So the this thread started out using two transistors and one IC to make a limiter. Now we are talking about using a PC and its millions of transistors. LOL Your are right there are DSP limiters that do a very good job (in software). I do have a number of DSP audio effect boxes.
there are monolithic vca's that manage good control signal
Most certainly!
DC coupling and using the distortion reducing resistors results in a loud POP when a fast attack time is used.
Probably so. I used two 10meg resistors (no cap) and run the MOSFET at about 1k to 100 ohms so the "pop" would be very small. Doing the math. We have 20meg to the "gain" voltage and 500 ohm to ground. So not much of the gain voltage will get into the audio.
Something like this would ( see attachment ) allow control of the MOSFET from 0 volts upwards.
Gordon, Sorry what you show injects some part of the negative supply into the audio. At low gain most of the negative supply is in the audio and at high gain a small amount of supply is in the audio. When the AudioGuru talked about a pop when the gain changes. This is it in a big way. The source of the MOSFET must set on audio ground. You need to add the voltage to the gate.
Here is the way I got the gate DC worked out. I do not like it but.....
MOSFET M1 and M2 need to be a matched pair.
M2 is to make a voltage that just gets the MOSFET turned on. (actually twice that voltage) The gate is at turn just on voltage, and via the two 4.7meg resistors "Vref" is at twice that voltage.
The top of R15 is at ground DC. This is where audio is injected at mV levels. R15, R12 make a 2:1 divider so this MOSFET is also at just turned on.
So really what I did was take R13 (470K) and remove it from ground and bump it up to a voltage where the MOSFET is just starting to work.
R12, R15 were 10meg in the limiter/compressors I built 1975-1979. I can't remember the transistors but I know it took many volts on the gate to get it turned on.
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R7 should be 0 to 1k. I am using that to adjust the attack time. 100k is wrong.