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Old 1st July 2008, 11:32 PM   #1
LarsXI is offline LarsXI  United States
diyAudio Member
Join Date: Jun 2008
Default Basic op amp optical compressor questions

I'm working on a project where I need a very simple and compact compressor for some onboard guitar circuitry. Good tone and fidelity are not important, as the signal being compressed is not to be sent not an audible output, but used for purposes where a uniform signal is desirable. In short, I'd like to clamp down on dynamics and try to force things into uniformity.

I'd like to work with the first circuit on this page. I'm aware that it's sketchy, and I've heard reports of high noise, however I'm impressed with the simplicity.

I've had trouble finding good information on how to set up optical compressors. I thought I'd read up for a bit and then tweak the above circuit. Unfortunately I haven't found anything relevant. If anyone knows of someplace I missed I'd greatly appreciate it. Otherwise, I have some rather fundamental questions due to my lack of experience.

The guitar signal will come from a preamp with an output impedance of 6k ohms. I'm assuming that this calls for an input resistor of 6k ohms?

As far as the photocell goes, should I look for one with a light resistance close to 6k ohms? What would be an acceptable dark resistance? Would a low dark resistance (around 500k) be good for the sake of not pushing the op amp too far, or would it be fine to go much higher (20meg, for instance)?

I understand that the resistor and LED work to follow the gain of the signal so it can be compressed accordingly; however, I'm unsure of how to determine appropriate values for the resistor and the LED.

I'm not looking for outstanding tone from this; however, are there better op amps out there that will do this more efficiently, faster, or with less noise?

Thanks for your time
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