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The original was posted on /r/askelectronics by /u/ideal_f on 2023-08-07 20:28:23+00:00.

Hi, I added an inverting opamp stage at the output of an audio effect circuit, that is there to inverse the phase of the signal at unity gain (parts in the dashed box are added by me, SW3 shall bypass inversion and restore original output characteristics).

I ran a 1000 Hz sine through the whole effect and adjusted its main gain stage (not shown) to measure 4.0 volts AC at SW3 Pin 2. However, putting in 10 k for both R19 and R20, i’m getting only 3.5V at SW3 when engaging the inversion stage. Using 15k for R20 gives me the full 4.0 volts.

What is the reason for that? I expected unity gain for R19=R20…but but rather I get some attenuation here?

Vcc/2 is divided from Vcc by two 10k R’s, an a 100uF C in parallel to the ground-connected R of this voltage divider.