Any Electronics Gurus Here? PAIA Tube Mic Preamp Trouble

soundchaser59

Reluctant Commander
Anybody here who might know what to suggest?

I think I accidentally applied phantom power to the XLR output of my PAIA tube mic preamp. Does anybody know what parts are most likely to have been damaged by this??

The output has a few resistors and a couple of 33uf / 16v caps, but also an IC chip. I assume the caps and the IC are the most vulnerable to damage of this nature??

Otherwise the input and the lights and tube drive and everything appear to be working fine, I just get a signal out that is next to nothing. I've already eliminated the tube and external factors. Please keep in mind that I do not have an o'scope, I only have a VOM. I do have an identical TMP that is working correctly to compare it to, if someone can tell me what I might check and compare.

Any suggestions?

PS: Attached a schematic. The output XLR jack "Bal Out" is on the right. I assume that caps C21 and C22 rated at 16v each would have been or could have been fried by the 48v phantom power. I also assume that the caps would not allow the dc power to pass thru to the IC3 chip, so I'm hoping the IC is ok and it is only the caps that got damaged.
 

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The caps would block DC until they fail due to rating (and note they got the phantom from the drain, where the voltage rating is something like edit: 15% the forward rating). However, they will pass a DC pulse as a low-frequency transient, and they could fail open. Thus, the IC is suspect.

Anyway, change the caps first, they are cheap. Then if it doesn't work, change the IC.

You could add a pair of zener diodes on the outputs in a limiter configuration, say 9V or so, to prevent future mishaps.

Edit: Ah, checking up on the advice below I see I was off, the standard for electros only provides for 15% reverse voltage, and then it is still not advised to be constant. Thus you should swap for bipolar caps if needed.




Original not too good advice: You could also change the caps to 63V rating and reverse them, that way they should be able to take 16V from the IC, and phantom from the output.
 
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Thanks! I was afraid of that.

I may actually have a couple of caps for that in my box, I'm getting the flashlight out now......
 
soundchaser59 said:
Thanks! I was afraid of that.

I may actually have a couple of caps for that in my box, I'm getting the flashlight out now......

Man.... Hosing a preamp and losing power all in the same day? :D :D :D

Sorry. Couldn't resist. :D
 
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