Sunday, August 18, 2013

AttenuVCA mixer -- Auto Panner --Simon Pegg

I recently moved to Portland,OR from Ann Arbor MI.  A little before the move I was working on a newer version of my 10 step sequencer but I ran into a bunch of problems and although it turned out OK, I won't do a post on it until I get it figured out completely.  That being said, my first project after my move was to build a simple 4 Channel VCA to replace my earlier VCA.

There were a few things I wanted to improve on the older, modified MFOS VCA I built.  I figured it would be cool to design in a simple Mixer circuit to allow the module to be used as voltage controlled mixer.  I also thought it would be nice to be able to invert the individual channels CV so I could use the module as an auto panner or subtractive VCA.  To accomplish this, I include attenu-verters on all the inputs.  The VCA I designed is mostly based on the MFOS VCA and the VCA that Ray put in his MAKE book, which came out this year.  I decided to skip the whole LIN/LOG aspect of the VCA and just make it a simple LIN VCA.  This kept the overall circuit much less complex.

There is a mistake in the schematic above.  the NPN transistor was switched out for a PNP.  the LED connects positive side to ground and negative side (shorter) to the emitter of the PNP.  the collector is connected to -15v via a 2k4 resistor.  the base is the same,  connected via a 100k resitor to pin 7.

the build... 

I never know how much time I should spend explaining how to assemble this stuff because I'm not sure that anyone actually builds it besides me.  Below is the etchable PCB and a PCB Parts layout.  One thing that is not on the PCB parts layout is the 4 x 3906 PNP transistors, so there are four sets of 3 open pads near the bottom. these are for the PNP's. The square pads are collectors. 

The panel mount pots are all 50k.  The leads that should be soldered to the circuit board are labeled on the PCB. For example, on the volume pot, the CW(clockwise) and C(center) pads are labeled.  the remaining pot pin CCW (counter clockwise) should be grounded.  same goes for the inverter pot, CCW and C are labeled,  the CW pin should be grounded

I used switching jacks for the Input and the CV input.  I connected the jack in such a way that if you plug an audio source into channel 1 and nothing in channel 2, then the signal will be repeated in channel 2.  the same goes for the CV input relationship between channel 1 and 2.  I also wired channels 3 and 4 this way.  Makes it easier to use the module as an auto panner.  

the four separate VCAs are arranged in columns on the PCB.

There are 3 trim pots per column.  the near the bottom, (50k) is for adjusting the attenuverter.  use an oscilloscope connected at the output of the attenuverter to compare the normal signal versus the panel mount pot all the way left and all the way right.  they should have equal amplitude, adjust the trim pot accordingly.  *pin 7 and GND or 8 and ground on the TL074 are where you should connect your oscilloscope depending on which VCA you are trimming

The middle trim is to set the DC offset of the CV input on the VCA.  plug an audio source into the input. leave the CV input unconnected and make sure the panel mount volume knob is all the way left.  connect your oscilloscope to the output. adjust the trim pot until the audio source just barely disappears.  

The top trim pot is used to adjust the DC bias of the output.  leave the oscilloscope connected to the output.  disconnect the audio source from the input and adjust this trim until the oscilloscope reads 0 volts. 

do this for all the channels. 

I forgot to mark the polarity on power supply capacitors below.  the left one is negative towards ground and the right one is positive towards ground.  that should be everything.  

I did a Panel layout with Simon Pegg.  if you want to etch it,  consult my earlier blog posts about etching aluminum.  the image below will need to be inverted and flipped horizontally for etching.  the PCB above is ready to transfer. 

These are some photos from the process. 
I was using an iron to do my transfers but I recently started working with a presto griddle.  It will eventually be more like a sublimation press but that's a project I'm still working on.  The griddle is an excellent heat source though.

 This isn't my best panel etching.  The ferric chloride was very fresh and etched a lot faster than I anticipated, that being said,  it still turned out cool and gave Simon "zombie eyes" which isn't out of theme from the picture, which was borrowed from Shaun of the Dead.

I came up with an idea to use those like right angle brackets for soldering all the wires and panel parts.

After I finish soldering, I can remove the brackets and mount the panel without much fuss to the stand off bolts. 

And finally,  here's a video of it in action.  


  1. Hi Charlie,
    Welcome to Portland. My Modular is here:
    I still can't get my 4 MFOS VCAs to behave, so props to you. Maybe I'll see you around at Control Voltage.

    1. thanks man. I had a problem with the MFOS LFO too for me, I didn't notice that the op amps diagram switches which side the Inverter vs the non inverting inputs. for example IC1-A has the inverting input on the top, and on IC1-B, the inverting input is on the bottom. that was my problem anyway.

  2. Hi Charlie,
    What is the CV voltage?(0 to 5V) or (0 to 10V)

    1. -5v to 5v. you can use the offset know to bias the VCA on and then use negative voltage to turn the VCA off.

    2. Thank you very much!
      But if you apply 10V like the yusynth ADSR?

    3. the "attenu-verter" aspects of the CV control means that you can attenuate the incoming CV signal. so, you can adjust attenuate a 10v signal for 5v or less.