[sdiy] droop of sustain level in analogue ADSR?

[Tom Wiltshire]

No-one's mentioned yet the diode-drop problem that I came across a few weeks ago and mentioned here ("Analog envelope generator offset"). That problem affects the sustain level too, and could account for "Sustain droop" in analogue ADSRs.

The problem is that as the voltage decays towards the sustain, the voltage across the diode drops below its forward voltage and the diode stops conducting. Obviously this happens one diode-drop above the sustain level. The capacitor still manages to reach the sustain level eventually, but only because the non-conducting diode has some leakage, so it takes a good while. This leakage discharge took 30 seconds or so in my circuit, but it obviously depends on the capacitor size. The effect is of a circuit that decays normally, but then droops after (which is actually what I initially had thought was going on in my circuit, until I investigated a bit more).

This affects any envelope that uses diodes to separate the stages and which doesn't compensate the diodes ( thats all of them, bar one design by Ian Fritz, as far as I could find!). Envelopes that use CMOS switches instead are immune from this problem.

HTH,
Tom


On 14 Nov 2014, at 07:38, Maarten <diy at artefacts.nl> wrote:

> Uh? I have never seen nor hear this. When the envelope is in the sustain stage the cap is directly connected to the sustain voltage, a potmeter in a normal envelope, a voltage in a vcadsr. This voltage is constant so no droop.
> 
> 
> Michael Zacherl schreef op 14-11-2014 3:20:
>> Hi, had a rather intense discussion with a colleague today.
>> He claimed there are analogue ADSR-designs which suffer from a voltage droop in the sustain stage.
>> Much like a sample&hold. I didn't exactly believe that.
>> Does anyone know of such a design?
>> thanks, Michael.
>> 
>> 
>> --
>> http://mz.klingt.org
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