Carnhil VTB9045 (10468) Microphone Input transformer. What ballpark resistance should I be seeing across XLR pins 2 and 3?

[777funk]

I'm getting around 30 ohms and it's dropping my phantom power from 52VDC to 40VDC (via a pair of 6.8k resistors feeding XLR pin 2 and 3). Is 30 ohms a normal input resistance on this transformer?

 

[emrr]

yes. I assume your mic load is what's actually dropping your voltage, also normal.

 

[777funk]

yes. I assume your mic load is what's actually dropping your voltage, also normal.

The 40VDC is without a mic connected to the preamp (just the transformer loading).

 

[emrr]

The transformer doesn't load if connected correctly. You're sending identical 52V to both sides of a floating winding, there's no load. Impossible to know from the evidence.

 

[777funk]

The transformer doesn't load if connected correctly. You're sending identical 52V to both sides of a floating winding, there's no load. Impossible to know from the evidence.

Yes you're right. I hadn't looked at a mic input transformer schematic in a while. Not sure why I have a voltage drop without a mic connected. Will need to investigate a bit.