Phantom Power through my vintage xformer inputs

[northsiderap]

Hello all... My first post...

I have racked up 8 vintage discrete fairchild microphone preamps with transformer inputs...

I have already made a phantom power bus with 6.8k resistors, one on each leg of the input xformer and a polar cap to shunt the AC on the 48v bus to ground.

The Question ~~~

Is this the right thing to do? The mic input transformers are the tiny AMPEX or BEYER mic inputs, probably 200 R to 30k R or so without a center tap on the input.

I am thinking that since there is no potential from one leg of the xformer to the other (both are at 48v) there should be no problem. Or will there be?

Any suggestions would be helpful... I don't want to fry any of these vintage xformers, I can't afford to... Especially since I am trying to gear up for a session in a week or so...

Thanks for y'alls help!!!!

[Havoc]

If you value your transformers, put a cap in series.

When all is well, there is no danger. But it could be a problem when a single wire is faulty. Like a short to ground, or even an open. Then there is a possibility of current flowing through the transformer. It will be limited by the 6.8K, but if you are not sure if it can handle that I would not take the risk.

[loonatron]

The cap in series may form a HPF?

[moamps]

Originally posted by northsiderap
....
Is this the right thing to do? The mic input transformers are the tiny AMPEX or BEYER mic inputs, probably 200 R to 30k R or so without a center tap on the input....

Yes.
Here's an example from Jensen site:

http://www.jensen-transformers.com/as/as016.pdf
and
http://www.jensen-transformers.com/as/as017.pdf

Regards,
Milan

[northsiderap]

I don't know if anyone is still looking at this post, but....

I used the Jensen schematics and everything works great.

48v at 500ma to feed 8 channels seems to work. The PSU wasn't the best in the world, but by the time it gets past the filter caps and into the mic it apparently is plenty clean enough.

No 'pops' on start even...

Thanks everyone...

[BrianL]

At the bare minimum you should use 1% resistors
matched to 0.1% as spec'ed by Jensen. Personally,
I'd troll surplus places (Surplus Sales of Nebraska or others)
for 0.01% tolerance (or better) Vishay resistors.
One could also get them from Texas Components
(second source for Vishay) or from Caddock.

Even buying 0.1% resistors (Mouser is handy) and
matching to better than that, if you can, would be
highly recommended.

Not only does matching of these two affect CMRR, but
they also affect the amount of DC that might flow
where you don't want it.

[northsiderap]

I usually hand-match them with a multi-meter...

Group them up into ohm ranges and usually get well under 1%..