VCA-1 multipurpose VCA pedal

From the vendor

VCA-1 multipurpose VCA pedal

Aug 15, 2022 15 min read

Updated: Nov 26, 2022

I've had the idea of an envelope-controlled VCA effect pedal for a few months. Of course, many existing circuits fall into this broad definition, including compressors, the boss slow gear and tremolo effects, for the VCA part. So what would set mine aside? One focus is on versatility. Once you have a way to control your output with a voltage, you can do whatever you like with it: swells, plucks, tremolo or even compressor if you try hard enough. I wanted to have at least a few of these modes at my disposal.

This project can be split into two main separate challenges: settling on a way to control the signal amplitude, and setting on a way to generate an envelope from the input signal. Other design choices and goals will be explained along the way.

The signal path

This part of the circuit is actually the most trivial to analyze. We just have an op-amp high impedance input stage with enough gain to compensate the loss of the following LDR-resistor divider when it's at the minimum attenuation. The LDR-resistor pair is the voltage controlled attenuator (hey, I never said the A stood for amplifier, but there's no practical difference here), which I've chosen because of its apparent simplicity (more on that later), low noise (compared to an OTA for example) and low distortion (unlike other methods, it doesn't require prior attenuation to keep distortion levels acceptable, which would otherwise harm the SNR).

The LDR I'm using has a wide resistance range, going below 1k at high light levels and above 60M at low light levels. Light resistance was measured at a current of 3.2mA with a 5mm bright red LED to be about 460Ω. To measure the dark resistance, I've thrown a shirt over it as soon as I was done with the LED and quickly got the measurement, since this will keep creeping back up for a long while otherwise.

The gate generator

Usually, in envelope-controlled pedals, the sidechain isn't much more complicated than an amplifier and a rectifier/peak detector giving you the control voltage. The envelope of this control voltage is simply decided by the charge and discharge curves of the hold capacitor, usually with a series and a parallel resistor.

While I think this basic scheme is effective in most situations, there are a few reasons I opted for something different.

First, unless the amplifier is saturating at all input voltages (which requires a way to gate any noise), the maximum output voltage depends on the input amplitude. Sometimes this is exactly what you want, for example it makes a filter controlled this way touch sensitive, but I don't think it's the case here. For one thing, it would mean that the gain depends on the input amplitude, thus making quiet notes quieter and loud notes louder (this is also basically how a compressor works). It also means that the attack and release times depend on the input amplitude too.

Second, this kind of envelope generator can