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Tube Mic Fried DIGI 001 Preamp?
Old 2nd February 2009
  #1
Here for the gear
 
🎧 10 years
Tube Mic Fried DIGI 001 Preamp?

Recently, I plugged a tube mic into one of the DIGI 001 mic pres. The mic/power supply cable that came with the mic had been wired improperly (perhaps applying 250V where it should not be). Ever since doing this, the following problem has appeared:

- The gain of the DIGI 001 mic preamp has been reduced 50%
- Turning the phantom power on or off produces a terrible crackling/hiss noise that dies down over a minute or two.

Initial troubleshooting notes:
- The phantom power crackle is happening before the gain pot
- The DIGI 001 uses combo jacks--balanced & 1/4"--and I believe it has separate gain stages for each. The 1/4" high-impedance preamp has no loss of gain
- The other mic pre (Input 2) is fine. It shares phantom power with Input 1, and does not have the crackle/hiss noise.


Without a schematic, my intuition says that I should start replacing components--all electrolytics and ICs in that first gain stage. My intuition gets me in trouble sometimes, and I'll be needing a magnifying lens to work on that PCB, so I thought I'd run it by some other minds first. Any "input" is appreciated.
Old 5th February 2009
  #2
Gear Addict
 
Minion's Avatar
 
🎧 15 years
The input of your preamp has Capacitors that should block DC from entering the curcuit , they are used to block the Phantom power , if these Caps get too much Voltage like the high plate voltage from a Tube mic then these Caps would most definately Blow which would definately affect the sound as the audio goes through these Caps and would explain why there is more noise when the Phantom is engaged.......

So I would look at the input DC blocking Caps , they should be the first caps after the Input XLR , they are probably between 22uF and 100uF and at least 50v , they could be visibly blown there for easy to find....

Cheers
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