I am working on the follwing circuit, and have found all values for the components, except for C3 which i have a hard time finding.
My requirements are that it should be able to handle 40-16k Hz with a flat frequency response, and that i can change the gain of the amplifier.
Because of this i would preferably have a cut off frequency of 160k Hz in the circuit:
where R3 = 7,3k ohm R4 = 1k ohm P1 = 0-242k ohm
The thing is that i see the last part of the circuit as a low pass filter, with the output on the negative terminal of the opamp. The problem is that R4 is a part of it, and i will always have a gain lover than 1/sqrt(2) and therefor a cut off frequency does not seem to make sense.
How can i determine a value for C3?