[sdiy] Analog envelope generator offset

Harald sdiy at haraldswerk.de
Sat Nov 1 17:04:59 CET 2014

...and i can not see or find it.

Am 01.11.2014 um 15:51 schrieb Matthias Herrmann:
> no problem here. it is a png file...
> matthias
>> -----Ursprüngliche Nachricht-----
>> Von: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
>> bounces at dropmix.xs4all.nl] Im Auftrag von Harald
>> Gesendet: Samstag, 1. November 2014 15:04
>> An: Synth-diy at synth-diy.org
>> Betreff: Re: [sdiy] Analog envelope generator offset
>> I am sorry but i can't find or see the schematic. Not on Muff's and not
>> on Ian's site.
>> Harald
>> Am 01.11.2014 um 14:08 schrieb Tom Wiltshire:
>>> That's very neat, at least for a simple AD/AR case. And it works
>> beautifully. I've tried it in LTspice.
>>> Can you explain a bit more how it works please? I sort-of get it in
>> general (I think) but I'd like to properly understand it, if I'm to adapt
>> it for my ADSR situation.
>>> What I think is going on is something like this:
>>> Imagine a gate signal going into the non-inv input of U1B. U1B provides a
>> Gate-signal-with-gain at its output. The cap then charges towards this
>> boosted voltage level. Once the cap voltage gets within one diode drop of
>> the charging level, the diodes around U1B start to conduct, and the gain on
>> the Gate drops towards unity.
>>> I don't really get how the gain around U1B is limited to the-gate-plus-
>> one-diode-drop, although I can see that it's clearly because of the back-
>> to-back diodes in the feedback loop. I just don't see how. This is where a
>> lack of formal training in any of this stuff starts to be a pain in the
>> neck - again!. Still, I'm always learning.
>>> Thanks,
>>> Tom
>>> On 31 Oct 2014, at 18:36, Ian Fritz <ijfritz at comcast.net> wrote:
>>>> I just posted a typical scheme at Muff's:
>>>> http://www.muffwiggler.com/forum/viewtopic.php?p=1708948#1708948
>>>> No idea why other designs don't worry about this ... used to drive me
>> crazy.  This solution was published in EN many years ago ... can't remember
>> offhand who first came up with it.
>>>> Clipping the envelope to zero when it gets to a small value also makes
>> sense.
>>>> Ian
>>>> At 10:38 AM 10/31/2014, Tom Wiltshire wrote:
>>>>> 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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