Here's another one fired off today:
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As posted to rec.audio.pro by David Satz
<<
Pawel, please do not try to use the Schoeps CMC 6-- amplifier (whether
it is the xt version or not) with a power supply that offers neither
standard 12 Volt phantom powering nor standard 48 Volt phantom
powering. What will very likely occur with a 30-Volt supply is that the
voltage received by the microphone's circuitry will be in the 20s or
lower (because of the 6.8 kOhm supply resistors), and the microphone
will switch over to its 12 Volt mode.
In that mode, however, the microphone draws about 10 mA and that will
very likely bring down the power supply regulator in the recording
device, quite possibly causing damage to the MicroTrack if its circuit
is not protected. I have personally seen that occur with another
lightweight, portable preamp/ADC in which the "phantom power" supply
was not up to standard.
M Audio should certainly know about this issue; they experienced it in
the design of the DMP3 preamp, since its predecessor the DMP2 could not
properly power a pair of modern condenser microphones--not even
Neumanns at 2 - 3 mA apiece, let alone a Schoeps (4 - 4.5 mA) or any of
the types that require more current, such as the Shure KSM series (5 -
6 mA), the original AKG C 451 (6 mA), the CAD Equiteks (8 mA) or
Earthworks (10 mA). I had an email exchange with the designers, and the
eventual model DMP3 was a definite improvement (also in regard to its
input overload margins).
>>
Also posted to sci.electronics
<<
Actually a repair problem for a low noise microphone pre-amp but seems to be
a general design flaw.
Pre-amp uses a Burr Brown INA103 very low noise instrumentation op-amp.
In this M-Audio Omni i/o preamp and an outline design application in the
Burr Brown book show much the same circuitry.
The 48Volt phantom supply to the mike is protected by 6.8K limiter
resistors.
But to block the 48V DC to the op-amp there is a 10uF/100V electrolytic in
each line directly to the inv & non-inv i/p of the op-amp .
If , as seems in this case, a balanced line microphone with a short to
ground is connected to such a system
then the +48V / 0V across the elecrolytic will instantneously go to 0V
/ -48V with -48V
directly connected to the op-amp i/p powered from +-15V rails and according
to the databook
can be taken to only +-12V.
Blown input to this op-amp due to just the owner connecting a microphone.
Anyone familiar with this, adding limiting diode pair at each input ?
I see no point in replacing this 15 GBP/ 25 USD IC until this design flaw
is attended to or it will happen again should a fault to ground develop in a
mike or lead while the 48V power is on.
>>
I hope this is helpful in troubleshooting this problem. Please let me know when/if you plan to address this issue in a later revision of the Microtrack. Thanks!
--
Jonny Durango
www.jdurango.com"If the key of C is the people's key, what is the key of the bourgeoisie?"