Simple VCA

The Simple VCA is a single-chip Voltage Controlled Amplifier module, in Eurorack format.
Simple VCA

It’s a nice first DIY VCA that you can build in very short time, with very few, off-the-shelf, components.

4 HP Front panel

This VCA can act as a variable volume, variable amplifier, mixer…
There are 3 Audio/signal inputs which are mixed together.

Triangle signal, modulated in amplitude by a triangle CV
The CV signal is shown in blue.
The CV response is linear.
Single chip VCA module!

Circuit description

The Simple VCA uses only one LM13700 (U1). It’s a dual Operational Transconductance Amplifier (OTA) in a single package, and 2 independent Darlington transistor buffers.
An OTA, unlike a classic Op-Amp has the ability to modify its “amplification” gain, with a dedicated input pin.

Also, the output of an OTA is a current output, not a voltage one like on Op-Amps.

The main circuit

The circuit is really simple: the input signals are mixed together by 3 resistors (R4, R5, R13). The mixed signal is then drastically lowered by a voltage divider (R8) as the LM13700 can only accept small voltages.

The signal enters U1A at pin #3, and then goes out, as a current, at pin #5, with a gain set by the amount of Iabc current inserted on pin #1.

The signal is then converted to a voltage, and offset, thanks to R11/R14. It is then boosted by the Darlington transistor buffer stage (U1C).

Finally, R15 protects the output against short circuits.

As OTA’s pin #1 requires a current (Iabc), and, as we have a spare OTA available, U1B is used as a voltage-to-current converter.

The CV signal is a mix of the voltage coming from jack J1 and an “Initial” setting by potentiometer R1. The mix is done by 2 resistors (R2, R3), and then the signal is drastically lowered by the voltage divider (R6).

Resistor R9 sets a fixed “amplification” factor for the CV.

Finally, U1B outputs the Iabc current at pin #12 and feeds it to pin #1.

Darlington buffer U1D is unused in this circuit.

PCB, Top side

Simple VCA

Documentation

6 thoughts on “Simple VCA

  1. Nice touch having the voltage test points on the PCB!

    Potential builders should be aware the DIP version of the LM13700 went out of production a year or so ago. As of late 2022 you can still get it but not as readily as before — DigiKey and Mouser no longer have it, Tayda and Thonk do. The SMD version is still produced, so when DIP supplies dry up entirely, a converter board like the Chip-Quik PA0005 or PA0005C might be usable to put the surface mount chip in a through hole footprint, depending on how much clearance there is around the footprint.

    1. Thank you Richard for pointing this out!
      However, there seems to be some clones available: Friends reported that they were working.
      Xinluda XD13700 at LCSC for example.

      Eventually, if the DIP package turns into pure unobtainium, I’ll do a SMD PCB version.

  2. Thanks so much for putting this up! I just built one from the schematics on a protoboard; it was a nicely straightforward build using parts I had on hand. The pot on mine is a slider, since that was the mountable 100k I had available quickly, so it ended up a 6HP unit. I need to make a final faceplate — the current test version is printable transparency film — but it otherwise works great. Thanks again!

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