[sdiy] Decay envelope from triggers/gates

Liam Wall liam.wall at gmail.com
Sat Aug 22 23:54:50 CEST 2020

Dear Esteemed sdiy'ers,

Apologies if this is a fairly noddy question for this list.

My design objective is something that will take a trigger or gate of any
length, and generate a consistent decay envelope (independent of gate or
trigger length).

I have a design here based on piecing together a circuit from Horowitz and
Hill, and one from Thomas Henry's Drum Synth Cookbook (with some amended
values). It's got three sections.
1. Pulse generator taking a rising edge to a roughly 1ms 11v pulse
2. Darlington pair to avoid loading the pulse output
3. Pulse to decay envelope (from the Thomas Henry book)

Based on ngspice simulation, it seems to work, but I don't really know what
I'm doing --- is there anything obviously bad or sub-optimal in this

Two things that I'm unsure about:
1. After the pulse completes, the base of Q5 goes to 0, Q5 and Q6 are off
and C2 is discharging through R6 (the decay stage) ---  at this point Q5
and/or Q6 are reverse biased across the base/emitter. I think the reverse
breakdown voltage is 6v, and ngspice shows more than that for Q5. (Q6 is
actually mostly _not_ reverse biased and its base mainly stays a diode drop
above its emitter, though I don't understand why). The simulation actually
shows ringing of around +/-200mV around zero at base of Q5 after the pulse
2. The arrangement on vcc for the darlington pair --- I noticed that on the
attack phase there is a pulse of current drawn peaking at 190mA (to charge
C2). This seems a lot to abruptly demand of the power supply even if it is
just for <=20uS. Is this a reasonable approach?

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