I recently took another look at this module and did some more adjustments to it, the following blog was written during an in between stage. You may want to look ahead in my blog for a more current version of this build. HERE
Basically, the best resource for getting started in building Modular Synthesizers is Music From Outerspace. Most of my modules came from this site, as well as a bunch of core ideas I applied to other modules. The only thing that can get in the way with building modules from this site, is, most of the schematics are for a +/-12v power supply. My Modular is the MOTM style with +/-15v. a lot of the projects are scaleable and don't require any modifications but some do. The next project has a couple small modifications I made to the MFOS dual VCA project which I found, made it better for a +/-15v system. REMINDER: I am an amateur and I don't guarantee anything.
so, if you feel inspired to build this circuit, I encourage you to visit the MFOS page where you will find more information and PCB layouts for the project.
I still had this problem with the volumes between the modes being really different. so I played with some of the other resistor values trying to make this better. It occurs to me now, after looking at the schematic again, I would approach this problem differently. What I ended up doing was changing out R9 from a 120K to a 1K. That allowed a lot more current through and brought the volume up. If you've got this schematic breaded up and you feel like trying some stuff, I would try changing the R3 and R4 to 10K resistors and see how that affected the circuit. I wouldn't be surprised if I wrote another blog about this circuit in the future with other changes.
Overall, the circuit works quite well, so I went ahead and etched 2 of them into a pcb to make a quad version of this module.
Making sure my parts would all fit on my double wide panel
I decided to add some leds to my amplifier. flashing lights make everything cooler. I built this simple 3904 led buffer circuit on a hand drawn PCB. basically, the Base of the 3904 is connected via a 100k resistor to the signal output of the VCA circuit. and the correct resistor value for the LED of your choice should go between the collector and +15v, and the LED should go between the emitter and ground. I always use a heat-sink when soldering transistors
I didn't have enough spdt switches, so I've got one dpdt switch in there.
I added a jumper area to the left of the original layout. I found it's nice to have all the wires leave the circuit board from the same area, makes things cleaner but also adds to the work. That's also where I've got my power hook up (top left)
This video is on the boring side, but I didn't really know how to effectively show the VCA in action without becoming convoluted with the operation of other modules. so basically, I change the envelope on 3 channels and also switch between Log and Linear VCA.