q3 when using a telecaster.
Just a note: Runoff Groove's newer circuits use clipping diodes after each stage in a way that ensures smooth breakup and to avoid hard clipping the FETs, but I decided to skip that little innovation because I found they were generating clipping earlier than I liked in this particular instance.
Compression!A miniature compressor stage, based on the one created by Aquataur for his Umble build (http://aquataur.hilpold.net/aquataur/musicstuff/umble.html#Intuitive_Sag_Circuit_For_The_Umble), is created by R10-14, D1, and C6. D1 generates a negative voltage, which travels through R7 and pulls the gate of the FET negative (by about 2V at max from my measurements), dropping the input level to Q3.
The result is a bit of sag reminiscent of a tube amp at higher gain settings. The compression kicks in and becomes measurable and observable around noon on the gain dial on the "high" compression setting (at least with my strat; other guitars might trigger it earlier). There's an accompanying reduction in distortion and some added perceived sustain. The compression also swallows a bit of treble when it turns on, which creates a smoother sounding distortion sound. It also means that the effect will bright up a bit when you roll off your guitar volume, which is something I've been dying to find a way to do in overdrives that don't rely on guitar loading for a while now.
The attack time in mine is 10mS, and the decay is 100mS. The switch disconnects the compression in the center; one setting has a threshold-limiting resistor (220K) to ease things up on higher gain settings, and the other setting bypasses that resistor and lets through all of the signal.
It's worth noting that there's no capacitor to set the cutoff frequency of the compression, but I did consider adding it. Unfortunately there's only so much signal to go around, and in any case I like that the compression works more on chords (which have more bass and also need more clarity) than on single notes.
Tone (Treble) ControlThe tone control, Mark Hammer's Stupidly Wonderful Tone Control, is after Q3. The pot is a continuous 100K series resistance between Q3 and Q4, and as the resistance increases between lugs 1 and 2 (pot turned clockwise), more treble is dumped through C8. The values were calculated to roll off treble starting at 15KHz down to 685Hz. It can get a little brighter and a little darker than the bypassed signal. It's not extreme (no "playing under water" sounds), but it's effective at controlling the harmonics going into the power tube section. R15 sets the highest cutoff frequency of the treble cut (combined with C8).
"Power Tube" -- Mosfet soft clipping stageAfter the tone control comes the "power tube" section. This is a MOSFET amplifier with MOSFETs arranged as diodes in its feedback loop. The MOSFET amplifier part is a pretty standard way to hook it up without needing a Vb reference voltage. I used a MOSFET here because it didn't distort as much as a transistor, and it also sounded a little less harsh when it did overdrive.
I loved the sound of MOSFETs as clipper diodes in the Mossy Sloth, but the arrangement is a little different here. MOSFETs conduct in two directions when you connect the drain and gate together: One way is the body diode (a simple silicon diode -- and no, it has no special sound) and the other way is ... something special that soft clips over a very wide range of a couple volts before it finally hard clips. By connecting them in series at their source pins, the body diode in each one prevents the diode from conducting in reverse, and the result is a very high, soft clipping threshold (about 2V and up).
One last trick to soften the clipping was the use of a limiting resistor (R18) in series with the clipping arrangement, similar to how it's done in certain op amp based designs like the Bluesbreaker and AMZ's big muff mod with a "warp" control. Big thanks to tca on DIYSB for lots of help (and diagrams) in understanding this (plus he suggested the MOSFET as Q4!). Although I don't
fully understand everything R18 is doing, but the gist of it is that once the diodes conduct, they appear in parallel with R16, and they will lower the negative feedback. R18 therefore helps set the clipping threshold and ALSO sets how "rubbery" the clipping will sound, AND just for good measure, also acts a bit like a clean blend. Values between 10K and 100K seemed to work with roughly similar overall results, but it's not quite as simple as a lower value = more distortion. I ended up not really being able to decide between 22K and 33K and went with 33K in the schematic simply because there were a bunch of them in the schematic already. So why isn't this a trimpot? I dunno, I guess it could be, but I'm not totally sure it's worth the trouble.
C9 sets the cutoff frequency of the clipping. It's bit enough to definitely pass all the frequencies on the guitar (and might be overkill, but I'm not sure how to calculate it). Smaller values (e.g. 1uF or 100nF) will let through more bass.
Output sectionR20 and R21 form a voltage divider, like a volume control permanently set to half. Adjusting their ratio to each other can boost or cut the output. I found it was sufficient to get a small boost to output even at very low gain settings.
An output buffer comes after that, followed by the volume control. The output buffer could be unnecessary (after all, the MOSFET probably has lower output impedance), but I thought it sounded very slightly better with it there. I'm not sure if it added some small amount of distortion, changed the bass or treble content subtly (perhaps cutting the lowest or highest frequencies a bit extra), but I figured it was only four extra parts and I had room on the layout.
Here's my build:
(http://jonpattonmusic.files.wordpress.com/2014/03/snow-day-od.jpg?w=271)(http://jonpattonmusic.files.wordpress.com/2014/03/snow-day-od-guts.jpg?w=423)
And here's my perfboard layout:
(https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20mini%20layouts/Snow%20Day%20Overdrive/Snow%20Day%20Overdrive.png)
If there's enough interest, I could do a small PCB run. Also, I do plan on making an etch layout, but I want to see first if I need to make a PCB layout (because I might be able to do them both in eagle instead of just making the perfboard layout into an etch image).
As always, I welcome any suggestions or questions about the design.
This looks fantastic.
Have you tried it with a bass at all?
Quote from: Bill Mountain on March 10, 2014, 08:45:14 AM
This looks fantastic.
Have you tried it with a bass at all?
Sorry, Bill, the best I can do for a bass is an octave pedal after the guitar. The bass player in my band has a perpetual loan of my bass (for all intents and purposes, I've given it to him).
It has a cut at 72Hz cut "sort of" at C5 and a 67Hz cut at C2. Those are really the only places with meaningful bass loss; every other cut is below 20Hz (I think). So I think you'd have to double both of those caps.
In any case, I imagine it would be quite a bit dirtier with bass, so I'm not sure what other changes would have to be made. Probably raising R19 at the very , perhaps even using a FET with higher Idss in Q3 for less dirt in the preamp.
I'm probably not the best guy to ask about modding some effects for bass, since I'm quite bad at playing that instrument. :)
Looks interesting Jon.
The MOSFET arangement looks pretty cool. What's the thought process behind tapping the drain in the middle of the two 10k resistors?
I haven't seen that sag/compression circuit before - is it useful enough to warrant its own effect, e.g. a Fetzer stage with a sag control?
In your schematic, it looks like you are missing a connection to V+ from the collector of Q5.
Quote from: samhay on March 10, 2014, 09:20:09 AM
Looks interesting Jon.
The MOSFET arangement looks pretty cool. What's the thought process behind tapping the drain in the middle of the two 10k resistors?
The pedal was INCREDIBLY loud without that. It's just the basic arrangement in the Fuzz Face, Rangemaster, Fuzzstainer, etc. It seems to be nicer to the volume knob behavior and treble content than a resistor in series with the volume pot.
I've also read that doing a divider like that results in a small drop in midrange. I couldn't find enough reliable information about it, though, so I didn't want to put my foot in my mouth in the "How It Works" section. It does sound a little different when I change the ratio; it gets fatter as you tilt it toward the transistor. So there must be something going on there.
QuoteI haven't seen that sag/compression circuit before - is it useful enough to warrant its own effect, e.g. a Fetzer stage with a sag control?
Unfortunately, not really, unless you're okay with adding another active device. You really need a lot of drive before it kicks in, enough to overdrive a typical FET. So I would say either at least two stages (it COULD go after Q2 with a bypass cap on either or both FETs) or just add a transistor to drive the envelope.
Oh, the other thing is, you need a small gate resistor to let through the small amount of negative voltage, so if it were right at the input, you'd need an input cap AND it would heavily load the guitar pickups. No idea how it would act. That has you up to at least two active devices regardless, with enough drive that it's probably not totally clean anyway. And increasing the decay will do funny things to the tone ... There are too many good simple compressors for me to really think this one's worth porting to its own effect!
QuoteIn your schematic, it looks like you are missing a connection to V+ from the collector of Q5.
Yup, I'll fix it ASAP! Thanks for looking out. :)
Another great project. Thanks for the backlog Jon. :icon_biggrin:
It must have a fair output to direct record to the interface. A nice feature and a great help to those recording in apartments and small studios that want a tube sound without the decibel count from a real amp. Nice job.
Lots of tone variation too. The compressor is a nice touch too.
I really like the zen drive with the mosfets in the feed back loop. I understand it's a different set up seeing as the zen uses op amps. But I like the touch responsiveness of this and your set project seems to do similar.
Great work man.
Rich
Also it reminded me of some of the tones in Snowy Whites Bird of Paradise.
I can't post you tube links from this machine else I would. Sorry.
Great work man and nice sound too.
I had a question; can I bias all JFETs using fetzer?
Quote from: sajy_ho on March 10, 2014, 11:18:14 AM
I had a question; can I bias all JFETs using fetzer?
The FETzer valve setup will bias any FET, yes. You can't necessarily just use any FET with the bias trimpot and source resistor values shown, though. The Runoff Groove article has more information than I can possibly regurgitate here: runoffgroove.com/fetzervalve.html
Is Q5 missing a V+ connection?
Quote from: Bill Mountain on March 10, 2014, 12:03:28 PM
Is Q5 missing a V+ connection?
I really will fix that, I promise. :icon_lol:
Quote from: midwayfair on March 09, 2014, 11:01:09 PM
EDIT 3/10/14: I will add the missing Va connection to Q5 ASAP.
Quote from: samhay on March 10, 2014, 09:20:09 AM
In your schematic, it looks like you are missing a connection to V+ from the collector of Q5.
Hey Jon,
I'm playing with a guitarist who's currently recording his clean electric straight into his digital recorder. It sounds godawful so I'm putting together a preamp pedal for him to run through first. Thoughts on using this, possibly with a condor sim after it?
Thanks for your thoughts Jon.
Sounds like I need to play with the MOSFET drain tap approach, but will add the sag control to the long list.
Quote from: Jopn on March 10, 2014, 01:02:17 PM
I'm playing with a guitarist who's currently recording his clean electric straight into his digital recorder. It sounds godawful so I'm putting together a preamp pedal for him to run through first. Thoughts on using this, possibly with a condor sim after it?
Well, check the audio only recording for the straight-to-console sound to make sure you like the sound of this particular pedal. I would still want to EQ the sound for direct recording, so he's going to need a decent DAW to go with it -- and most decent DAWs these days have at least competent amp simulation. I really like Logic's amps for clean sounds, but they do get kind of gross on the dirty sounds. Basically, this pedal is like everything up to the speaker hook up, and as we all know there's a lot more going on after that point!
FWIW, I tried this into the tweed, vox, and blackface sims on my computer and it sounded much more realistic than the pedal by itself, but the distortion from the pedal was better than anything I was getting out of the sims.
I can also HIGHLY recommend the Azabache and Britannia for what you're talking about -- they are both more specifically made to sound like particular amps and they both sound amazing. And they both already have some cabinet simulation.
My goal with this project was less to emulate the sound of any particular amp than to add the feel and breakup of an amp to an existing platform. Even using a solid state amp is not the same as direct.
I can't answer the question about how it will pair with the Condor specifically, as I haven't built that project, but I'm guessing it will be a big help and a good alternative to a plugin amp sim -- the EQ response of the Condor is very, very similar to the EQ shape on many of the amp plugins.
Lastly, I would venture a guess that even just a buffer will be a big help with your friend -- chances are the input impedance of the recorder is too low to handle a guitar. Keep that in mind for your preamp. :)
Thanks a tonne Jon! I had Britannia half built for this purpose when I saw Snow Day pop up. I'll definitely be building both, but I'll probably just continue on with Britannia for this current (and urgently needed!) build.
Cheers!
John
(http://media1.giphy.com/media/GaiXm1EOHlgek/giphy.gif)
:icon_mrgreen:
Quote from: Kipper4 on March 10, 2014, 10:22:25 AM
Another great project. Thanks for the backlog Jon. :icon_biggrin:
Ha ha! :D
todo_list = todo_list + 1
Very nicely done, Jon, as always, and the out-of-the-box thinking is most welcome. But, honestly, I'll never get to this if someone doesn't do a PCB. :icon_wink:
I just wanted to give people a heads-up that there will be a PCB for this. I won't be the one making the layout. That's the most I can say about it for now, though; I don't want to spill the beans about who is going to be working on it. But I will say that as usual, it will be someone in the community with more talent than me.
Hey Jon, hope you're OK
I'm trying to breadboard your circuit, but as I couldn't find any 2n5457 around, I'm using J201s for all the fetzer stages; my problem is the high gain. It sounds very brittle and I want a bluesy type OD like yours.
So what do you think? Can I use 9V voltage presented in the circuit for fetzers to lower the gain and run the rest of it on 18V, wouldn't it interact with the saturation switch? or just use a voltage dividder to do that?
Quote from: sajy_ho on March 17, 2014, 12:39:48 PM
Hey Jon, hope you're OK
I'm trying to breadboard your circuit, but as I couldn't find any 2n5457 around, I'm using J201s for all the fetzer stages; my problem is the high gain. It sounds very brittle and I want a bluesy type OD like yours.
So what do you think? Can I use 9V voltage presented in the circuit for fetzers to lower the gain and run the rest of it on 18V, wouldn't it interact with the saturation switch? or just use a voltage dividder to do that?
Do you have any other FETs around? You could mix and match a couple lower gain FETs with a single J201 in Q2 and get something really close. J201s more than twice as much gain. If you're going to use those, I'd say run it on 9v, and increase the 33K between each stage a little (I'm not sure how much, but try 47K). You might lose a bit of treble, but the gain will be similar.
The big thing that will be affected is that you're going to be VASTLY reducing the dynamic range. You're going from an average of -1.58V Vp (the voltage at which it clips) to only 0.83V.
The compression shouldn't be affected. You just want to get down to approximately the same gains.
EDIT: I'm not sure what'll happen with the MOSFET stage. I finalized that after moving to 18V.
Quote from: midwayfair on March 17, 2014, 03:26:58 PM
Do you have any other FETs around? You could mix and match a couple lower gain FETs with a single J201 in Q2 and get something really close. J201s more than twice as much gain. If you're going to use those, I'd say run it on 9v, and increase the 33K between each stage a little (I'm not sure how much, but try 47K). You might lose a bit of treble, but the gain will be similar.
The big thing that will be affected is that you're going to be VASTLY reducing the dynamic range. You're going from an average of -1.58V Vp (the voltage at which it clips) to only 0.83V.
The compression shouldn't be affected. You just want to get down to approximately the same gains.
EDIT: I'm not sure what'll happen with the MOSFET stage. I finalized that after moving to 18V.
Thanks for the quick reply, I looked in my parts box and found a 2n3819 with Vp of -3.44V and it'll give 1.5x voltage gain in fetzer. I calculated the gain of three 2n5457s and it's around 131x, so if I use two j201s + one 2n3819 I'll get 133x gain, pretty close to yours. so can I use this 3819?
Ps in wich fetzer I put 2n3819 for the most dynamic range and harmonics?
Thanks
Quote from: sajy_ho on March 17, 2014, 03:47:36 PM
Ps in wich fetzer I put 2n3819 for the most dynamic range and harmonics?
Thanks
I'm going to say q3. You can probably just lower the gain of the J201 in Q1 to avoid overdriving q2 too much.
Mind the pinout, the 3819 is DGS I think.
Thanks alot.
Quote from: midwayfair on March 09, 2014, 11:01:09 PM
"Power Tube" -- Mosfet soft clipping stage
After the tone control comes the "power tube" section. This is a MOSFET amplifier with MOSFETs arranged as diodes in its feedback loop. The MOSFET amplifier part is a pretty standard way to hook it up without needing a Vb reference voltage. I used a MOSFET here because it didn't distort as much as a transistor, and it also sounded a little less harsh when it did overdrive.
I loved the sound of MOSFETs as clipper diodes in the Mossy Sloth, but the arrangement is a little different here. MOSFETs conduct in two directions when you connect the drain and gate together: One way is the body diode (a simple silicon diode -- and no, it has no special sound) and the other way is ... something special that soft clips over a very wide range of a couple volts before it finally hard clips. By connecting them in series at their source pins, the body diode in each one prevents the diode from conducting in reverse, and the result is a very high, soft clipping threshold (about 2V and up).
One last trick to soften the clipping was the use of a limiting resistor (R18) in series with the clipping arrangement, similar to how it's done in certain op amp based designs like the Bluesbreaker and AMZ's big muff mod with a "warp" control. Big thanks to tca on DIYSB for lots of help (and diagrams) in understanding this (plus he suggested the MOSFET as Q4!). Although I don't fully understand everything R18 is doing, but the gist of it is that once the diodes conduct, they appear in parallel with R16, and they will lower the negative feedback. R18 therefore helps set the clipping threshold and ALSO sets how "rubbery" the clipping will sound, AND just for good measure, also acts a bit like a clean blend. Values between 10K and 100K seemed to work with roughly similar overall results, but it's not quite as simple as a lower value = more distortion. I ended up not really being able to decide between 22K and 33K and went with 33K in the schematic simply because there were a bunch of them in the schematic already. So why isn't this a trimpot? I dunno, I guess it could be, but I'm not totally sure it's worth the trouble.
C9 sets the cutoff frequency of the clipping. It's bit enough to definitely pass all the frequencies on the guitar (and might be overkill, but I'm not sure how to calculate it). Smaller values (e.g. 1uF or 100nF) will let through more bass.
Bringing this thread back from the dead to discuss and help break down the interesting power stage's clipping section. I'm more interested in how we implement clipping as opposed to how we're using Transistors as clippers. For all intents and purposes, I suppose we could effectively replace them with our usual anti-parallel diode pair.
Ignoring the clipping "diodes" for a minute, we've got a MOSFET stage whose gate is biased by a large 1meg resistor (R17) tied to the drain. I forget where I read this...but is our gain effectively going to be the Value of the "Feedback" resistor divided by the input resistance (-RF/Ri)...which in our case would be the 100k Tone pot? Can anyone please confirm?
Ok, so now Let's add in the Clipping section, but pretend we're shorting the 33k clipping limiting resistor (R19) to eliminate the effect of clipping limiting. I'm assuming the diodes conduct when then signal at the (let's just guess) 5V Q Point exceeds the forward voltage of the diodes, So 5V +/- Vd? Once those diodes conduct, we effectively short out the 1meg (R17) resistor. Shorting that resistor effectively cuts off the gain stage. So to me...this clipping arrangement more closely resembles clipping diodes to ground as opposed to clipping diodes in the feedback of a non-inverting op amp.
These are a few assumptions I'm making based on my understanding of the circuit. Can anyone please weigh in and clarify or correct some of these issues?
Much appreciated!
YouAre, make sure you're accounting for the fact that when the diodes conduct they are creating negative feedback with the input signal in addition to the signal that's bypassed by the diodes. It's not just straight-up clipped, since the input signal level changes in addition to the output signal level (if I'm understanding it correctly). That, to me, makes it a little different from clipping diodes to ground, but I can say that it certainly sounds and feels different.
I had a thread around here recently discussing the feedback loop in a BMP stage. I was specifically asking about the cap in series with the diodes in the BMP (and even after all this, I'm still not sure exactly hwo to calculate it ...), but you might get some good information on the general behavior of that part of the circuit there:
http://www.diystompboxes.com/smfforum/index.php?topic=109438.msg1000958#msg1000958
And of course any other threads discussing the BMP or the same diode arrangement:
http://www.diystompboxes.com/smfforum/index.php?topic=78111.0
To be honest, that MOSFET + feedback stage is above my paygrade. I played around with it until I got the sound I wanted, based on some suggestions from other people about how to achieve it. I could have used an op amp for that stage and the following buffer (like what ROG did with the Britannia) if I absolutely needed op amp-style soft clipping, but at the time I did this pedal I knew far less about op amps than I do now (which is not a lot) and it would have required more parts, plus I like what a MOSFET sounds like when it distorts compared with an op amp.
Quote from: midwayfair on January 13, 2015, 12:09:18 PM
YouAre, make sure you're accounting for the fact that when the diodes conduct they are creating negative feedback with the input signal in addition to the signal that's bypassed by the diodes. It's not just straight-up clipped, since the input signal level changes in addition to the output signal level (if I'm understanding it correctly). That, to me, makes it a little different from clipping diodes to ground, but I can say that it certainly sounds and feels different.
I had a thread around here recently discussing the feedback loop in a BMP stage. I was specifically asking about the cap in series with the diodes in the BMP (and even after all this, I'm still not sure exactly hwo to calculate it ...), but you might get some good information on the general behavior of that part of the circuit there:
http://www.diystompboxes.com/smfforum/index.php?topic=109438.msg1000958#msg1000958
And of course any other threads discussing the BMP or the same diode arrangement:
http://www.diystompboxes.com/smfforum/index.php?topic=78111.0
To be honest, that MOSFET + feedback stage is above my paygrade. I played around with it until I got the sound I wanted, based on some suggestions from other people about how to achieve it. I could have used an op amp for that stage and the following buffer (like what ROG did with the Britannia) if I absolutely needed op amp-style soft clipping, but at the time I did this pedal I knew far less about op amps than I do now (which is not a lot) and it would have required more parts, plus I like what a MOSFET sounds like when it distorts compared with an op amp.
Ah, so it's not exactly clipping the signal as much as it is...subtracting some signal from the input to the MOSFET? (Basically my rudimentary and probably flawed understanding of how negative feedback works in this design).
I'm asking because I'm trying to apply a form of "soft clipping" (ala non-inverting opamp with diodes in feedback), to a mu-amp/mini-booster type circuit. I want to make it so that if the diodes clip, we lower the gain of the pedal to avoid any hard clipping.
Quote from: YouAre on January 14, 2015, 02:06:57 PM
Quote from: midwayfair on January 13, 2015, 12:09:18 PM
YouAre, make sure you're accounting for the fact that when the diodes conduct they are creating negative feedback with the input signal in addition to the signal that's bypassed by the diodes. It's not just straight-up clipped, since the input signal level changes in addition to the output signal level (if I'm understanding it correctly). That, to me, makes it a little different from clipping diodes to ground, but I can say that it certainly sounds and feels different.
I had a thread around here recently discussing the feedback loop in a BMP stage. I was specifically asking about the cap in series with the diodes in the BMP (and even after all this, I'm still not sure exactly hwo to calculate it ...), but you might get some good information on the general behavior of that part of the circuit there:
http://www.diystompboxes.com/smfforum/index.php?topic=109438.msg1000958#msg1000958
And of course any other threads discussing the BMP or the same diode arrangement:
http://www.diystompboxes.com/smfforum/index.php?topic=78111.0
To be honest, that MOSFET + feedback stage is above my paygrade. I played around with it until I got the sound I wanted, based on some suggestions from other people about how to achieve it. I could have used an op amp for that stage and the following buffer (like what ROG did with the Britannia) if I absolutely needed op amp-style soft clipping, but at the time I did this pedal I knew far less about op amps than I do now (which is not a lot) and it would have required more parts, plus I like what a MOSFET sounds like when it distorts compared with an op amp.
Ah, so it's not exactly clipping the signal as much as it is...subtracting some signal from the input to the MOSFET? (Basically my rudimentary and probably flawed understanding of how negative feedback works in this design).
I'm asking because I'm trying to apply a form of "soft clipping" (ala non-inverting opamp with diodes in feedback), to a mu-amp/mini-booster type circuit. I want to make it so that if the diodes clip, we lower the gain of the pedal to avoid any hard clipping.
I'm going to go with both. The signal is clipped, and it creates some subtraction. People refer to it as soft clipping, and it sounds soft, but I don't have a scope so I can't comment on what it looks like compared to other forms of soft clipping.
What you're describing would, I think, work better with diodes from the gate to ground, like what Runoff Groove has been doing lately. You could also check out my "Clipper Ship" for an example that uses the gate diodes with a mu amp design ... it does sound very good. :) There's also a thread about the Azabache where someone knowledgeable explained how gate diodes produce soft clipping rather than hard clipping. There's also a way to use them as partial negative feedback by putting a small resitor between the source and the bias resistor + bypass cap in a FETzer valve, and then running the diodes to the junction of those components. It produces very nice results but is more components and requires the bypass cap to work really well.
For anyone who didn't know about it ... Madbean made a PCB project for this design! Go check it out:
http://madbeanpedals.com/projects/Flabulanche/Flabulanche_2015.pdf
He also made a couple small changes to the circuit.
Nice one, Jon. I have a HYUUUUGE backlog at the moment, but with the new stuff that Brian's putting up, I probably need to have a "visit" sometime soon.
Tagboard Effects did a vero layout:
http://tagboardeffects.blogspot.com/2015/11/madbean-flabulanche-snow-day-overdrive.html
:)
Here's a perf and PCB layout of this one. Thanks for sharing, Jon!
(https://lh3.googleusercontent.com/-H-nkr7J5emw/VmhW3n1tj0I/AAAAAAAABg8/kWQjlAIolas/s1024-Ic42/Snow%252520Day%252520Overdrive.png) (http://effectslayouts.blogspot.com/2015/12/jon-pattons-snow-day-overdrive.html)
Wow, great layout! Thank you!
"Snowverdrive"?
Quote from: MrStab on December 14, 2015, 02:42:09 PM
"Snowverdrive"?
I considered calling it the polar bear briefly, but I think I'm just going to stick with Snow Day to avoid any confusion.
Per my recent thread after encountering some problems with a 1N5817 for the compression rectifier, if you've used a 1N5817 in the Flabulanche, I highly recommend swapping it for pretty much any other Schottky. A BAT4x has extremely low leakage and the .25 Fv ones are probably the best choice. A 1N60P will also work and will have lower Fv.
You don't need to if your bias on Q3 doesn't change more than a volt (or two at most) when you flip the switch, but if it divebombs, definitely swap out the diode.
Anybody have the scheme from this project? Seems to have been folded into the ether.
Quote from: mth5044 on February 18, 2019, 03:30:36 PM
Anybody have the scheme from this project? Seems to have been folded into the ether.
Try this:
http://www.madbeanpedals.com/projects/archives/DistBoostOD/Flabulanche.zip
Here are Jon's pictures from his original post:
(https://i.imgur.com/ay5uzka.png)
(https://i.imgur.com/B7a2K5N.png)
Quote from: EBK on February 18, 2019, 03:44:04 PM
Quote from: mth5044 on February 18, 2019, 03:30:36 PM
Anybody have the scheme from this project? Seems to have been folded into the ether.
Try this:
http://www.madbeanpedals.com/projects/archives/DistBoostOD/Flabulanche.zip
Oh! Thanks for that, didn't realize it got turned into a Madbean project.
Quote from: bluebunny on February 18, 2019, 03:46:27 PM
Here are Jon's pictures from his original post:
Thanks for this as well! For some reason I can see your files fine, but the OP is blank.
Quote from: mth5044 on February 18, 2019, 06:56:51 PM
Thanks for this as well! For some reason I can see your files fine, but the OP is blank.
When I said "original", I meant "my copies of Jon's originals". ;) I reposted on Imgur so you could see them.
The projects here go back to the mid-2000's. a lot of the original schematics are lost because places like PhotoBucket stopped sharing, or the author's DropBox account closed, or other vagaries of life on the web. Makes it very frustrating at times. It would be nice if everything could be archived through this web site/forum, but I'm aware of just how much money that takes. Big shout out to the moderator/owner and the folks that keep this thing running!
Quote from: mth5044 on February 18, 2019, 06:56:51 PM
Quote from: EBK on February 18, 2019, 03:44:04 PM
Quote from: mth5044 on February 18, 2019, 03:30:36 PM
Anybody have the scheme from this project? Seems to have been folded into the ether.
Try this:
http://www.madbeanpedals.com/projects/archives/DistBoostOD/Flabulanche.zip
Oh! Thanks for that, didn't realize it got turned into a Madbean project.
It was for a time, but the project kind of ran its course so I discontinued it. Jon was kind enough to let me license the design from him (really generous of him!) It's a terrific circuit and a highly recommended build.
Quote from: Sooner Boomer on February 19, 2019, 06:02:53 AM...lost because places like PhotoBucket stopped sharing, or the author's DropBox account closed, or other vagaries of life on the web....
Just FYI: _your_ avatar here is blank.
Investigating: you actually point to an image stored on some other forum. We must be logged-in *there* to see it.
Works for you (you see it) because you are probably logged-in forever.