[sdiy] droop of sustain level in analogue ADSR?

[Ian Fritz]

At 05:28 AM 11/16/2014, Tom Wiltshire wrote:
>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.

I tried to send a post to that effect two days ago, but it didn't go 
through.  (iPad Comcast app doesn't allow plain-text in mail!)   I believe 
that what Tim just mentioned is basically the same effect, except the diode 
drop is from a transistor buffer.

I always use an AD + AR mixture for my ADSR envelopes, so I definitely see 
the "sustain droop" in that configuration, unless the diode switch 
compensation is included. When using an AD to give a pitch "chirp" the 
version of my circuit with a "truncate" switch puts two diode drops in the 
feedback path so the cutoff is sharper.
http://electro-music.com/forum/topic-21723.html&postorder=asc&highlight=envelope+generator+ian

Ian


>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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