On 05/22/2010 09:43 AM, Scott Dorsey wrote:
Bill wrote:
I live in a neighborhood where there is horrendous noise to the point
where I can't receive weak signals. My setup has an RME DB-20 R.F.
preamp feeding the Hammarlund. The antenna is just a wire I ran through
the trees away from any power lines. Some of the noise was coming from
early generation CFL's, some from my SCR touch lamp which makes noise
even while off. Even cell phone battery chargers and AA NiMH chargers
seem to have switching supplies in them and they probably all came from
China. I turned off my plug strip feeding this mess and the noise went
down but was still there. I turned off everything in the house and still
had noise.

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.

Now the hard part.
Can I put a noise blanker between the preamp and the radio?

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

I know that Hammarlund had a noise blanked circuit but these are very
hard to find and usually a lot of $$$$.

These actually go into the IF strip, and to be honest they aren't really
very effective against the kind of noise problem you are encountering.
They are great for the occasional impulse from things like arcing power
lines or ignition noise, but the whines and drones from switching supplies
aren't so easy to deal with.

I am/was an electronics engineer for over 30 years but I find myself at
a loss as to what I should build or buy.
A noise blanker on the antenna input to the preamp seems like a good
place to put a blanker but the signal off the 25 foot 'long' wire might
not be enough to work with. My thinking is that it would be good to
intercept the noise before it even gets into thee tuned circuits and
causes ringing or some other side effect.

What's happening is that those side effects are happening inside your
preamp. Why do you need the preamp at all? The receiver sensitivity
should be okay by itself, and the dynamic range of the receiver front end
is better than any solid state preamp.

The preamp really is only useful above 20 MHz, so it isn't even used
most of the time. The Hammarlund has great sensitivity below 20 MHz but
can use the boost above that. The only thing I have been able to receive
up there is CB chatter, so that is probably not worth it.
With my scope synced to 60 Hz I can see two big spikes and lots of
little ones that take a while to settle down.
Thanks for the reply,
Bill Baka
--scott