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



Mike Rivers wrote:

Paul Stamler wrote:
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.

But if you want to measure the PREAMP - and that's what I thought we
were talking about, you don't want a measurement that varies with the
mic that you use with it.

But that *is* a measure of how good the preamp is. e.g a fet input pre-amp will
both measure quieter and be quieter in real use when measured using the resistor
method than a bipolar one measured into a short circuit which may appear on the
basis of shorted input noise to be better.

And if you measure with a resistor, then you
need to specify the test setup in order to make the results meaningful.

One does. Typically 150R or 200R is stated together with the measuring bandwidth.

And we all know how many "specs" are published without knowing how the
data was taken.

Not me.

It would obviously be an advantage to a manufacturer to make his
measurements with a shorted input.

I see Mackie's doing it now ( for one ) IIRC but says so.

If it's common knowledge that a real
mic will raise the noise level by 6 to 12 dB, then those with the
knowledge can expect more noise than the published value.

Those with knowledge ! Ahem !!!

If the
increase in noise with a current path at the input is fairly consistent
(as it seems to be in my experiments and as stated by others here),
knowing the short circuit noise will allow making adjustments for real
world sources.

It's been suggested that certain "poor" preamps are not so predictable.
If you find yourself with one of those, it can probably just be put in
the "crummy preamp" pile and there's no real need to compare it to
others in that pile.

In truth it's just as easy to state both figures. But then again for practical
purposes it's the one with a resistor that makes sense. The other figure is simply
specmanship for its own sake - you can't use it that way.

In any case it's the resistor figure that's been the norm for decades. Why worry
about it now ?

Graham


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