[AMRadio] 51J3 or 75A3?

Donald Chester k4kyv at hotmail.com
Sat Mar 4 18:40:58 EST 2006

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

This message was typed using the DVORAK keyboard layout.  Try it - you'll 
like it.

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