[sdiy] Analog envelope generator offset

Ian Fritz ijfritz at comcast.net
Fri Oct 31 15:19:29 CET 2014


If it uses diode switching, it is because the diode resistance becomes very 
large at the end of the decay.  There are standard ways to compensate for 
the diode drop so you only get the true exponential RC response.  A very 
old and well-known problem.

Ian

At 05:47 AM 10/31/2014, Tom Wiltshire wrote:
>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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