Topband: Re: eliminating radio hum/hiss

[Tom Rauch]

Martin,

If you can reduce or eliminate the hum or hiss in or just after the audio
amp that might be a good thing to do.

Be careful before working with gain distribution! Many radios have hiss
because they have high gain after the narrow filters. They do not have
selectivity after the high gain, so the broadband noise from the amplifiers
has two sidebands and extremely wide noise bandwidth hitting the product
detector. In many cases the hiss is irritating more than anything else. It
might show up as noise on a meter and be "tiring" to listen to, but it
almost never affects weak signal copy by experienced ears.

The cure is worse than the hiss if you increase gain in front of narrow CW
filters and reduce gain after the narrow filters. That will always reduce
narrow-spaced blocking and IM dynamic range of a receiver.

Hiss can often be totally eliminated without reducing last stage IF gain and
increasing gain in front of narrow filters by adding a simple cheap audio
filter. The advantage is this does not compromise close spaced signal
performance.

I cannot think of many reasons to go inside the receiver if you do not have
to.

73, Tom W8JI