On 31/08/2010, at 5:35 AM, Rob Danielson wrote:
> If this exercise is futile, free feel to say so.
Hi Rob,
It isn't futile.
What the input clipping tells you is the hottest signal the preamp will ac=
cept before clipping, and this occurs when the gain is turn to it's lowest =
point. So this isn't much use for our purposes.
The mixers are fairly straight forward - gain is measured as the difference=
between input signal and output signal so it is possible to give an absolu=
te figure specified at max gain for instance. The MicPre specs show a maxi=
mum gain of 66dB throw in an additional complication in that it shows that =
gain drops by 10dB when phantom power is engaged. A quick check of archive=
.org indicates that the 10dB drop with phantom engaged only applies to MixP=
re's with a s/n 501x or 502x. The MixPre design was changed with s/n 503x a=
nd the 10dB drop does not apply to these models.
- Quadmic manual claims "Gain +10 dB up to +60 dB adjustable per channel" u=
nder the brief decription heading.
- Symterix SX202 specs claim a gain range of +20 to +60dB. (BTW a different=
SX202 spec sheet shows +26dB to +66dB and has the note "specifications may=
change without notice")
- Lunatec V2 manual states +10dB to +60dB in 5dB steps.
So based on this information you'd expect both A (QuadMic) and B (SX202) to=
produce very close output levels. If you hooked up a pre s/n 503_ MixPre v=
ia balanced outputs at the same input levels you'd expect the saturation t=
o be lower that A or B (+60db vs +56dB). However if the MIxPre was s/n 0503=
_ or greater you expect the opposite outcome due to the MixPre's higher out=
put with phantom engaged (+66dB).
(2) is dependant on the line-level settings of the recorder. In the case of=
the SD 7-series recorders when you have the line level set at 0dB on the d=
isplay you are attenuating the Line input.
The confusion arises because the SD recorders display gain referenced to -2=
Message: 0dBFS.
Subject: Sound Devices measures all it's recorder specs to -20dBFS so to be =
able to compare with the gain specs given for recorders you need to discoun=
t the SD gain figures by roughly 20dB. SD include a spec that confirms th=
is is the case. If you look at the Input Clipping Level for the line input=
you'll see that this is given as +26dBu with gain fully down. As the cli=
pping point must be 0dBFS this indicates that the line input is attenuated =
by 26dB with gain fully down. The gain range for the line input is given a=
s -6 to 18dB input to -20dBFS. Subtracting 20dB to estimate the gain range=
referenced to 0dBFS we get -26dB to -2dB input to 0dBFS. So a +26dBu inpu=
t attenuated by 26dB will give us 0dBFS.
So if you have the 744 line in set to 0.0/"Unity" gain on the display you a=
ctually have +20dBu input to 0dBFS, or 20dB attenuation.
If this is the case your total gain is:
A & B: 60dB + -20dB =3D 40dB
D: 56dB + -20dB =3D 36dB (pre s/n 503_) OR 66dB + -20dB =3D 46dB (s/n 50=
3_ or later)
SD 744 mic preamp gain: 47.4dB (spec gain figure)
If you had the Line-In set to maximum gain ( or in reality -2dB attenuation=
):
A & B: 60dB + -2dB =3D 58dB
D: 56dB + -2dB =3D 54dB
Your relative rankings would fit the specs if you had the line-in at 0dB (-=
20dB actual) setting.
cheers
Paul
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