"With the "mushy" preamps, were you using any kind of bandlimiting
filters ahead of them. I am wondering if you have any local BC that
is loading up the preamps."
"Your rigs would high pass filter it out, but a cheap preamp with a
wide open front-end could start to
overload on strong BC and you might not realize it. The good preamps
with very high IP3s would be transparent even in the presence of
strong BC. I have this kind of problem when I use my JPS ANC-4 noise
cancellor."
"If there is any strong local BC present, the ANC-4's internal preamp
starts to overload
as it has no bandlimiting filters ahead of it. If I put a high-pass
(BC reject filter) in front of it, it cleans right up."
Mike,
The nearest AM BC stations are in Austin, about 40 miles south of us.
I don't think they were overloading the preamps I tested. It's also
interesting that some, although not all, of the poorest performers had
bandwidth-limiting circuits ahead of the active components. None of
the broadband preamps with good IP3s sounded bad or stretched pulses
excessively.
The Palomar preamps that allowed the user to tune to a specific
frequency did poorly. The Palomar PA-360, a broadband device, did
much better. The AR P1-30 and Aiken AMP 1200-9 preamps that did very
well were broadband units with no BC-reject or bandwidth limiting of
any kind. The KD9SV and K9AY preamps displayed some pulse stretching
(they had tuned/filtered input circuits), but they sounded much better
than the Palomars did.
As Tom has suggested, the pulse-stretching I observed is due to the
group delay characteristics of the filter networks. It looks as if
you've either got to design a preamp with a good IP3 number and leave
it wide open, or you've got to design the front-end filter(s) as best
you can and accept compromise elsewhere.
Brad, KV5V
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