Friday, February 1, 2013

MFOS Dual VCA (Quad VCA) module for MOTM


UPDATE!!!!  2/13/13
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.

So, like all projects, I breadboarded it up first and tried it out.  It worked well but there was a huge volume difference between Log and Linear CV response.  Also, I own an Synthesizers.com VCA and it had a noticeably lower volume over all.  using my Synthesizers.com VCA as a standard  I decided to mess with the resistor values and see if I couldn't get a similar volume.  Well, as you can see above, Ray actually made some modifications to the circuit in red, and I thought, not knowing exactly what I was doing, I would try reverting those values back to their orignal values to see what effect they would have.  At least then , I could understand how they improved the circuit. I Found that going back to the original values for R15 and R16 gave me more overall volume. I liked that.  R18 I left at 220ohms and R14, I ended up using a 27K resistor because it's what I had.

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.
 I built this handy circuit board holder for the project.  Made soldering a whole lot easier.  I pretty much always use those chip sockets because my soldering iron doesn't have a temp control and I've been known to burn up chips in other ways too.


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)




All finished!!!


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. 

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