The GAC circuit in the Ethos Overdrive is driven by the output of the op amps in the presence circuits. On the clean side, the output of the presence op amp is dumped right in; on the OD side, it first goes through a lowpass RC filter made with a 47K resistor and a 22 uF electrolytic, which in theory is also loaded down by the GAC circuit. I ran two simulations, one with that initial RC filter for the OD circuit, and one without; the frequency response of the GAC circuit is almost the same either way.
While the resistor values were read off of the parts, the capacitors C40 and C41 were read with a multimeter. The C40 reading of 0.22uF looked pretty solid, but the 0.02uF reading of C41 was a little iffy looking. I used 0.022uF for C41 in the results shown here; using 0.02 uF didn't change the result significantly.
The result is that GAC is a basically passive bandpass filter with a center frequency of 400 Hz. When it's activated, the following op amp is set up to provide a gain of two, I think to compensate for the "insertion loss" of the GAC filter.
When the GAC circuit is switched off, the GAC circuit technically loads down the op amps feeding it, but the amount of loading seems to be quite insignificant.
[Incidentally, I discovered some errors in my previous posts during this experiment... R49 is 39K, not 3.9Km, and in the handwritten GAC schematic I previously posted, the capacitor in parallel with R50 is C41, not C50.]
Here's the OD version LTspice schematic:
gac_from_od_channel_schem.png
Here's the clean version LTspice schematic:
gac_from_clean_channel_schem.png
Here's the OD version frequency response:
gac_from_od_channel_freqresp.tiff
Here's the clean version frequency response:
gac_from_clean_channel_freqresp.png
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