vca schematic
8.0 Bipolar Transistors
Although it's entirely counter-intuitive that a normal small signal bipolar junction transistor (BJT) could possibly work as a VCA, they have been used. This isn't something you'd find in professional equipment, but BJT based limiters used to be fairly common in portable cassette recorders, and it's entirely possible that newer digital versions use the same thing. Performance is decidedly low-end, but distortion can be acceptable as long as a) you have low expectations and b) the voltage across the transistor is limited to no more than 20mV RMS or so. This is already a significant limitation because the working voltage is so low.
Figure 8.1 - Bipolar Transistor Based Voltage Controlled AttenuatorControl voltage feedthrough may be an issue with this circuit, because the collector voltage will always vary a little, along with the base voltage. However, in the arrangement shown the CV feedthrough is not overly obtrusive, showing around 3mV momentary DC offset when a 50mV input is reduced to 10mV RMS. With an input of 50mV RMS the distortion will remain below 2% at almost any output voltage. This is hardly a wonderful result though, and there are no easy ways to apply distortion cancellation. The parallel PNP transistor driven with the opposite polarity control voltage helps (a bit), but that negates the extreme simplicity … and it still has poor performance. The simplest version has no inverter, and a single NPN transistor. Distortion is at its maximum with around 6dB of attenuation.
Although interesting, this is not a recommended approach due to excessive distortion. It was used in low-end equipment for one simple reason - it's very cheap to build. Bipolar transistors are a few cents each at most, and this was the driving force for the use of a BJT in budget gear. In a basic voice recorder with low fidelity at the best of times, no-one is likely to even notice the extra distortion. It's really a voltage controlled attenuator, but that's a moot point as many other schemes are no different in this respect.[1]
add this to the kick signal before it gets inverted to adjust the attack and release for the vca cv signal
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https://therepaircafe.wordpress.com/2021/04/11/envelope-follower-attack-release-ar-generator/