SIMPLE VCA PROJECT: Over several posts, I address different aspects of the circuit design, simulation, implementation, and design iteration of my simple VCA. Here’s a quick list of related posts.
- Simple VCA Circuit Explanation
- Simple VCA Module Build
- Simple VCA LTSpice netlist
- Simple VCA V.2 Circuit Changes
Simple VCA based on Gilbert Cell:
I was initially inspired by the simple VCA on yu synth, but ultimately returned to the more basic concept of a Gilbert Cell. This design, being half of a Gilbert Cell, is something my former analog teacher, Rick Campbell, referred to as the “Dilbert Cell”. The circuit works as a differential amplifier whose gain can be changed by adjusting the quiescent current through the differential pair. I’m not going to go through all the details of how the circuit works, there are poorly written books for that, but rather, just point out a few practical things.
The Design Below is optimized for my large format modular, but it will work at +-12v euro rack voltages as well.
Signal Input Conditioner:
Pro LTspice tip: you can use the “E” Component, voltage dependent voltage source, as stand-ins for op amps, so long as you check the current and voltage to make sure that they are reasonable. Reasonable current for an op amp is usually around 1 mA or less. They are more idealized than an op amp model and they allow LTspice to run faster. You can also use voltage dependent current sources in the same manner.
The Modular is designed to work with audio signals of around 5Vpp. The 2n3904 is considered a small signal transistors and tends to be more linear at lower signal levels. This initial gain stages brings the large modular signals down to small signal level. This could also be accomplished with a simple voltage divider, but there are benefits to buffered inputs.
Variable Gain Differential Amplifier:
Q1 and Q2 form the basis of the differential amplifier. Some people might say that these transistors need to be matched, but it has been my experience that the circuit works perfectly well so long as the transistors are the same temperature. This can be achieved by gluing the transistors together with thermal paste and heat shrink. I suspect that heat shrinking them together would even be enough.
The gain of the amplifier is determined by the R1, R2 and the bias current. You might try to match the load resistors R1 and R2, but regardless, you will need to adjust the load balance trim pot, represented as R18 and R19 (LTspice doesn’t have a potentiometer component). If you build this circuit, you can just adjust this trim to center the output at 0V when no signal is applied.
R23 and R24 make the response of the amplifier more linear. They are not strictly necessary and can be omitted for the quick and dirty version.
Control Signal Conditioner and Current Bias:
Q3 along with R4 and R3 determine the current flowing through the differential amplifier, which in turn, controls the gain of the amplifier. The Bias trimpot, represented by R12 and R13, is used to set the “no-signal” current on the amplifier to just under being “on”. CV input potentiometer, represented by R21 and R22, allow the input signal to be trimmed.
As voltage is applied to the base of Q3, the transistor turns on more, allowing more current to flow through R3 and this allows more current to flow through the load resistors, R1 and R2, in the differential amplifier.
Differential Amplifier:
The last differential amplifier is used to get rid of the common mode between the inverting output and the noninverting output of the variable gain differential amplifier as well as condition the signal to the correct voltage for the modular system.
Finally, this is what the signal looks like at the output.
A quick prototype of the circuit on a breadboard to make sure it works. I skipped some of bias circuitry, but even so, this circuit has very few components. I’m a bit of a component minimalist and I tend to block the entrance rather than the exit.
My Oscilloscope doesn’t photograph well, so I took a video. You can see that the response isn’t perfectly linear, but still good enough for synthesizers.
I’m working on a dual VCA module, I’ve already sent the board out to Osh Park
More to come as the project progresses.
Sagittronics 
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