[sdiy] Analog envelope generator offset

[Jean-Pierre Desrochers]

Tom,

What would be doable is the following;

Any ADSR circuit that has that 'ending' release offset problem
get it at some 'ending level'.
Let's say it's at 10% of the max release level.
So from a max release of 5 volts it would be around 0.5v
Ok. The thing that could be done is to monitor the actual
ADSR output voltage only during the RELEASE time
thru a comparator/schmit trigger that would add a 'damping' resistor
parallel to the release cap to 'accelerate' its release time
only there, solving the sticking offset voltage issue.
This comparator reference would always be the actual max release voltage
passed thru a resistor divider to get it at a level of here 10%.

Just a possibility.
J-Pierre




Le 2014-10-31 13:19, Tom Wiltshire a écrit :
> Yes, it is, but the offset droops over time as the cap gradually
> discharges (presumably through diode leakage). So after 20 seconds or
> so, the compensation is making the situation worse, not better.
> 
> On 31 Oct 2014, at 16:54, Rob Spencer <spencer.rob at slvi.co.uk> wrote:
> 
>> Is it possible to remove the 0.5v DC bias at the next stage? Maybe 
>> using a trimmer pot to put -0.5v in after the diode?
>> 
>> Cheers
>> 
>> Rob
>> 
>>> On 31 Oct 2014, at 16:38, Tom Wiltshire <tom at electricdruid.net> 
>>> 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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