Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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N Cook
 
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Default Omni i/o 2 channel preamp to pc a/d

No abuse apparently, owner plugged in a new microphone and blew one channel.
Seems to have knocked out expensive instumention op-amp ( comparing the 2 ).
I don't like the look of the electrolytic at the input, no input dropper
resitor and a 48V
phantom supply involved. Anyone any ideas ? don't fancy spending out on
an expensive Burr Brown IC if it is only going to happen again.
I've not often dealt with kit with phantom supplies - can any one
direct to a resource of generic probs associated with 48V on signal inputs?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/




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N Cook
 
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"N Cook" wrote in message
...
No abuse apparently, owner plugged in a new microphone and blew one

channel.
Seems to have knocked out expensive instumention op-amp ( comparing the

2 ).
I don't like the look of the electrolytic at the input, no input dropper
resitor and a 48V
phantom supply involved. Anyone any ideas ? don't fancy spending out on
an expensive Burr Brown IC if it is only going to happen again.
I've not often dealt with kit with phantom supplies - can any one
direct to a resource of generic probs associated with 48V on signal

inputs?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





There is 2 6.8K resistors from the 48V line to each of the balanced mike
lines , so protecting the phantom supply.
Each of these mike lines is connected via a 10uF/100V electrolytic directly
to + and - inputs of the op-amp.
So if a short to ground at the mike then the +48V side of the electrolytic
goes to 0V and the other side must go to -48V
or at least try to , op-amp supplies +-15V.

Modification of clamping diodes at the + and - opamp inputs?


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