Adam, you wrote,
>There has been a lot of discussion but as far as I know no hard
>comparitive data in this area. This would answer the questions of 1)
>What is the measurable advantage to using an external mic preamp (if
>any), 2) Does a particular recorder benefit from an external mic
>pre, and 3) What (if any) is the noise penalty using a consumer
>(Sony, Sharp) MiniDisc recorder. To be completely valid, these
>measurements will need be done on separate components and also
>including the entire signal chain -- external mic pre connected to
>the recorder in question.
Great. Do you have the NSS Newsletter report of my mike preamp tests
of several years ago? It's time for an update.
>...Mic preamp noise is measured in EIN (equivalent input noise), which
>is the increase in noise contributed by the mic preamp over the
>theoretical noise level of the source impedance. Most professional
>quality mic preamps claim an EIN of 1.0dB or less, which means they
>shouldn't contribute any significant noise to the signal, and the
>noise heard would come almost entirely from the microphone circuitry
>or capsule itself.
The EIN is a signal level usually expressed in dB, like "-129 dBm."
The difference you describe is called something like "noise figure,"
I think.
>This brings up another complication of getting hard data that
>translates to real field usage -- the actual source impedance of the
>microphone may vary with frequency from the nominal 150 ohm spec, so
>measurement of the source impedance of some popular microphones and
>the EIN of mic preamps over a range of impedances and frequency
>would be in order. Anyone have some mic test heads for various
>models (I have one for the Schoepps)?
I used an oscillator and a pad to set a standard gain, and a 150-ohm
dummy input load to measure the no-mike noise (same as manufacturers
use for 200-ohm mikes). A low-output dynamic mike at the end of a
100-ft cable out on the hillside provided a real-world noise
comparison.
-Dan Dugan
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