Sound lab VCA + MFOS ADSR

hopelessennui · Joined: Mar 23, 2008 Posts: 17 Location: Los Angeles

Hello All!

I built a patched out soundlab and am now adding more modules to my setup.

I built the ADSR and seems to be working correctly. however when patched into the soundlab VCA i cant get the VCA to be silent when the ADSR isnt being triggered, it still lets some sound through, even when turned all the way down.

The MFOS ADSR output volts go from about 1.4 mV to 10.05 V

the soundlab AR goes from -7mV to 7 volts. (the soundlab AR works fine with the VCA by the way)

Is my Soundlab VCA the problem? THANKS!

haxster · Posted: Thu Jul 10, 2008 11:42 pm Post subject:

If you never get it to work, try building the dual vca and use that. Did you build the SL plus? Some resistors do change for the VCA cv inputs, but my guess is that it has to do with those resistors. I read somewhere that the SL VCA is not fully balanced and that the DUAL VCA circuit is a bit better for Modular and more compatible. If this gets solved I would like to know, I was thinking of doing exactly what you have right now.

hopelessennui · Joined: Mar 23, 2008 Posts: 17 Location: Los Angeles

I built the regular sound lab.

It looks like the sound lab plus uses 150k resistors on the cv inputs while the regular SL uses 470k (R16/R20)

The SL plus also has a 47k and 6.2k resistor between the +/- power and the initial gain pot (R223/R224) where the regular SL has none.

also the resistor on wiper of the initial gain (R18) is 100k on the SL plus and 470k on the regular.

land R17 is also changed 150k to 27k on the SL plus.

I dont know what all this means, but im hoping someone does. If i change any of the resistors will i be able to lower the initial gain down so that no audio passes through?

Im not sure what a "balanced" vca is either?

hopelessennui · Joined: Mar 23, 2008 Posts: 17 Location: Los Angeles

Haxster,
Out of curiosity are you going to build the soundlab into a particular modular format? I went with frac rack because i didnt want to deal with giant 1/4 inch patch cables and bigger module size. but it seems like theres so much more panels and projects for MOTM. I guess im still second guessing my decision...

filterstein · Joined: Mar 17, 2007 Posts: 42 Location: The Netherlands

Hi,

You say the build in adsr puts out a negative voltage when it's low. The voltages of the two adsr's are very close, so it could be just your meter.
But, it might be that the output of the mfos adsr is just a bit to high to fully close the vca. You could try to offset it with a smalll negative voltage added to the vca input.

hopelessennui · Joined: Mar 23, 2008 Posts: 17 Location: Los Angeles

would i do this by modding the adsr somehow?

filterstein · Joined: Mar 17, 2007 Posts: 42 Location: The Netherlands

just add a small negative voltage to the point where the adsr control voltage enters the vca. for example put a 100k trimmer between positive and negative supply. trim it to -1v and tip it to the vca cv input. if the output lowers you are on the right way, just have to pick the right voltage. maybe you could also trim the adsr output, but i have not looked into that.