Phantom Power-Vintage Pres

[craigmorris74]

I've read an article by Oliver at TAB where he discusses how phantom power will ruin the input transformer of Telefunken tube pres.  My questions is what type of input section or input transformer is succeptable to being harmed by phantom power? 

The reason i'm asking is that I have a new UA 610 pre, and a UTC O-1 trasformer.  I was thinking of trying the O-1 in the 610 to try to better replicate the original 610 sound, but was concerned about ruining it.

Thanks,

Craig

 

[emrr]

I have never seen an american input transformer ruined by phantom power.  Many are designed with the old carbon mic standard of feeding battery power through a center tap, which is also the earliest recommended practice for implementing phantom power:  single resistor feeding DC through transformer center tap.  I can't address the problems with certain euro transformers.

 

[jensenmann]

The problem Oliver mentioned has to do with the high ratio of these transformers. They have 1:20 and since it´s impossible to wind them perfectly there will be a little CMRR mismatch. If you turn on your (unramped) phantom power it is actually a stepfunction. Though the CMRR mismatch is very low the steep rising transient of the stepfunction will induce a very high secondary voltage which is dangerous for the isolation of the secondary winding. In the 50s they used some sort of laquer for isolation which degenerated over time and can´t stand the above mentioned high voltage.

If you have modern trannies you don´t have to expect any trouble at all. If you use ramped phantom power you won´t even have any trouble with these old transformers.

 

[craigmorris74]

Thanks for your replies.  I'll measure the phantom supply on this pre and check if it's ramped or not.

 

[Michael Tibes]

I remember this thread of Olli, but I understood it a little different. What I concluded was, that the phantom power might create little arcs, 'jumping' over the anyway very thin isolation (the laquer you mention which might have gotten even thinner over the decades) and causing shortcuts between windings. I assumed that this was a problem due to the very high ratio, for which they used  very thin wire with thin insolation. I decided not to use phantom without coupling capacitors any more on these units.

But maybe I just got it wrong or drew false conclusions.

Thinking about it again, if I remember right both the V76 and the V72 normally have a cap in the primary center tap (unless it is especially disabled). So there should be no dc be possible through the primary windings anyway? I'd have to recheck the schemo to verify that...

Michael