# Help understand the Compressor circuit of a Yamaha MG166CX console.

#### Yami

Joined Jan 18, 2016
352
Hi everyone I was hoping that you could help decipher the workings of the compressor circuit of the Yamaha console. I have attached a pdf of the preamp board and a photo of the circuit of interest. Would greatly appreciated your help any further tips on what to read/research would be awesome.

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#### ronsimpson

Joined Oct 7, 2019
1,465
R129 and Q103 makes a voltage divider. (much like a volume control) When Q103 is open (doing nothing) all of the signal passes though. If Q103 is turned on so it has 27k the volume is 1/2. If Q103 is turned harder it eats up more of the signal. Q103 is used as a voltage controlled resistor.

IC103 looks at the input signal. IC104 almost looks at the output signal. One looks for compression while the other looks for limiting. The output of these two ICs sets the voltage on Q103.

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#### Yami

Joined Jan 18, 2016
352
R129 and Q103 makes a voltage divider. (much like a volume control) When Q103 is open (doing nothing) all of the signal passes though. If Q103 is turned on so it has 27k the volume is 1/2. If Q103 is turned harder it eats up more of the signal. Q103 is used as a voltage controlled resistor.

IC103 looks at the input signal. IC104 almost looks at the output signal. One looks for compression while the other looks for limiting. The output of these two ICs sets the voltage on Q103.
@ronsimpson Thanks so much for the explanation.
I was hoping you could help me clear some more doubts.
1.How does VR102 the pot which controls the amount of compression comes into 'play'?
2.In which configuration are the opamp used IC103 and IC104? Inverting amplfiers?
3.What is the use of D102 in the feedback of IC103?
4. What is the use of D101?

Thanks so much again for the help really appreciate it.