Jürgen Haible VCA/Distortion question

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wackelpeter
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Jürgen Haible VCA/Distortion question

Post by wackelpeter » Tue Mar 30, 2021 1:42 pm

I have built the Jürgen Haible VCA/Inverter/Overdrive but somehow there are some issues or things i'm not quite sure if it works as intended.
Schematic can be seen here:
https://electro-music.com/wiki/pmwiki.p ... -Overdrive

First of all it doesn't really get muted, if no CV is applied. Secondly i can't get unity gain.

The driving circuitry for Iabc of the 3080 or 13700 does look almost like the one in for example Thomas Henry's OTA VCA. Except the biasing of the noninverting input of the opamp, the cap and that there is a much smaller resistor 2k2 in the feedback path instead of 100k in TH's design. Otherwise one uses a 2n3906 while he uses a bc560 (BC560 has more gain i think, correct?). One output current goes directly from the collector into Iabc and the other goes via 15k resistor.

That's the Thomas Henry VCA i'm talking about as a reference:
hardly readable graphic
https://electro-music.com/forum/post-315292.html

or linked downloadable as zip on the original site
https://www.birthofasynth.com/Thomas_He ... VCA-1.html


Also in the J.Haible design the audio in goes to the - input of the OTA while in every other VCA it seems to go into the + input. Is this intentional because otherwise the distortion/inverting trick wouldn'T work? Turning the drive pot to it's outmost positions i get one heavily clipped and large signal, while turned the other way around it's inverted but also heavily distorted. Between mid position and the point when clipping occurs, i get either the original (albeit much smaller amplitude) signal or it's inverted version.

Should/ or to what extent could i increase the range by for example increasing the 2k2 or reducing the 15k if i want it to use more prominently as an VCA but keep the benefit of the possibility of distortion and inversion?

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Re: Jürgen Haible VCA/Distortion question

Post by guest » Tue Mar 30, 2021 2:15 pm

do you still get bleedthrough if you turn the drive all the way down? that pathway is always active regardless of the OTA gain setting. you could try removing one of those connections and see if its the OTA or that bypass causing the issue.

the signal going to the - pin of the OTA is because the next stage is a inverter, so the signal comes out in the right polarity. this also makes it easy to mix some of the original signal at the summing junction.

if you want to use it more as a VCA (reduce the effect of the bypass pathway), then incrase the 22k. the OTA bias current shouldnt be driven too high.
openmusiclabs.com

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Re: Jürgen Haible VCA/Distortion question

Post by wackelpeter » Tue Mar 30, 2021 2:23 pm

Thanks guest, will try that out tomorrow... and well with no cv or even with CV (+5V) the drive knob does as mentioned above in one direction does produce a clipped signal and in the opposite end an inverted but also clipped signal.

edit: woosh! :doh: as you say it that the second opamp stage is an inverting one... i have gotten it the other way around, as a non-inverting stage... think i'll have to correct that first tomorrow... didn't noticed that while soldering...

nope... i was somehow confused... it's correct, i have it as an inverting stage... don't know what led me to say otherwise... :hmm: But anyway i'll try that suggested changes and report back.

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Re: Jürgen Haible VCA/Distortion question

Post by wackelpeter » Fri Apr 02, 2021 1:11 pm

Okay... fixed it and as almost always, at least in my cases, it was not a design flaw or error in the schematic, just simply a little bit of incompetence on my behalf to bring the schematic properly onto a stripboard... :party: :roll:

After having once again checked the correct orientation of the BC560 i finally spotted my mistake and wondered why the heck are two resistors connected to the collector. Well obviously it turned out that the resistor-trimmer combination that should have connected to the emitter, somehow took advantage of a moment when i was paying less attention or perhaps making coffee in the kitchen and connected itself to the collector... such a wicked gang, who would have guessed that?

...ehmm yes, and obviously the circuit is working fine and as intended... going to just add a few more control inputs like manual gain control and a second CV input

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Re: Jürgen Haible VCA/Distortion question

Post by guest » Fri Apr 02, 2021 4:00 pm

glad you found the problem!
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