> I have a long list of such "mysteries" that I would very much like
> to
> have explained, at least on a basic level.
Klas,
Here's one possibility. Many mics have low impedance outputs and are
quoted for running into a higher impedance than the rated 150 or
Message: 200ohms.
Subject: There is a mysterious parameter called "noise impedance"
which is often different from the mic output impedance or the
mixer/recorder input impedance. The noise impedance is the optimum
impedance for minimum noise.
Some mics quote a minimum input stage impedance of 2000 ohms or
higher. If a recorder input is lower, say 200ohms, it will match the
mic output impedance for a good frequency reponse but may not be
optimal for the noise impedance.
Another possibility is noise from a phantom powering circuit which
should balance out. If the mic or the input amp is not fully balanced,
this is a route for different noise levels from those quoted.
Transformer inputs balance the input and feed this into an unbalanced
front stage. Direct transistor balanced inputs use positive and
negative input stages and add these to balance the audio, but not
necessarily the noise.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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