MST Dual 2164 VCA

Synthrotek is pleased to offer a complete line of eurorack synthesizer modules, power, cases and accessories in both complete units kits and panels & pcbs. We now offer a very complete analog modular monophonic voice with many accompanying utility modules. This new complete voice is the product of a collaboration with synth guru George Mattson, who now joins us in releasing our new MST (Mattson – Synthrotek) line of premium modules.



The MST Dual 2164 VCA is a precision AC (audio) & DC (gate triggers or control voltage) Voltage Controlled Amplifier in 6HP.  Each channel has a linear or exponential response switch, the exponential resulting in a “poppier” response.  Adjustable output level and bleed if you want to mix in your unaffected source audio.  Butters your toast all the way to the edge.


Two identical VCA circuits

Capable of AC and DC signal processing

Linear and Exponential response select switch

CV input (1:1 input-output ratio at 5VDC CV input)

Signal Input

 CV 1 normalled to CV 2

Signal 1 normalled to Signal 2

Bleed level adjustment per channel


Current draw: V+ max 20mA, V- max 17.5mA

Width: 6HP  Depth: 3.5 cm (1.75 in)

MST Dual VCA Calibration Instructions

General Patch Setup

The patch we use to calibrate the MST Dual VCA is relatively simple, but calibration requires an oscilloscope, a module that can generate a sine wave, a module that can generate a slow saw wave, a mixer, a mult, and something that can generate a 5v CV signal. The modules that generate your sine wave and slow saw wave can not be the same module, as they need to be a different frequencies.

Plug the sine wave into the mult, and run one output from the mult to the mixer, the other to the VCA. Next, put the output of the VCA to another channel of the mixer. You will need to be able to plug both of these into your oscilloscope, and you can either use a mixer with switches, or just plug and unplug each channel separately.


The first thing we need to set is the overall output level. Make sure you have:

  • No CV input
  • Turn level fully CW
  • Turn Bleed fully CW

With just the sine wave going into the VCA (channel 1), in linear mode, turn trimmer RT201 so that the output level of the VCA matches the output level of the sine wave. This will set the output for both channels.

Channel 1

Now we will set the CV for channel 1.

  • Turn Level fully CW
  • Turn Bleed fully CCW
  • Add 5V signal into CV input of channel one

With the sine wave still in channel 1’s input, adjust trimmer RT202 so that the output level of the VCA matches the output level of the incoming sine wave.

Channel 2

Now we will set the CV for channel 2. Bring each plug from channel 1 to the corresponding jack in channel 2.

  • Turn Level Fully CW
  • Turn Bleed Fully CCW
  • Use the same CV input from the previous step, plugged into channel 2’s CV in.

Now adjust trimmer RT203 so that the output level of the VCA matches the output level of the incoming sine wave.


Exponential Calibration: (works for both channels)

  • Keep sine wave input in channel 2 input
  • Plug your sawtooth wave into channel 2 CV in.
  • Turn Level Fully CW
  • Turn Bleed Fully CCW
  • Put the switch in Linear mode. (down)

With it set up like the above bullet points, look at the oscilloscope’s screen. You should see something similar to the picture below. We call that the ‘Linear Christmas tree’. Yours should look very similar.

MST VCA Linear
Flip the switch on the VCA into exponential mode, and then adjust RT200 until you see a new ‘christmas tree’ that looks like the one below. The edge of it should have a nice exponential curve to it. Make the level of the exponential either match, or about .5 volts higher to get a nice ‘pop’ sound from your VCA.

MST VCA Exponential


  1. David Ellis says:

    I’ve just come across this. What if we don’t have an oscilloscope? It would be really helpful to have a rough & ready way of calibrating this and other MST modules.

  2. Steve Harmon says:

    Hi David, you can use your ears to adjust the module to taste if you don’t have a scope. It will ‘work’ even if not calibrated to our settings.

  3. Paul Geffen says:

    I built this module from PCB and I haven’t been able to get it to work. A schematic would help. Have you published one?

    – Paul Geffen

  4. Patrick Kelly says:

    Hey Paul,
    Unfortunately due to contracting and licensing issues as well as other factors, we can not release the schematic for the Dual VCA. Sorry


  5. ian rhodes says:

    Hey one of my VCA chips blew so i replaced it (on first try of the module). it might have been put in bssackwards but i replaced the chip and it still doesn’t work, the diodes looked a bit weird so i replaced those, still nothing… I live in Portland so if i walk into your storefront will you guys check it out?

  6. Steve Harmon says:

    Hey Ian,

    If you want to drop it off at the shop, we got repair forms up front and we’d be happy to try to help you out!

Leave a Reply

Your email address will not be published. Required fields are marked *