Question about 911 Envelope "Thump"

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dykesh
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Question about 911 Envelope "Thump"

Post by dykesh » Tue Nov 27, 2018 6:51 pm

Maybe a silly question but I have a 911 from Mos-lab and Synth-werk being fed a gate from a Cirklon and I notice that I have to turn the attack past 10ms or more or I get a very noticeable thumping sound from my VCA. I have attached an mp3 with all knobs at 0, then attack: 0, Decay 45ms, Release: 25ms, Sustain at 15%. I understand I could be getting this from a zero attack but I did not think I would have to back off so far. Is this behavior the same for all envelopes? Any help is appreciated.

I am also surprised that the Mos-Lab and Synth-werk set to the same timing settings send out a very noticeable different envelope. The Synth-werk is much quicker. Strange?
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911 Env Test.mp3
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CZ Rider
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Post by CZ Rider » Wed Nov 28, 2018 9:20 am

Have a set of the original 1969 revision modules here. I would guess the SW version might be an exact clone?

If I understand correctly, the way the 911 works is the peak voltage output, about 5.5 volts. Is the voltage the T1 and T2 will attack then decay to. So even setting both values to 2ms will still spike to that same 5.5 volts. The Esus value is the voltage that T2 will decay to and is not an attenuator for the overall peak voltage. So a 2ms 5.5 voltage spike should make a click.

A few ways to eliminate this effect. An attenuator could reduce the overall voltage output. On my system a click could indicate the 902 VCA needs adjustment. Moog did make a "click filter" on the CP3 output for such. I tried it and the best it did was reduce the click effect when modulating a filter with a square wave from the 901 oscillator.
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jkjelec
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Post by jkjelec » Wed Nov 28, 2018 12:50 pm

Does it produce a thump sound even when there is no input signal to the VCA? If so, there might be a DC offset that has crept in. Most classic VCAs have an offset adjust trimpot whose purpose is to eliminate any DC offset and minimize or eliminate the thump.

Edit/Add: If the VCA does indeed have a DC offset in it's output, then faster envelopes will make the thump have more high frequency content and thus make it more "clicky" and noticeable. Slow attack and release times will mask it, to the point where it might not even be noticed. So two different envelope generators, if they have different minimum attack times at their fastest settings, could result in different sounding thumps/clicks.
Last edited by jkjelec on Wed Nov 28, 2018 5:37 pm, edited 2 times in total.

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ranix
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Post by ranix » Wed Nov 28, 2018 5:19 pm

can you stick an oscope on it and record an image of the thump? That will tell for sure if there is an offset or if something else is going on

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dykesh
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Post by dykesh » Wed Nov 28, 2018 7:32 pm

Thank you all for the help on this. I have made some progress but have created new questions. Using my Cirklon cv gate output which is 5 volts creates the thump heard in my audio file. But when I run the Cirklon gate to an STG attentuator and set the volume knob to around +1 or so, I get a much more correct sounding envelope. I also sent a Mos-lab LFO output to the 911 and it worked correctly. I sent a synthesizers.com LFO++ to and could get it to mostly work correctly but had to turn the amplitude knob to around 8 out of 10.

Does anyone know what peak voltage the 911's are needing? I am guessing 6 volts but I cannot find any literature.

Is the peak level going into the 911's that delicate to get correct? With the volume knobs low, I get no envelope, then a small range of correct envelope, then I get thumping turning the volume up more. Any advice is appreciated.

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