On 20/08/2010, at 5:29 PM, Rob Danielson wrote:
> Hi Paul--
>
> I do understand that gain measurement isn't as easy as one might
> suspect and that the manuf. supplied specs are often lacking
> sufficient criteria. 50 dB max gain for the 7XX's sounds very low
> based on comparisons with SD pres rated at 66dB and other pres I use
> regularly rated at 60dB of gain. The SD7XX is hotter than both. It
> would be good to get some reliable numbers, but it doesn't impact my
> test outcomes.
Hi Rob,
Gain measurements just require the right test gear. Connect a signal source=
to the mic input, increase the level of the test signal until you just hit=
0dBFS or clipping, then measure the level of the test signal with a true =
rms multimeter or an oscilloscope. The measured voltage of the test signal=
is converted to dBU, and this gives you the gain measurement.
Both Raimund's measurements and the Sound Devices specs point to a similar=
maximum gain for the SD 7-Series, so it would seem that the discrepancy ha=
s it's roots in the comparative assessment, and the logical thing to do wo=
uld be look for the source of this discrepancy.
The MixPre has a "headline" maximum gain of 66dB but this only applies to t=
he balanced line output connected to a balanced input. If connected line-o=
ut -> unbalanced this drops to 60dB gain, and connected via tape out maxim=
um gain is 56dB. The SD 7-series has a line input which has a maximum gain=
of -6 to 18dB to -20dBFS, which means when referenced to 0dBFS the gain is=
-26dB to -2dB - so effectively provides attenuation only. If you have the=
MP-2 hooked up to a SD 7-series recorder at maximum gain on both devices y=
ou'd see a total of 64dB gain. If you set the gain on the SD recorder line =
input to 0dB you'll be attenuating the MixPre by 20dB, resulting in an over=
all gain of 44dB. At this setting the MixPre would appear to have about 4dB=
less gain than the SD744. Same principle applies to any preamp/mixer run =
via a line-in.
cheers
Paul
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