Bizarre Phantom Power Problem

[jrmintz]

Hi all,

This is what's making me crazy today: I built a 2-channel N*ve 1272-type preamp with Carnhill transformers and Joe Malone's 1272-to-1073 boards. I have switched phantom power hooked up to both inputs, one channel is fine, the other one gives me 2.5V on pin 2 and 0V on pin 3 of the xlr. There is 48 volts before the 6.81k resistors on both channels and both channels are wired exactly the same way. Yet one channel passes the phantom to the input connector and one doesn't. Both channels of the preamp seem to work fine with a dynamic mic, sound good and sound the same, and measure the same distortion and noise. Could this have something to do with the primary of the input transformer? I'm baffled. Any ideas?

Thanks.

:sad:

 

[NewYorkDave]

You didn't ground the centertap of the input transformer primary, did ya?

 

[Svart]

are those numbers under load or just floating? phantom is pretty low current and will be pulled down pretty easy. check for solder shorts?

:guinness:

 

[jrmintz]

Dave,

I don't think so, but I'll have to check. I did check to see that both input transformer primaries are wired the same way.

Svart,

Under load and not. I checked the wiring several times and didn't see any shorts.Doesn't mean I didn't miss one, though.

 

[tmbg]

hey svart, I hate to beat a dead horse, but something that's "low current and will be pulled down pretty easy" has a high output impedance!

 

[jrmintz]

Dave,

If I had grounded the center tap by accident (I don't think I did, BTW) wouldn't that change the ratio of the transformer and cause the two channels to have different gain?

:?

 

[NewYorkDave]

No, but it would definitely kill your phantom power on that channel.

 

[jrmintz]

[quote author="NewYorkDave"]No, but it would definitely kill your phantom power on that channel.[/quote]

Back down to the cellar!

Thanks guys.

 

[Svart]

hey TMBG, i see that as low output current sourcing! I guess it's the same animal, just different sides. the current supplied isn't enough to support the voltage with such a high resistance at the load side.

also I am used to seeing that DC is characterized by resistance, AC by impedence. so this would be resistance in this instance.


:green: :guinness:

 

[jrmintz]

Well, Dave was right - the center tap was shorted to ground. The interesting thing is that it was not external - it's happening inside the transformer somewhere. If I put a little pressure on the center tap it shorts to the shell. I might have deformed it a little when I originally screwed it in or something like that, so I enlarged the screw holes and used nylon washers to screw it in. Or it might have been borderline defective. So far so good, anyway. I have phantom power as long as I don't stress the thing.

:thumb: :thumb:

 

[NewYorkDave]

It might be possible to slip a piece of heat shrink over the centertap lead and slide it up partway into the transformer case.

 

[Svart]

i's still change it or it'll fail when you need it most!

Murphy's law!

glad you found it! :guinness:

 

[rlaury]

I was going to suggest, just use the channel thats working on
phantom and the other for dynamic mics.

Just kidding guy's ...It's Friday! and my first weekend
off in the last 2 month!

RonL

http://www.nashaudio.com

 

[jrmintz]

[quote author="NewYorkDave"]It might be possible to slip a piece of heat shrink over the centertap lead and slide it up partway into the transformer case.[/quote]

They're not leads, they're pins that stick out through what looks like masonite. The transformer is potted in silicone. Hopefully I've relieved the stress on it.

 

[tmbg]

[quote author="Svart"]hey TMBG, i see that as low output current sourcing! I guess it's the same animal, just different sides. the current supplied isn't enough to support the voltage with such a high resistance at the load side.

also I am used to seeing that DC is characterized by resistance, AC by impedence. so this would be resistance in this instance.


:green: :guinness:[/quote]

yeah it's all the same stuff it is only able to source so much current because it has a current limiting resistor in series -- the output impedance!

resistance IS impedance! Impedance is characterized as a complex number made up of resistance on the real axis and reactance on the imaginary axis... a pure resistance has no reactance, but it's still an impedance with 0 reactance

 

[Svart]

oh a funny man eh? :green:

:guinness: