> "I must emphasize what Bob says about different headphones."
Unless it masks another problem, then it's a design issue for which I extend
no forgiveness. There's no excuse for hiss, noise, or high-distortion
levels in any receiver, especially those designed within the last decade.
It costs a manufacturer no more to design and include a low-noise,
low-distortion AF stage. Any compromise is flat-out sloppiness, often
attributed to . LM-386 class of amplifiers have no place in today's
receivers.
An extreme example of a "fix" was designed by Yaesu engineers into the
FT-950. The residual hiss is so high they implemented a relay to cut off Rx
audio when the AF control is full CCW. The activation of the relay can be
heard just short of the CCW stop. Yaesu actually made the transceiver more
costly by utilizing the relay as a crutch. In its place, they could have
used a low-cost, low-noise op-amp, followed by a complementary-symmetry
PNP/NPN pair. The op-amp is responsible for low-noise in the high gain
stage while the NPN/PNP pair allows for zero gain but high AF current to
drive a speaker and headphone. Doing it right means adding one or two
1N914 cross-over distortion-cancellation diodes. The result is no hiss and
audiophile-grade low distortion with power to drive just about any speaker.
Such a circuit can be seen on my QRZ.com page.
Dave's suggestion of using an in-line audio attenuator will work with
residual hiss (i.e., hiss that is independent of the AF control). However,
as the AF control is advanced in unison with the attenuator, be careful of
clipping. Most transceiver AF stages do not produce much power before
clipping occurs.
Paul, W9AC
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