Re: Is this a good way to test pre-amp hiss of a digital recorder?

The thing is, if you measure with a shorted input, you're measuring the
voltage-generated noise of the preamp. Period. With a preamp whose input
stage is made from tubes, or FETs, that's fine, cause that's all there is
(okay, not quite, but close enough for folk music).

But if you're measuring a preamp whose input stage is made from bipolar
transistors, all bets are off. That's because, in addition to voltage noise,
bipolars produce a significant amount of current noise. That's only
measurable with a >0 source impedance. It's entirely possible that if you
measured preamps A & B with a shorted input, and A was quieter than B, you
could measure them with a resistive input (say 150 ohms) and discover that A
was *noisier* than B, because while it had less voltage noise, it had more
current noise. That's why a shorted input isn't really a meaningful test for
bipolar-transistor preamps. A 150 ohm resistor isn't a perfect source, but
it's closer to reality than a short circuit if you want to predict how a
preamp will perform in use.

Peace,
Paul


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