> Not quite up to anechoic chamber standards, but under several duvets
> > here are the spectrum plots:
>
> http://www.donaldingram.com/Audio/8070vs40spectrum.jpg
Donald,
I got the same curves but the difference is levels between the mics
should have been 13 dB as the spec suggests. That should stomp across
any input channel noise. (I'm assuming no attenuators are switched
in.) Both curves show a rising "thermal noise" profile at the upper
end but I don't know how close this is to the free air thermal noise
without a reference.
The relative noise specs are within a dB between the two mics so the
relative levels are important. The CCIR468 weighted spec is the
important one for hiss. See
http://en.wikipedia.org/wiki/ITU-R_468_noise_weighting
I use random noise peaked at 1KHz for level testing and frequency
response curves conventionally use warble tones. Both iron our local
peaks and troughs.
I would pursue the level discrepancy as this should give you a benefit
of 13 dBs unless it it a mic problem. Also check out your input noise
with an open circuit and a dummy load of 150/180 ohms or if you don't
have one, 0 ohms or a paper clip. The 2000 ohm minimum impedance is
normally for the maximum output without distortion and shouldn't
affect low level signals but there would be a fault condition if the
mic is sensitive to loading. Try a 1000 ohm resistor across the mic
input if you want to check this.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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