Improving the MFOS Dual Log/Linear VCA

The Peasant

I first built a MFOS dual VCA a couple of years ago when I was working on my first big modular project. At the time I made some modifications to the circuit, and as I am currently building another of these modules, I recently revisited these changes. If anyone wants to tweak their VCA they can try this.

I had trouble completely nulling the output voltage offset, as well as the control feedthrough, even using the optional bias trimmers, possibly due to different LM13700 characteristics. To correct this, I added a 100K output offset trimpot in series between R21/R43 and ground. R21/R43 were changed to 430K and R22/R44 were changed to 59K as well.

When setting up the VCA, with no inputs connected, set the front panel offset control all of the way down. Next adjust the output offset trimpot for zero volts DC on the VCA output. Then turn the panel offset control slowly up, adjusting the bias trimpot so that the DC output stays at zero volts as the VCA turns on. You should be able to get no noticeable change in the output offset until just before overdriving the control input, when it will drift down slightly.

I also changed R14/R34 to 24K and added a 470K resistor from the LOG terminal of the LOG/LIN switch to -12V. I didn't write down why I made these changes, but I must have had an issue with the LOG control. Trusting my earlier efforts, I made these modifications to my current board, and it is working fine.

Below is a pic of the pcb with the new trimpots and resistors, one channel connected. I also added ferrite beads to the power inputs.

Take care,
Doug

mfosvca.jpg

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The Peasant · Posted: Thu Aug 03, 2023 8:54 am Post subject:

It has been brought to my attention that there is an error in the instructions above. Funny that nobody noticed this for almost 14 years.

Anyways, it should read "To correct this, I added a 100K output offset trimpot in series between R21/R43 and +12V."

Hope this clears things up!

Take care,
Doug

PHOBoS · Posted: Thu Aug 03, 2023 11:17 am Post subject:

for correcting an error after this amount of time. I did actually build a quad MFOS VCA years ago which I never got to use much.
Not sure if I ran into any problems but maybe I'll dust it off someday and now I know where to look if it's not functioning properly.