[sdiy] Analog envelope generator offset

[Harald]

Hi Tom

For the Elekor Formant it was recommended to build a dedicaded ADSR for 
every filter or VCA. The offset voltage was then compensated with the CV 
input circuitry of the filter or VCA for this special module. This leeds 
to unwanted dependencies for best performance between the modules.

Harald

Am 31.10.2014 um 17:38 schrieb Tom Wiltshire:
> Ian, Harald,
>
> Ok, thanks. That sounds exactly like what I'm looking at. The relevant part of my circuit is very similar to this one, though I've done the control logic very differently. The offset compensation is pretty much the same too.
>
> http://www.cgs.synth.net/modules/cgs78_env.html
>
> Please correct me if I'm wrong, but what I understand is the following:
> As the cap decays towards zero, the voltage across it drops below the 0.5V or so forward voltage of the diode, and the diode basically stops conducting, or conducts only very slightly. This means that the final part of the RC curve doesn't use R, but rather R+some big diode resistance. This messes up the curve and makes the final portion take aaaaagggggeeeeessss!
>
> So how can one prevent the diode from shutting off when we still need it, without  making it conduct all the time? And if this is a very old and well known problem, why do most of the current designs either not bother with any compensation or only use a voltage offset compensation? It must have been solved way back, no? But I can't find anything, aside from a few linear envelopes which use op-amp integrators.
>
> Thanks,
> Tom
>
>
> On 31 Oct 2014, at 15:16, Harald <sdiy at haraldswerk.de> wrote:
>
>> A common problem if it uses diode switching. I tried to compensate that for the Elektor Formant ADSR here: http://www.haraldswerk.de/NGF/NGF_ADSR_F/NGF_ADSR_F_110.html. Sorry its in German i still have to translate this site, but look at the schematic for IC2C and IC2D.
>>
>> Am 31.10.2014 um 12:47 schrieb Tom Wiltshire:
>>> Hi All,
>>>
>>> I'm playing with an analog envelope generator at the moment. This is something new for me since all the envelopes I've done thus far have been digital.
>>>
>>> I noticed that there seems to be a 400mV offset on the output voltage. However, when I started testing it, it seems like it is just the very last bit of the release curve. The output rapidly falls from the sustain level to about 300-400mV, but then takes another full 20 seconds to reach something measurably close to zero.
>>>
>>> I realise that in theory it should *never* reach zero, but do all analog envelopes behave like this? When you trigger a quick series of envelopes, it amounts to a considerable offset (it would be several semitones) in the interval between the envelopes. Are there tricks used to eliminate this effect? I've checked several available ADSR schematics and none of them seem to do anything different - a cap feeding a TL08x voltage follower seems to be standard, and the cap just gets discharged to ground. If what I'm seeing is typical, these designs should all have this "offset."
>>>
>>> I'm just looking for some pointers really, since I don't know what to expect.
>>>
>>> Thanks,
>>> Tom
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