> So it would appear that at max gain the USBPre has a far lower
> dynamic range than its cheapo comparison box.
> Does anyone know why this would be the case?
Ben,
For a start, the noise figures of both are well below acoustic noise
levels or even the dynamic range of the listener's ears. The cost of
the equipment is irrelevant. What is more imporatant is its
reliability in adverse conditions and ease of use, battery life, etc.
An input clipping level of -7.5dB may mean an input attenuator may be
needed if using high gain mics, especially if you are recording
thunder. BTW an impressive sound is a tropical storm approching
through canopy forest even without thunder. Also get near water to
make rain sound wet. Experiment with letting the peak thunderclaps
distort and here is a never-quoted parameter which is the recovery
time after an overload. A limiter "dropout" after a peak is very
uncomfortable. It may be better to record at low level and
clip the waveform later. If you hnow what your mic and mixer noise
sounds like, you can set the record level so the background noise just
swamps the system hoise.
The problem with relying on RMS noise figures is that these may not be
a good guide to noise as heard on a recording. LF noise, for instance,
is usually swamped by acoustic LF background but hiss is much more
prominent. The preferred weighting for noise is the "468" curve which
peaks at about 11KHz by around +12dBs. Few manufactureres quote this
as it gives noise figures around 11dB higher than the meaningless A
weighting.
Dummy loads can make a big difference to measured figures but the
actual output impedance of mics varies from 50 Ohm to 2KOhm, so a
dummy load of 150 to 200 Ohms is only a starting point. Alternatively,
you can do noise measurrements with a zero load and open circuit which
will also give the "equivalent noise impedance" of the input stage.
In my book the real noise test is with a particular mic in a quiet
place and by listening. You may be able to distinguish the mic hiss
from the input hiss in which case the input stage is noisy. With my
MKH's a power spectrum shows an upturn in the noise from around 8KHz
upwards which is thermal noise from the mic and possibly the air
itself. You can't get quieter than that.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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