>From Tom:
"I don't think this is a valid worry.
"The duration of a lightning discharge is relatively long, in the
order of
hundreds of milliseconds to a few seconds.
"Even if the pulse were very short and abrupt, the receiver's
selective
circuits and AGC system would absolutely overshadow any problem in a
preamplifier making it totally unnoticeable. When we have a receiver
with
.25 to 3kHz selectivity stages and multiple stages, it dominates both
ringing and distortion in the system."
>From Brad:
My own listening tests have been very subjective. I think we'd need
some "blind" tests to validate the differences I think I hear. The
poorest of the preamps I tested make the receivers (a Mk V and an
'817) sound more "mushy" under noisy conditions. (Preamp gain and
receiver attenuation were adjusted to prevent overloading.) But the
effect is certainly down at the "Maybe it's there, maybe it's not"
level. The best of my preamps, an Aiken Industries AMP-1200-9 ($1725
current, single-unit price), is completely transparent to the ear.
In doing the tests, I was asking whether there might be ways of
defining preamp fidelity other than the usual IMD, IMDDR, IP2, IP3,
etc. The methods we usually apply define amplifier behavior in the
presence of transmitted signals. But we listen to the signals in the
presence of non-coherent, random noise. Amplitudes vary widely, and
rise and duration times can be very short. Are there any tests we
might apply to define how a preamp handles this noise?
You're right in pointing out that the behavior of the selective stages
and AGC in a receiver can have a much greater effect on what we hear
than the preamp does. On the other hand, we can change most of these
behaviors by changing bandwidths and adjusting AGC TCs, but when it
comes to the preamp, we've only got two options--turn it on or turn it
off. If you need it to pull a signal out of the receiver noise floor,
then you're going to want a preamp that generates as few artifacts of
its own as possible.
I'm aware that most lightning events are considerably longer than 20
nS, but they're usually made up of strings of shorter events. This is
the reason that when we do EN 61000-4-4 powerline transient immunity
testing in our EMC lab, we apply bursts of pulses that are only 50 nS
+/-30% wide. Rise times are on the order of 5 nS, and the bursts last
15 mS. Of course, this noise is presumed to have been generated by
interruption of inductive loads, but these noises are in our receiving
environment. Moreover, I understand that the University of Florida
guys have recorded lightning discharges with similar rise and duration
times.
Let's keep the question open a little longer. Do the coherent signal
tests fully-define preamp behavior. Could there be some value in
tests that evaluate how preamps handle random, broadband signals? The
impulse (or transient?) response test is my first stab at an answer.
Suggestions anyone?
CUALL at the Topband Dinner,
Brad, KV5V
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