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Differential Distortion

Notes by Gary Burchett:
I think the circuit sounds pretty good with the "stock" setup. Changing the 100k resistor at the emitters of Q2 & Q3 to 10k change (detailed at Guitarkit.com, see link below) gives a decent volume boost. This circuit would not be good for use as a booster for a tube amp. The overall sound is fat! It is very chunky and has a slightly trashy high end. I think a small pF cap from output to ground would help to reduce the "bacon sizzle." The circuit is more amp-like than distortion box.

The circuit is very touch sensitive and cleans up well with picking dynamics and volume knob on the guitar. Listen to the last part of the sound clip for examples of this. The lead part has some cool dynamics changes without the somewhat typical volume loss.

Harmonics are decent and sound smooth. Pick harmonics are pretty good, but not great with a Stratocaster. Think early Billy Gibbons pick chirps, not EVH squeals. Maybe a humbucker would change this.

There seems to be an inherent mid-scoop EQ in the circuit. The lows are sometimes over-powering and the highs are sometimes harsh. This is similar to how I hear distorted Fender Bassman amps, though not quite as "flubby" as a real Fender.

There is not a ton of dirt available. This is not a thrash metal circuit. It would be interesting to add a booster in front of this circuit or maybe adjust the bias of the 2N5088 for more push. This is low to medium intensity distortion, similar to a JCM800 or similar master volume amp.

The circuit has some feel. It has a "spongy" attack. This adds to the tube amp-like feel. You can adjust the trimmer on the NPN to enhance this feel at the expense of some volume and sustain. For the most part, the trimmer needs to be set close to 100k. Maybe a larger trimmer should be used for more adjustment. If the trimmer is set too low, you will get no signal.

I don't always like a lot of low end, so I swapped the 0.01uF cap over the PNPs. I changed it to 10uF to see what it would do. The sound didn't really change much but it did have an even looser feel. I suspect one would have to increase the size of the in and out caps to realize the full effects. I worked my way through values all the way to 150pF As you go up above 0.01uF, the low end gets less and less massive. I like values around 0.001uF for a good cutting sound with a Strat. The circuit sounds similar to a friend's Silver Face Super Reverb cranked. At 150pF, you have a huge treble boost. I think I'll stick between 0.01uF and 0.001uF. The trimmer on the NPN will have to be adjusted for different cap values. Also, I tried the Guitarkit.com suggestion of adding a 10M to ground from the 470k/0.01uF junction. It made it feel more hi-gain, but I don't feel it is a sound I would use. The sustain is greatly reduced and notes feel "choked off."

Listen to the Differential Distortion!

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