[sdiy] Analog envelope generator offset
tom at electricdruid.net
Sat Nov 1 14:08:01 CET 2014
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.
On 31 Oct 2014, at 18:36, Ian Fritz <ijfritz at comcast.net> wrote:
> I just posted a typical scheme at Muff's:
> 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.
> 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.
>> 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.
>> 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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