Rats and other distortions

There’s been some Rat discussion over on the Mail Day Thread, thought maybe it should have its own topic. I’ve been thinking about doing a Rat module but it won’t happen right away, so I figured I’d write up a little of what I’ve thought of in case others want to chime in or use it. I haven’t done any breadboarding so this is all theoretical or based on a simulation.

Base design: I’ve been working off this schematic:

Levels: I’d want to make it synth level compatible by reducing the level on input and boosting it on output, using more or less the Ken Stone design.


One thing puzzling me about that is the input is divided down using a 22k:1k divider. Why not more like 100k:4.7k for a ~100k input impedance? Are there guitar pedals with low enough impedance that you need the smaller resistors? If so, that doesn’t apply here, so the larger resistors I think would be preferable.

For the output level, I did a simulation and it looks like the level after the JFET is about 1 Vpp. If that’s right, then Stone’s ~20x boost is too large. His 470R should become 1k for a ~10x boost.

Stone uses one bypass capacitor between the two rails, I’d change that to two, from each rail to ground.

I don’t like output attenuators, so I’d drop that pot.

Power: We can use a 9 V regulator (L78L09, T92), presumably in its datasheet configuration with 330 nF and 10 nF capacitors, and get rid of everything left of R11 (protection components and bypass caps, not needed here). I’m not sure how necessary C13 is (or what it’s for) so I’d leave it in. In my simulation I tried changing the 9 V to 12 V and the outputs seemed to change very little, so maybe that’s a simplification that can be made.

Redundancies: With the input voltage divider, I see no need to keep R1. Nor do we need Stone’s 1M and 10 uF after the Rat output.

Op amp: Rat mythology notwithstanding, there’s very little difference if you substitute an OP07 ($0.49 at Tayda) for the LM308 (unobtainium and widely faked). (See this video.) Of course that’s up to the builder, the circuit design isn’t affected.

Diodes: People like to experiment with different diodes and maybe it’s worth throwing in a switch to select between different diode options. I’ve been thinking of a DPDT to select either 1N4148 or LEDs. The LEDs could be on the front panel but they don’t have to be; it’d complicate things to have them light up while not affecting the distortion, doing it the simple way means they would not light up if the 1N4148s are selected. Of course the output level would be maybe 4x higher with LEDs than with 1N4148s, meaning the 10x output boost would be too large. So you’d have to either provide a way to change the gain of the output stage (no, not just an attenuator after it!), or put a voltage divider after the LEDs that gets switched in and out along with them. I prefer the latter. With an on-off-on switch you could also select no diodes, but there again you’d have to reduce the level compared to the 1N4148 path, and that gets more complicated to do other than by just manually reducing the output stage gain. Or you could go nuts with a rotary switch, selecting 1N4148, various color LEDs, or no diodes, each with suitable voltage divider.

Panel size: I’d favor 25 mm, with a perpendicular PCB using horizontal board mount pots, Molex to connect the switches and jacks. Of course such a board could be mounted behind a 50 mm panel too, if that’s your preference.


I’ve been looking at the Synthrotek schematic here: https://synthrotek.com/Schematics/308_Eurorack_Distortion_Module_Schematic.pdf

It includes a vactrol-based CV for the drive, which is fun, although I don’t know why there’s all that extra circuitry on the input side of the vactrol, all the ones I’ve used in the past have been just input → LED. I don’t know why that doesn’t apply here. Actually, looking at it now, I actually guess it’s to invert it, so that it’s always on and signal turns the LED off → resistance up?

This design also 4 switches which I think are internal to their module for diode selection, but I’d probably simplify that to two and have a switch on the panel. Pedal heads have a thing about tracking down rare Soviet diodes and that sort of thing that I’m not really into, but two modes is fun enough and people can just use whatever diodes they want if they’re into rare diodes.

Great find with the OP07, I don’t like doing new work around discontinued chips.

I guess the main thing I like about this design is that there’s no need for a 9V regulator, which I’d like to avoid if possible as it’s kind of a specialty part that I don’t think anybody will have unless they’re doing pedal->module conversions a lot.

I personally like 5cm for reasons I’ve outlined elsewhere - max fun. Also, with the passive fuzz and the safety valve in my box already, I do already have quite a lot of distortion…


I don’t see that the Synthrotek design does anything to accommodate the difference in signal levels between what the Rat was designed for and what you get in a synth. In which case I don’t really understand how their drive pot and vactrol would work, since even at very low gain you’d still get lots of clipping. And unless I’ve overlooked something I’d think the output level would be quite low. Also, C9 (in the schematic I posted above) is 22 nF and in the Synthrotek it’s 2.2 nF for whatever reason.

Putting a CV directly into a vactrol isn’t a good idea. CVs are not meant to supply power, just to be reference voltages. In addition, the LED brightness would not be proportional to the CV, in fact it would be off for CV values below some minimum. I’m not familiar with the design they’re using but presumably it’s an LED driver that supplies a current proportional to the CV. I haven’t been thinking about CV and am not at all sure whether throwing a vactrol at it would work well (but I’m largely vactrol skeptical).

As I mentioned above, it may be fine acceptable to just omit the 9 V regulator and use 12 V instead, with no other changes required. In the simulation the outputs were not quite identical but from what I saw they seemed very very close. you do get rather different outputs at the low end of the distortion pot:

(grey = 9 V, green = 12 V)

If you do use 12 V it might be worth considering reducing the input level reduction factor from ~1/20 to about 1/15, that’ll bring back some of the distortion lost at the low end.


From a guitarist perspective, and kinda of a Rat head, having the classic controls is something I prefer. The master volume on the pedal allows you to have more control the signal from the guitar. I have 4 different Rats and Keeley Mod, Fuzzrocious, and KATZENKÖNIG. Out of those the 1988 Rat is the one still on my main pedalboard. Classic design with maybe the Keeley diode mod switching.



It’s just an attenuator, and attenuation can just as well be done on the input of the downstream module — or in an interpolated attenuator module, if necessary. But I like having 10 Vpp on all my outputs, with any needed reduction happening where it’s needed rather than upstream. Minimizes noise that way, and allows splitting between modules that need more signal and ones needing less. It also means the output impedance is constant; with an output attenuator, it varies. Certainly you do not want a 100k pot on the output as in the pedal schematic, that’s WAY too large.


Yeah I understand that. But when you are using multiple synths and other instruments to play live you don’t want to be looking up or down steam. You do you, I’ll definitely try whatever you design but I still want a classic Rat with classic controls.



Some more historical and useful info on a great MOD here.


I have a modern rat pedal, the Proco I think. I’ve only used it for synth a couple times but it can make my baby loog guitar sound super heavy :rofl:


Ok so the rat is amazing but what other distortions do you all use? :slight_smile:

I’m looking for something nice for my 606 style drums


I use this on my QSR and other synths. It can get crazy but has a sweet spot for drums sometimes, the EQ gives you nice control. The Safety Valve sounds nice on drums as well FYI.


I have a FC meth amp, its basically a voltage-controlled Big Muff. I haven’t explored it fully, but in my first impression I wasn’t amazed by it.


This is also awesome.


Thanks for the tips :slight_smile:

I see the metal zone and big muff pop up a lot.

I have a behringer heavy distortion lying around somewhere, i think it’s based on the metal zone or one of the other black boss pedals. I’ve only tried it on bass and guitar though, i should give it a try on drums then :slight_smile:

Haven’t tried the safety valve either, should have thought of that as i am quite obsessed with tubes :slight_smile:

I see some different opinions on the muff as it can be lacking in the bass content. I wasn’t 100% convinced by the demos.
Might justify getting a bass big muff because bass guitar is still my main gig :slight_smile: the deluxe looks very nice as it has a clean blend knob and also a crossover.

The lyra looks amazingly cool but a bit over my budget :frowning:

To share my own experience so far, i like slamming the kicks into my mixer really hard. I have a wam290 micro mixer (mine says sound city but it gets rebranded a lot) that i use for pre mixing my kicks. It distorts nicely and i put the level very high and put that into a mic channel on my inkel mx-991 mixer.
It’s a nice sound, but i’m still looking for a bit more :slight_smile:

I have a blackstar ht-drive but i don’t really like that one, only for guitar.

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Also, i use a starved tube line stage from the valve wizards design.
It’s the last one in his ‘triodes at low voltages’ pdf.
I’m not always a fan of starved tubes, but this one does not really sound like it. It’s not meant to be a distortion anyway

At 12v it has a very limited output swing, so i use it as a limiter between my main mixer and my amp. It adds a nice warm saturation to the entire mix :slight_smile:

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Been thinking about doing a Kosmo version of this distorting EQ: Monotropa v3 – Reverselandfill

based on the Boss GE-7 EQ


You should do it! Any modular distortion with EQ adds versatility.

Touch pads on this are really cool.


I am thinking about trying some darkglass stuff. I found schematics for the duality and the b3k. I like the b3k for bass, and I am interested how the duality might sound with synth o.O


Skull and Circuits has a new thing


This sounds pretty great.