Hi Adam and thanks for the below comments!
I would say that amps in general perform their best noise values at
max gain, say 60 db.
For the naturesound recordist, I would say that the ideal preamp has
two gain settings, say 30 and 60 db, and a low value (600 Ohms??)
potentiometer at the output. Normally you could run such an amp at
max gain, having the 30 db gain for parabol recordings.
The PIPamp prototype, powered by a 9V battery, gives 2V RMS at the
output, which should be enough for every "normal" nature situation.
I happen to have a HQ volt / db calibrated meter with 0db at 0,775 V.
This means that I can measure dBu directly, but you are right - it is
very difficult to get rid of induced hum and noise from all cables
and lamps around. I only feel safe when I have had a close look at
the scope before reading the meter.
Measuring battery powered amps is not that difficult, but measuring
microphone self noise down to 12-13 dbA is almost impossible. With
the mains electricity switched off, working with a flash light, the
slightest wind outside the building (which you cannot hear) ruins the value=
.
Do you know of any simple trick to make a "silence chamber"??
Klas.
At 21:39 2007-01-11, you wrote:
>Hi,
>
>Make sure that the 150 ohm resistor is mounted inside a metal
>connector with the case on it -- if it is open, you will pick up hum
>and interference.
>
>---
> In Dan Dugan <> wrote:
>
> > Where there is no analog output, like an MD recorder, I dunno!
>
>You would inject a known tone level into the recorder mic input (say
>-40dBu), adjust the record level to just under zero on the meter (so
>it doesn't clip), and record it. Stop the recorder, substitute a 150
>ohm terminated dummy plug for the signal, and record some noise. Then,
>play back the tone, and adjust your playback system for a calibrated
>output level. Then when the tone stops, you'll hear the section with
>noise.
>
>To measure noise, you'd measure the level difference between the tone
>and the noise, and then subtract this from the input level (-40dBu in
>this case) to get the EIN. Without measurement instruments, you'd have
>to do the same test on different recorders, setting the tone to play
>back at the same volume and then compare the noise subjectively.
>
>This tests the total system, which is probably more relevant anyway
>for a recorder. Note that you'll get different results when you use
>different input signal levels -- obviously more noise with a lower
>signal level like -60dBu, but the EIN will change also and the
>relative ranking of the recorders could vary.
>
>What signal level is relevant to nature recording? That
>complicates things, as the record gain will depend on what you are
>recording, and the output level of your microphone. And then, another
>factor is to determine the noise level of the microphone, and at what
>point that swamps out the recorder's preamp noise.
>
>At some point, I plan to do measurements on a bunch of recorders using
>my Audio Precision and write this all up in a paper. I might see what
>publications would be interested. So far, there are lots of reviews of
>different recorders, but many don't have real hard measurements, or
>the measurement methods are different so they can't be compared.
>
>- Adam Liberman
>Liberman Sound
>
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
email:
website: www.telinga.com
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