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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
CGS78 UEG small "residual" voltage while Gate is high
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tömbszelence



Joined: Aug 12, 2012
Posts: 5
Location: Hungary

PostPosted: Wed Jan 23, 2019 3:36 am    Post subject: CGS78 UEG small "residual" voltage while Gate is high Reply with quote  Mark this post and the followings unread

My CGS78 Utility Envelope Generator got trimmed for 2-3mV voltage offset on the output when there is no gate signal present.
But when there is a gate signal, even when i turn down the sustain pot all the way down to zero i got an "offset" of 200-300mV which is enough to keep my VCAs slightly open so i can't kill the output completely while i hold down a key or send a continous midi note on signal from my interface.

Can i trim this out somehow? I am open to add some additional parts.

On the topic of mods have anybody added an output buffer to this circuit? Since the circuit relies on the output signal to work when i connect the output to an attenuating input and turn it all the way down to zero (essentially connecting the UEG output to ground) it stops working, no LEDs or envelopes and it makes the whole thing less robust to use.
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Hubaswift



Joined: Jul 06, 2016
Posts: 76
Location: Canada

PostPosted: Fri Jan 25, 2019 11:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

An attenuator shouldn't ever short your input to ground. Say you have a pot, one end will go to your input, the other end to ground. the wiper in the center goes to your output so that the wiper is essentially scanning between the input and gnd. if you use say a 100k pot for your attenuator, the input will always have a resistance of 100k to ground. By the sounds of it you're using the wiper of your pot as the input of the attenuator when it should be the output. That aside, I have also built a CGS Utility EG. It's late for me now, but once I find some time during the weekend I'll look into your problem and check if mine exhibits the same flaw.
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tömbszelence



Joined: Aug 12, 2012
Posts: 5
Location: Hungary

PostPosted: Sat Jan 26, 2019 3:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Hubaswift wrote:
An attenuator shouldn't ever short your input to ground. Say you have a pot, one end will go to your input, the other end to ground. the wiper in the center goes to your output so that the wiper is essentially scanning between the input and gnd. if you use say a 100k pot for your attenuator, the input will always have a resistance of 100k to ground. By the sounds of it you're using the wiper of your pot as the input of the attenuator when it should be the output. That aside, I have also built a CGS Utility EG. It's late for me now, but once I find some time during the weekend I'll look into your problem and check if mine exhibits the same flaw.


Thanks for the answer, when i measured it i just connected a cable to the env/cv out and measured between the tip and of the sleeve cable.

You were right, i wired one of the CV pots wrong on that circuit which could mess with the UEG. (but since it had an initial CV level pot and also a trim for trimming CV i could trim out the error so i did not noticed what happened)
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