I pull audio off the af gain control of my 75A4 and send to an outboard
hi-fi amplifier and speaker, and it sounds pretty good. I changed some
coupling caps between the detector, noise limiter and af amp, and clipped
out the 510 pf caps Collins later added to the design to limit high
frequency audio response.
A suspect the main source of distortion is not so much the mixer, but the
mechanical filters themselves. The R-390A has similar distortion, but the
R-390 (with L-C i.f. selectivity) sounds quite a bit smoother. According to
the military, the conversion to mechanical filters in the 390-A was a
cost-cutting measure, not for improved selecivity.
But I find the 75A4, with modified audio and outboard power amp, an
acceptable compromise between AM audio fidelity and
selectivity/stability/dial accuracy.
Don k4kyv
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