Friday, February 15, 2013

MFOS VCA for MOTM +/-15v take 2!

You may want to consult my earlier blog about the MFOS DUAL VCA.  I wasn't totally happy with the results from that build, so I went back and made better, more informed modifications to the circuit.

I don't want to take credit for doing anything except changing some values to get the circuit to perform better with my +/-15v system.  If you're at all interested in building this circuit, I suggest you start by checking out the original VCA on Rays website.  There you can find the earlier version for +/-12v along PCB layouts for etching. 

This is the circuit and the values I ended up using for the VCA.
These changes gave gave both Log CV and Linear CV similar volumes, which was a problem I was having with the original circuit.  It also brought the overall output volume up in the range of my VCA which was nice.  

Somewhere along the line, I accidentally used a 220K for R18 instead of 220ohms. I didn't notice until I hooked it up to my Oscilloscope to compare it to my VCA. Basically, The mistake over drove all the incoming signals and turned all my waves into Square waves.  After hunting around, I found my mistake and fixed it.  It goes to show,  that not all mistakes like that, will make the circuit totally un-useable.  Sometimes those simple value mistakes can take a little while to manifest. 


  1. Thanks for the post. I'm adapting the same card to 15v right now.

    1. I built a new similar but different VCA in later post if you want to check that one out too.

  2. More thanks for this. I built Ray's latest version (2010) - experienced same issues as you did on +/- 15V. Incorporated your changes and now it's so much better!