Making a “Roll your own VCA”

If you’re looking for a simple but effective voltage controlled amplifier design you should check out the “Roll your own VCA”.

When it came to building a voltage controlled amplifier there were various things I was looking for. I wanted something that was simple and I didn’t want complicated setting up procedures. I found lots of designs to get started but nothing really sprang out. What was really putting me of was they mostly had complicated setting up. Then I came across what I had been looking for. The “Roll your own VCA”.

It was pretty simple, just four op amps and an operational trans conductance amplifier, the OTA is half of a LM13700 and the op amps are either LM324’s or TL084’s. Looking at the circuit, which has three VCA’s on a circuit board, the TL084’s mainly have the audio going through them and the LM324’s handle the control voltage.

I decided to build two VCA’s on a single PCB and have each one using half of an LM13700 and half of an LM324 (2 op amps) for the control voltage and half of a TL084 (2 op amps) for the audio part.

my version of the VCA from Roll your own

This is my version of the Roll your own VCA circuit built as a double VCA device. Right click on save as you a version you can view.

All of these integrated circuits were components that I had used before and was familiar with, so if I had any serious fault finding to do I was confident that I’d be able to work out what was going wrong.

Setting up looked really straight forward to do. Just a case off connecting voltages up and tweaking various pre-sets till the correct voltage was reading on a multi meter. Everything I wanted in a design, what could possible go wrong!

Each of the two channels is identical, with an input and output, a CV input and three pre-sets.

It could also be set up for a unity gain of 5V which was perfect for my system.

Only having two channels meant I wasn’t wasting half of an LM13700, the LM324 was being used just for CV and the TL084 just for audio so the laying out if the circuit board was pretty straight forward. The original circuit was powered from a + and – 12v supply so I stuck with the same and just put a socket on it to plug the supplies in. Once I had laid the board out and had it made the construction was pretty simple. Following the usual resistors and small components first and using sockets for the IC’s. I decided to have four VCA’s and a euro rack panel so that meant two PCB’s for the panel.

My take on the circuit for the Roll your own VCA built as a dual device.

I built a couple of boards up and powered them up without the integrated circuits in and measured the voltages on the IC sockets, everything was correct so I unpowered the board and populated it with the LM324, TL084 and LM13700. After powering up the board again I checked my power supply to see it was just supplying a few milliamps and touching the integrated circuits confirmed none of them were getting warm.

So far so good so and looking at the calibration procedure it was just a case of connecting the output to a voltmeter and each of the inputs to zero or 5V while tweaking the pre-sets. If the set up procedure worked then your VCA would be working

Initially you connect the input and the CV input to 5V and adjust the pre-set on the CV input on the LM324 (VR 2) to get a voltage on the output of about 2.5V

Then leave the 5V connected to the CV and connect the input to 0V and adjust the input pre-set on the TL084 (VR 8) till the output reads 0V

Now swap the input and the CV inputs over so you have 5V on the input and 0V connected to the CV input and adjust the pre-set on the second half of the LM324 (VR 5)till the output reads 0V

Then connect 5V to both inputs again and adjust the first pre-set you tweaked on the first half of the LM324 (VR2) till the output is 5V

Repeat steps 2, 3 and 4 again and keep repeating, each time there will be less and less adjustment until you can’t get it any better. I always think that’s a complicated way to explain it but when you’ve done it once you’ll realise it’s a lot easier to do than put down in words.

You then do the same for the other 3 pre-sets on the second half of the LM13700n. Once it’s set up correctly you’ll see that it basically functions like a sort of analogue “and gate. Connect 5V to the input and you’ll get 5V on the output when the CV input is 5vV but lower voltage on the CV input and you’ll get a lower voltage on the output.

So next stage is to connect an oscillator output to its input and monitor the output, varying the CV input will vary the output level and bingo there’s your working VCA.

Two boards mounted on a front panel to give four VCA's

Only thing was on of mine didn’t function, what’s more the same one didn’t function on the second circuit board. Because of the straight forward circuit design it wants too difficult to measure where the problem was. Pin 7 of the LM13700, which was the “BUFINA” input, wasn’t connected to -12V. I checked on the PCB layout confirmed that the “net” hadn’t been routed and it hadn’t told me either!

Luckily this was one of those easy if annoying faults to repair as there is a -12V on the pin next to it so it was just a case of joining pins 6 and 7 together. Not ideal but easy enough to correct with a link. Once this was done to both boards all four channels functioned correctly.

These VCA’s perform excellently, are easy to understand and set up and are as cheap as you could hope for on the component front with all parts being easy to obtain and that should remain the case for the foreseeable future.