Enveloping Timbre: envelope distortion

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Enveloping Timbre: envelope distortion

Nov 28, 2022 9 min read

Keep the clarity of your attack before letting the distortion embrace you: the best of both worlds!

At least that was the original idea behind this effect, to keep the gain low for the short-lived, high-amplitude note attack of a plucked instrument, then ramp up the gain and the clipping for the decay when the level becomes steadier. This is quite the opposite of what happens with conventional fixed-gain distortion circuits, where the attack of the note is the most distorted part, having higher amplitude. Of course there’s nothing wrong and much to like about that, but going the opposite way would be cool and unusual.

This circuit falls in line with my recent interest in unusual VCRs and level triggered envelopes, elements present in the VCA-1 and the PUP. This time, I thought it would be interesting to try to use plain old BJTs as voltage-controlled element. Nothing new so far, BJTs, and diodes, have been used often in the past this way, with the caveats of keeping the levels low and accepting a bit of asymmetrical distortion, in part caused by a single transistor having to rely on reverse beta when the collector-emitter polarity is flipped by the AC signal.

I thought they would be perfect in this case, since how I was planning to use them forced the voltage across them to be equal, at worst, to the guitar input, and a tiny bit of distortion, while unacceptable for hi-fi, would go unnoticed in a circuit that has distortion as its primary goal, and in the world of electric stringed instruments in general.

Without getting needlessly accurate about it, I tested this was the case by ear and didn’t notice any effect on the sound with a configuration equivalent to the one in the circuit coming below.

The signal path

The basic circuit is my own take on the usual diode clipping distortion recipe (you can call it overdrive if you want): high input impedance, going straight into a high gain amplifier for best noise performance; clipping diodes; a shelving treble control; a recovery amplifier and out. Unimpressive, although probably good sounding, but this circuit has other tricks up its sleeve to be interesting.

The side-chain

I’ll start with the side-chain before getting to what’s actually being controlled. Originally I had tried a high gain amplifier or comparator going into a rectifier and peak detector, not unlike in the VCA-1, in parallel with the clipping stage. While this works in principle, using the other half of the TL072 this way led to a quite unpleasant signal bleed from the saturating side-chain amplifier to the signal path, most noticeable when gain was low: this repeated with different op-amps and I’m confident to exclude capacitive coupling since the two sections were as separated as possible, unless I reduced the gain of the side-chain to barely anything; LEDs to prevent saturation, capacitors to limit the slew rate didn’t do much to help, so I think the cause is to