MFOS VCA not working

Hello there!

Recently i’ve built the VCA from this website: Music From Outer Space - Your Synth-DIY Headquarters

but the end circuit ended up a bit different

So i’ll describe the behavior of the VCA while turning the potentiometer connected to -12 and +12.
during the first half of the potentiometer i get zero volume, then there’s a glitchy zone where i can hear the signal coming through but also a low frequency oscilation. The rest of the potentiometer is at a constant volume without the glitching.

any idea what it could be?
all help is appreciated :slight_smile:

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The LM13700 seems to me to be connected strangely. I’m not sure which MFOS VCA this is based on — this one? — but I don’t think any of them use the LM13700 like this. The CV processing looks rather different as well. Hard to say where to start when the circuit seems so much different than the original, unless the original is some other one I overlooked.

One thing is that in Wilson’s VCA (the one linked above) the signal goes through a 100k:~1k voltage divider reducing it to about 50 mV peak to peak. Yours goes through a 47k:100R divider which makes a 5 V signal into about 10 mV peak to peak, 5x smaller than Wilson’s.

Also Wilson mixes the CV through a 30k with -12 V through a 20k; you use 47k and 6.2k, which again is a rather substantial difference.

And Wilson’s Darlington buffer has a bias voltage on the base which is absent in yours.



I think it would be better for you to build this one. Its even easier!

The Iabc input expects a small positive current, it’s basically connected directly to the base of a transistor, so not getting any output if you feed in a negative current is expected. Could the low frequency oscillation be mains hum, maybe?

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analogoutput → well, the link i posted takes me to the circuit i based my VCA on
this one

Yeah_im_15 → that looks promissing! although i don’t have that 2N3906 could a BC558 work as well?

fredrik → yeah, the negative voltage giving me no sound is fine. It’s an easy fix.
Maybe it could be mains :thinking:. Maybe adding decoupling caps could fix it?

but i think the greatest problem is that the volume curve isn’t linear :thinking:

in the meantime i’ll try that other circuit and also add decoupling caps.

It takes you to a page with links to several different VCAs, including that one which I’d overlooked. The MFOS site is badly designed, based on frames so you can’t just copy the URL for a particular module from the address bar. Use this button
to get a shareable link:

So yeah, that basically is exactly Wilson’s design. I’m still kind of surprised at some of the design decisions. But presumably it worked for him.

You already have decoupling caps on the signal input and output. Are you talking about on the CV input? If you have line hum on your CV that’s something to be fixed upstream…

There’s a nice discussion of designing an LM13700 VCA, ending up with a pretty easy looking circuit (which I have not built) here:

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i was talking about decoupling caps for the +12 -12 lines

You mean bypass caps? I don’t think they’re likely to do much about line hum. But you could try.