Bill Baka wrote:
On 05/22/2010 09:43 AM, Scott Dorsey wrote:

Welcome to the New Era. Everything has a switching supply in it. None
of them meet FCC Part 15 specs. Nobody at the FCC gives a damn. Write
your congressman and complain about the enormous arrays of CFLs, touch
lamps, and TV sets for sale at Wal-Mart which are patently illegal.

Tell me about it. Damn near everything that comes out of Walmart is
Chinese and if I buy anything it seems to fail just after any hope of
warranty. The noise seems to come from anything Chinese so I try not to
buy that stuff, but in many cases China is the only source.

Don't blame the Chinese. The Chinese make cheap crap because Americans
demand cheap crap. If Americans wanted good products, the Chinese would
make good products. But the American importers are constantly after the
Chinese factories to cut costs, not to improve quality, and so this is
what you get.

No, and I think the preamp is probably a lot of your issue, that broadband
noise is saturating it. You might want to consider a more tightly tuned
preamp, possibly one with a front end that has outrageous dynamic range and
low noise (like, say a nuvistor or even a 6X8).

It does have switch so I can bypass the preamp and just go through the
radio. The preamp only gets overloaded by an AM station here on 1,600
KHz at 5,000 or 50,000 watts, even though the station is a good 5 miles
from me. If I hit that with the preamp on, yes it will overload, but
going straight to the Hammarlund still gives excellent tuning so in
order to get

We're not talking about catastrophic overload, we're only talking about
a little nonlinearity. It doesn't take much.

You have two problems:

1. Noise that is off-channel, maybe even out of band, which either winds up
being detected due to poor selectivity, poor shielding, or because
something in the front end or early IF (or in your case the preamp) is
mixing multiple noise sources together to form a beat product on your
channel.

2. Noise that is actually on-channel, on the exact frequency you are on.

The first one can be remedied by eliminating the preamp, making sure the
receiver front end is clean (ie. the first RF stage is perfectly linear)
and possibly adding a preselector.

The second one cannot be remedied. The noise blanker is one thing people
have used; it drops the whole signal out when the signal reaches a
limiting level and it good only for impulse noise that is stronger than
the signal.

One of the DSP boxes like JPS sells can help hide on-channel noise,
sort of.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."