Phantom power through a Y-cable

A few years ago, I was hired on to a low budget film, using the
production company's gear. The outside stuff called for an MKH70 on a
pole, into a Tascam DA-P1. They gave me a Y-cable and told me to just
split the mic cable into the 2 channel inputs on the DA-P1. My
preference is to use something else like a MixPre as a front end for
that deck, but that was not a budgetary option.

My real question here is what happens to the phantom power when it
comes out of both XLR
jacks of the deck, then combines through the Y-cable into a single mic
cable? I didn't hear anything odd at the time, but it made me feel a
little queasy (the owner of the deck & mic said not to worry about
it...so I tried not to). It just doesn't seem like the right thing to
do to a mic and/or the inputs of the deck, but I don't really know.

What do you guys think? Does a Y-cable to feed both channels of a
portable deck go together with phantom power? Or does that setup have
the potential of doing harm to either the mic or the deck?

Just wondering all this time...

Andy

On 2006-04-20 21:41:56 -0400, rolling_thunder55 DeleteThis @hotmail.com said:

> What do you guys think? Does a Y-cable to feed both channels of a
> portable deck go together with phantom power? Or does that setup have
> the potential of doing harm to either the mic or the deck?

Shouldn't do harm to either one. The phantom voltage (through some
current limiting resistors) will see the same phantom supply on the
other input (through equal resistors), so no current will flow between
the inputs unless those resistors are incredibly out-of-spec.

And these days, the input impedance of mic circuits is at least 2x the
output impedance of any condenser mic you're using in the field, so
there shouldn't be any bad audio loading either. (Line inputs are as
much as 10x the impedance they're supposed to be matching.)

Mics that depend on audio power transfer rather than voltage transfer -
i.e., dynamics and ribbons - may have audio issues when being split
through a Y-cable. But you wouldn't want phantom turned on with them,
anyway... (the few dynamic or ribbon mics that actually look for
phantom voltage have internal output amps, just like a condenser).

Now, Y-connecting two _outputs_ into a single input is another tale
entirely. Rarely good; sometimes disasterous. With line-level as well
as mic.



--
Jay Rose CAS
tutorials and other sound goodies at dplay.com
email is "jay@" plus the dot-com in the previous line.