CMOS Distortion Box

This is a schematic for a distortion box I made for myself, and a lot of other people.  I built the first one for performance use in 1978.  I used it exclusively for years as my working distortion box.  It has a warm, compressed sound that breaks over quite nicely, so you can solo by cranking the knob on your guitar, then back to a pretty clean rhythm sound.  I rarely changed the settings on the box during a performance, a sign of a good effects box.  It was a little noisy, but for Rock and Roll....

You can hear it best in this recording, one of my songs, Too Perfect.  Good for lead, and good for rhythm too - listen guitars in the at about 1:38 in the song for the rythm sound. .  


When designing with CMOS, we frequently used spare inverters as analog buffers, if we weren't too picky about their performance. They were used in this mode for all sorts of things.

Since they are MOSFET based, some people were using them to simulate tube amplifier characteristics.  The distortion boxes I had were very harsh, and I was looking for something that would not be such a punch in the face.  So I gave it a try, and it worked out pretty well.  

The first I built using vector sockets and flea clips on peg board, but a did a few simple ones using a standard Radio Shack prototype board (example).  I don't think any two of them were the same.

I have shown the version I used on stage.  You can build a simpler one, or add more stages with the spare inverters.  Just make sure to tie down any unused inverter inputs, or they will oscillate and cause all sorts of weird problems.  


Circuit Description

You can see the basic hookup of the inverter as an op amp.  This will work with any digital inverter.  The bipolar 7404s sounded like hell.   Motorola CMOS 4049s were ok, but the CD4069 is the one I settled on after listening to a few.  Some people liked the harder sound of the 4049, so I built some that way.  

I used a CMOS 4016 to switch the effect in and out.  This was because I had a spare switch on my Acoustic 260 footswitch, and I wanted to have all my footswitches together on the stage.  The box sat on the floor next to the footswitch, with the two controls facing up.  

As a working musician, reliability was very important.  Feet are hard on things.  A distortion box gets stomped on 40-60 times a night.  The Acoustic footswitch was a very robust casting holding only a on/off switch.   By using the separate Acoustic footswitch, the fuzz I built would not be stomped on, only the Acoustic footswitch, which was made for it.  The fuzz never failed in 5 years of use 4-5 times a week.    

Most units I built for other people used a mechanical In/Out switch on top of a very sturdy box.  They were also surprised that a box that can be stomped on is the most expensive part of the project.   

The original schematic had a 1N751power supply circuit so the output wouldn't have too high an output, but the output pot cut the level just fine, so I eliminated it from the circuit.

When I added the 4016s to use the external footswitch, I added the unity gain buffer on the input.  This made the analog switchers work silently, so that when I punched in, there was no pop.  C6 doesn't do much, but I was in the one I used regularly, so I have shown it here.  Sometimes I would put in C4 to limit the bandwidth to the audio range.  The one I used regularly did not have this.  At a gain of 10, the inverters pretty much are bandwidth limited themselves.  You can play with this value to smooth it off as you like, or even add a tone control pretty easily.  

If you don't use the analog switches, you probably don't need the input buffer.   You can build up a usable circuit just using the circuits around U2B, U2C and U2D, and the dc blocking caps C2 and C3, to see what it sounds like.

The inverters gain bandwidth product makes them linear only up to gains of about 10, so it sounds much better if you put them in stages, as shown here.  The last one goes all out.  

I used pots from a big bag that was given to me.  If you are going to buy them, I imagine you could calculate more appropriate values than the ones I used.

The circuit is a little noisy, so I experimented with a few ways knock the noise down.  At one point I added an inductor in series with the power supply, figuring that the battery was contributing.  It didn't affect the noise much (which is primarily from the MOSFETs in the 4069), but it did make the supply soft - like a tube rectified amp.  So when you hit it hard, it kinda groans, like an old Fender Bassman.   I put this in, and took it out.  Then put it in.   Depended on my mood.  Probably should have added a switch, but I never did.

Anyway, I make no promises about whether you will like the sound, but I used it for years, and it is very easy to prototype.  A 4069, a battery, a protoboard from radioshack, some resistors and caps and for less than $5, you are in business.  

Or you can use a POD XT Live, or Bidule with a Voxengo Boogex VST plugin. They are my current favorites. But they are a little more expensive, and weren't around in 1978.