S. Whitt, Audio Compressor, in: Practical Electronics, January 1976 🇬🇧

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modman
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Post by modman »

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Ben N
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Post by Ben N »

That looks too simple not to build.

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Post by mictester »

Basically, it varies the working point of the transistor by moving its bias, thereby varying its gain. This was an approach used in some communications receivers in the 60's as a crude "audio AGC". It tends to introduce lots of distortion (it'll be a slightly squeezed fuzz sound), which doesn't matter too much in a communications receiver, but is simply NOT hi-fi enough for most purposes.

Other - slightly unusual - approaches to gain changing stages I've experimented with include using a "diode attenuator", where the current through a couple of diodes varies their dynamic impedance. This works well, and as long as your audio is only a few tens of millivolts, it works without significant distortion.

Other approaches include using ordinary bipolar transistors as voltage-controlled attenuators, using LDRs (as in the Really Cheap Compressor), using transconductance amplifiers (as in the Dynacomp), and using transconductance amps in the negative feedback loop of op-amps (my favourite method).

If you try the above circuit, don't be surprised if you get LOTS of distortion and noise!
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