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I built 40m version QCX. It Works fine, I was surprised how this simple radio clear recieve.
But as radio has no AGC, I need use a volume knob a lot. I kept it most of the time almost on zero. The total gain is just too much for 40m (or 80m), I believe optimal for higher bands.

As a 'secure' solution I modified R37 from 1 K to 10K that decrease gain by 20dB. I kept R38 and C23 as it defines cutoff frequency (1326 Hz) for low pass filter.
It works fine, maybe sidetone is quiter. But I am still thinking how to lower gain on first stage IC5.
I cannot touch 100 ohm resistors R5-R9 (input impedance of amplifier) as they are tighten with quad. sampling detector. What about R7-C4, R10-C7 in feedback?
2k2+47n, gain -13dB
3k3+30n, gain -9.7dB
cutoff freq. remains the same

What do you think? Could it affect IP3 / sensitivity of whole in negative way?

73
Ondrej OM4DW

I reduced R38 (originally 120K) in my 80-meter QCX to 12K for a 20 dB reduction in audio gain.? It had way too much with the 120K.? About right now and I turned up the sidetone level a bit to compensate.? I'm think about ways to add AGC of some type to this really fine rig.? Seems like one could take advantage of the audio sampling that goes on with the audio1 line that is feeding one of the ADCs in the CPU.? I think that is the source for the S-Meter display data, so maybe use a JFET in the feedback where R38 is and shut down the audio gain that way.? More experiments to do!? :-)

72,

Jim, K8IQY