Snow Day OD - schem, layout, demo, discussion ... and name change?

[sajy_ho]

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?

[sajy_ho]

Ps in wich fetzer I put 2n3819 for the most dynamic range and harmonics?
Thanks

[midwayfair]

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.

[sajy_ho]

Thanks alot.

[YouAre]

"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!

[midwayfair]

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.

[YouAre]

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.

[midwayfair]

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.

[midwayfair]

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.

[bluebunny]

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.

[midwayfair]

Tagboard Effects did a vero layout:

http://tagboardeffects.blogspot.com/2015/11/madbean-flabulanche-snow-day-overdrive.html

[storyboardist]

Here's a perf and PCB layout of this one. Thanks for sharing, Jon!

[midwayfair]

Wow, great layout! Thank you!

[MrStab]

"Snowverdrive"?

[midwayfair]

"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.

[midwayfair]

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.

[mth5044]

Anybody have the scheme from this project? Seems to have been folded into the ether.

[EBK]

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

[bluebunny]

Here are Jon's pictures from his original post:

[mth5044]

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.

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.