DC on mic-input transformer primaries

Hello people.

Some microphone input transformers cannot handle any DC on their primaries. But, assuming the resistors providing phantom power are perfectly matched, there should in theory be no difference between the DC on one side of the winding relative to the other, hence there is no DC on the primaries.

So, if I make a bridge circuit to match resistors perfectly for this purpose, can I forget about this concern altogether and safely apply phantom power to transformers which normally can not handle DC?

Thanks in advance!

-E.

[quote author="electronaut"]So, if I make a bridge circuit to match resistors perfectly for this purpose, can I forget about this concern altogether and safely apply phantom power to transformers which normally can not handle DC?[/quote]

Both resistors should be tied to the same potential (48V). The reason people match phantom resistors is for symmetry/CMR. And even then, just measuring 1% resistors with your VOM and matching them within reason is good enough. Or buy .1% parts and be done with it :grin:.

Peace,
Al.

So, if I make a bridge circuit to match resistors perfectly for this purpose, can I forget about this concern altogether and safely apply phantom power to transformers which normally can not handle DC?

Click to expand...

I'd like to know that as well! Is there a definitive need for caps or is it ok if resistors are matched closely enough? I am about to rack a pair of V72 and a V76...

FWIW#1, the thought occured that when those delicate TXs have been subject to too much DC-current, degaussing could come to the rescue - just like in tape-heads & CRTs (someone remembers any of these ? :wink: ).

Anyone ever tried something like this ? Beyond 'it sounds good again', it's of course a bit more elaborate to verify if it's been done enough, but OK.


FWIW#2: recently needed to check if some old TXs didn't have open windings. The testcurrent of a multimeter is of course very small, especially on the higher ranges (measuring the 'Ohms-testcurrent' with another meter).

But some recent info here (I thought from Jakob) made me wondering about what would be worse - if it matters at all at these tiny current-values:
the DC-current of an analog meter or the pulses like DMM's often use ?

The former is DC, the latter AC...


Regards,

Peter

Thanks, cool info. Just the kind of confirmation I was looking for. :thumb:

Bye,

Peter

I asked this very question in an earlier thread. Bcarso told me that it shouldn't harm the transformer because there is no potential--the amount of voltage on the transformer is equal on each side, so no current will flow.

I spoke to someone at Edcor about this (I know, they're unshielded so they're not supposed to be used as mic input trafos, but I've been doing it anyway with no RF problems--I use a fully enclosed ps), and their tech support, albeit somewhat rude, told me pretty much the same thing.

Sowter, OTOH, told me that under no circumstances should I allow DC voltage to come in contact with the transformer.

Maybe it would self-degauss better if you short the secondary while applying signal??

"Sowter, OTOH, told me that under no circumstances should I allow DC voltage to come in contact with the transformer."

They are just being cautious. It's current that counts, and the only way it flows is via induction or conduction, and the only way there is conduction is when there is a potential difference, i.e., a differential voltage.

[quote author="bcarso"]"Sowter, OTOH, told me that under no circumstances should I allow DC voltage to come in contact with the transformer."

They are just being cautious. It's current that counts, and the only way it flows is via induction or conduction, and the only way there is conduction is when there is a potential difference, i.e., a differential voltage.[/quote]
With this in mind, it should be OK to feed Phantom to a primary center tap of the transformer, current should flow equal in both sides of the winding creating a zero net field on the core. I guess the only danger in this is if someone plugs an unbalanced source that has pin 2 or 3 shorted to pin 1.

I was told to feed the phantom through the 6.8k's to the +/- terminals of the transfomer, but not to the center tap. I was also told not to ground the center tap to the same place that the phantom power grounds to.

Thanks for the info about Sowter, bcarso. I thought it might be an issue with core material or something...I recall very little of the section on DC and inductors in E101, and now I'm paying for it...








*edited*

[quote author="featherpillow"]not to the primary.[/quote]

On a mic input transformer you have to feed phantom at the primary. Phantom power is a DC voltage - feed it at the secondary and it won't get to the mic.

[quote author="featherpillow"]I was also told not to ground the center tap to the same place that the phantom power grounds to.[/quote]

If you use PP, the center tap sholdn't be grounded at all! Remember all grounds eventually (should) come together.

Peace,
Al.

On a mic input transformer you have to feed phantom at the primary. Phantom power is a DC voltage - feed it at the secondary and it won't get to the mic.

Click to expand...

I meant center tap, not primary.

This topic has been worked to death a few times already. You just need to dredge it up.

I asked this very question in an earlier thread. Bcarso told me that it shouldn't harm the transformer because there is no potential--the amount of voltage on the transformer is equal on each side, so no current will flow.

Click to expand...

There are actually to issues going on, and I'm not unguilty of inserting the second one and making things confusing.

#1 the whole primary floating above ground is fine, as long as it's at the same DC-voltage (so no direct current flowing through the winding).

#2 this one was about TX-testing with say a multimeter. Checking for resistance is usually done by a current that flows through the object under test. There'll be a difference between analog multimeters & the digital ones (pulses - but dunno if all do it like that).

All clear, and indeed, as CJ said there's lots of 'previous info' to be found.

[quote author="BYacey"][quote author="bcarso"]"Sowter, OTOH, told me that under no circumstances should I allow DC voltage to come in contact with the transformer."

They are just being cautious. It's current that counts, and the only way it flows is via induction or conduction, and the only way there is conduction is when there is a potential difference, i.e., a differential voltage.[/quote]
With this in mind, it should be OK to feed Phantom to a primary center tap of the transformer, current should flow equal in both sides of the winding creating a zero net field on the core. I guess the only danger in this is if someone plugs an unbalanced source that has pin 2 or 3 shorted to pin 1.[/quote]
FWIW, JLM audio has phantom fed through the trafo centertap via a 3k3 resistor on most (all?) of the digrams on their site. As a matter of fact, I'd guess that they put a center tap on their custom mic input trafos for this reason. I've never tried it method myself because I like to keep the center taps available for Z switching. Carefully matched resistors are worth the peace of mind, IMHO.

Agree---I wouldn't want to count on the coupling coefficient between the winding halves necessarily, although if they are bifilar it is probably pretty good. A couple of closely matched R's can't be that painful.

Input dcr is usually pretty low for a 600 coil.