SubSub page: VCA differential pair based
design from 2017
Introduction:
Differential pair based VCA's are quite common but unfortunately I found no chips or designs that work at 5V single supply.
So I designed and build something myself for use in the MonoLadder synth.
This is a very simple and cheap differential pair VCA. I found a 30 cent differential pair (BC847bs), skipped all the temperature compensation (could be done by microprocessor) and had it working up to 100mVpp only (in the MonoLadder that is what the VCF also uses)
At higher input levels it is not hardclipping but produces very nice sounding rounded waveforms (see simulation).
Control signal crosstalk is really low because matching is quite good and I used a trick to prevent signal input offset, also that by the input biascurrents of the transistors.
It goes like this: for dc both inputs are connected to the input signal via identical value resistors (1k). This supresses dc unbalance by input biascurrents
and signal offset without any trimming needed.
The 10uF cap C2 on the base of Q2 creates an ac-only differential input signal (20Hz up).
I made a circuit simulation model (SPICE, Microcap) to test the design.
The simulation results as a "scope trace" sweeping the control voltage (gain) from 0 to 5V, input signal 100mVpp:
The simulation results in the frequency domain, clearly showing the constant dB/V controlcurve (dB axis):
The simulation results as a "scope trace" sweeping the input signal amplitude from 20mVpp to 200mVpp, gain=20dB:
note the very smooth overload behaviour without harsh clipping.
More text will be added here