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schematic

simulate this circuit – Schematic created using CircuitLab

I am having trouble understanding the feedback loop for this gain stage for a guitar distortion pedal. I am completely new to electronics. I tried looking for similar looking RC filter topology and have found none. I though at first that it might be a twin t notch filter, but it isn't that one.

Based on what I have read, \$C_4\$ is used for power supply decoupling. What would be the correct formula for its frequency cutoff?

$$ {1\over 2\pi R_6C_4} $$

or

$$ {1\over 2\pi R_{ps}C_4}; R_{ps} = (R_3+R_4+R_6) $$

What is the function of \$C_2\$? How do I simplify the feedback loop resistance only, without the filter for the power supply decoupling? Sorry if I am doing it all wrong. Any pointers would be greatly appreciated. Thank you.

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Start out by removing the series RC 33K/10nF.

The remaining 10nF ensures the DC gain is ONE.

The 100pF ensures the very high frequency gain also is ONE.

The 100pF in parallel with 10 Kohm is 1uS tau, or 160KHz F3dB.

The 100pF in parallel with (10K + 100K) is 11uS tau, or 14 KHz F3dB.

Now add back in R5 (33K) and C3 (10nF); Tau is 3.3e4 * 1e-8 = 3.3e-4 => radian frequency of 3KHz, F3dB of 500Hz (2 octaves above middle C).

R5 and C3 being parallel with R6, provides 4dB boost in gain above 500Hz.

What is the gain? The minimum gain is set as 1 + 10K/27K or 1.4 or 3dB.

The maximum gain is set as 1 + 110K/27K = 5 (14dB)

And do not forget about the boost in gain, from R5+C3

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I'm not sure about the whole circuit but having a capacitor from the output to the negative input indicates to me that it's a low-pass filter of some sort. Texas Instruments has a fantastic document on OPAMP design with many examples called Op Amps For Everyone. If it's a common circuit it'll be in there. But if it isn't then the tools to solve it will be.

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