Differential Distortion  Schematic (AMZ).

A differential pair amplifies the difference between 2 signals, and has an inverted and non-inverted output. The DC offset bias on each input signal affect the way the output clips. It's a change from diode clipping anyway.

Aron Nelson had a soundclip of the differential distortion which I always liked. The tone areound the 21 second mark sounded like a jazz guitar fuzzbox. So I built one.

So what does it play like? It's touch sensitive, smooth fuzzy when you hit the strings hard. There are no controls on the schematic.

Distortion Control: Replace the 470k resistor with a 500k (or a 1 Meg) Log variable resistor in series with a 30k. You can go from clean to distorted to fuzz with this.

Volume boost: Reduce the 100k resistor which goes to the junction  of the 2 transistor emitters to about 10k. And/or increase the value of the 1k resistor at the output.

Volume control: I added a 100k volume control.

Bias Wierdness Switch: a 10 meg (yes 10 meg) resistor from ground to the junction of the old 470k resistor/0.01uF cap. This gives you noise gating and a vicious rhythm guitar sound.

The ouput impedance is really low, so I'm not sure how well a Big Muff tonestack and output transistor would work.

Verdict: It has a sound of it's own. It's as fuzzy as a Fuzz Face, and has a smoothness you can hear in parts of Aron's sound sample. At moderate distortion it's not bad, but nothing special. It's easy to build and draws very little battery current.