At 6:41 AM +1000 9/1/10, Paul Jacobson wrote:
>
>
>
>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 accept 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 absolute figure specified at max gain for instance. The
>MicPre specs show a maximum 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 MixPre's with a s/n
>501x or 502x. The MixPre design was changed with s/n 503x and the
>10dB drop does not apply to these models.
Hi Paul--
Thanks for taking the time for this. I'll check to see my ser #. Its
in the barn.
>
>- Quadmic manual claims "Gain +10 dB up to +60 dB adjustable per
>channel" under 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")
Thanks for all the extra research! I own four of these and haven't
noticed a 6 dB difference between any units so far.
>- 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 via balanced outputs at the same input levels you'd
>expect the saturation to 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 output 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 display you are attenuating the Line input.
>
>The confusion arises because the SD recorders display gain
>referenced to -20dBFS. 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 discount the SD gain figures by roughly
>20dB. SD include a spec that confirms this 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 clipping 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 as
>-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 input 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 actually have +20dBu input to 0dBFS, or 20dB attenuation.
Luckliy, I don't actually use the numbers displayed on the 744T for
anything serious like knowing where to set "unity!" :-(
>
>
>If this is the case your total gain is:
>
>A & B: 60dB + -20dB =3D 40dB
Me confused again. Are these 40dB values the absolute gain as would
be provided by the pres at max setting for a standard 600 ohm
balanced line input? I'm curious about the universal gain rating
itself, not adjusted for the 744T's meter display peculiarities.
>[C]: 56dB + -20dB =3D 36dB (pre s/n 503_) OR 66dB + -20dB =3D 46dB (s/n
>503_ 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 attenuatio=
n):
If I set the 744T's line input to the max setting,.. I think it will
over over modulate when I set the gain the Mix Pre to max.
>
>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.
I'm still baffled, for example, why a manufacturer prints "60dB" and
it turns out to "40dB." I've used audio interfaces with pres that
we're spec'd at 40 dB and I know they had much less gain than my
Symetrix. My previous understanding must have been omitting a very
basic factor,..
So, you are computing, apples to apples, actual dB gain ratings of:
A and B: 40 dB gain
C 46dB
D 54 dB
They do make sense relatively.
Rob D.
>cheers
>Paul
>
>__
--
|