Thread: Using Multiple Mics on the Same Console Input
From: Jim Brown
Subject: Re: Flute miking
When you're miking a lot of players in a section, you can
generally Y identical mics on like instruments in pairs
into an input.
Jim Brown
Audio Systems Group, Inc.
Chicago
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From: CTaipale
Subject: Re: Flute miking
<"When you're miking a lot of players in a section, you can
generally Y identical mics on like instruments in pairs
into an input.">
Jim,
A very helpful solution for combining dynamic mics, though
condenser mics don't tend to like sharing that phantom voltage.
What have you done to accomodate sharing the phantom voltage?
Curt
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From: Jim Brown
Subject: Re: Flute miking
<"What have you done to accommodate sharing the phantom
voltage?">
Nothing special, but some of the mics may have been using
batteries the times I've done it.
How things work with phantom will depend on the mic and the
phantom supply in the console. Many mics don't need much
phantom current, so they'll work fine, while others need a
bit more.
I would expect the major problem with running out of
phantom current to be a bit of pre-mature clipping (i.e,,
with lower signal levels). But I wouldn't expect to see
enough level from strings for this to be a big problem with
most mics.
What's your experience?
Jim Brown
Audio Systems Group, Inc.
Chicago
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From: Curt Taipale
Subject: Using mics with a Y-cord
<"What's your experience?"> (re: sharing phantom power with
two condenser mics on the same input channel)
Jim,
I remember one time trying to Y two Neumann U87's, and there
definitely was a major drop in level. The difference was so great -
unusable - that I didn't take the time to see if there was a
corresponding difference in the character of the sound. I want to say
that I've tried this other times, but it's been a while and I don't
really remember. By comparison, I've Y-d two dynamic mics many
different times with very good results.
On a side note, here's another issue. At the church I used to work
at, we had the house desk (a Harrison HM4) and the monitor desk
(a DDA) that shared all mic inputs on stage via a Y-cord splitter.
That is to say, because of a lack of budget at the time the mic
snake feeds were distributed to both consoles through terminal
strips mounted on a board under the stage - with no transformers.
We managed to get away with it, and the plan was to add transformers
a handful at a time. But those plans were never really seen through,
with only a handful of channels eventually receiving transformers.
We found through trial and error that whenever we would connect a
U87 (or any Neumann mic for that matter) to any channel without a
transformer, that the output signal from that mic would go way down
in level. It's been some time so I don't really recall how much,
but something on the order of 6dB to 12 dB. If we simply unplugged
the snake input to the monitor desk the mic would react
normally.
I never researched it much further. We found the way around it,
and went on to other things. Any ideas? I suspected that the monitor
desk was loading down the Neumann output. Interestingly enough, it
was only the Neumanns (U87 and KM84) that had this problem. The
AKG414's or any other mic we used had no apparent problems at all.
Blessings!
Curt
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From: Rick Chinn
Subject: Re: Using mics with Y cords
Curt, I've wye'd my U87's and KM84's between house and monitor desks
without trouble.
One possible problem: these are both phantom powered mics, and both
depend on the phantom voltage to be somewhere in the vicinity of 48V.
I'll try to keep this brief: take the case of two boards, both of
which have global phantom power switches. there is no convention for
the circuit location of the phantom power enable switch. Sometimes it
is at the very input to the phantom power supply (ac from the power
transformer). Other times they let the regulator run all the time,
and switch the feed to all the phantom resistors.
If the designer took the first option, then with the two consoles
paralleled via the Y cord, the phantom power from console 1 (phantom
supplier) feeds back into console 2, through its phantom feed
resistors, back into the power supply. I call this phenomenom
"backwashing the power supply."
Now there's nothing to guarantee how the supply will react to having
power shoved into a terminal that ordinarily is its output. So,
usually, the supply just acts like a load on power coming through the
phantom feed resistors. Net result: it loads the phantom supply coming
from console #1.
With the Neumann mikes, this would lower the phantom voltage
presented to the mike, which also means that the polarization
voltage delivered to the capsule would be lower, and the
output level would suffer as a result.
If the designer took option 2, then it'll probably work.
In the Mackie SR24.4/32.4 boards, I took option 1 as it was easier
from a wiring standpoint. BUT I put a diode in series with the
output of the phantom regulator so that you could backwash it
as described above without the regulator presenting a load
as described. So, here you can get away with it.
The VLZ mixers (1202, 1402, 1604) mixers use a different phantom
regulator and from inspection, look like they'll withstand being
backwashed. But I have not tried this, so there are no
promises here.
So, if you're presented with two boards that took option 1, the
solution is to turn the phantom power on at both boards. This
makes twice the current available, and it doesn't seem to matter.
I've done this many times, with mikes ranging from cheap EV
electrets, AKG's and Neumanns. BTW, the Neumann's, drawing
very little current, would be affected by double phantom power
sourcing the least. The AKGs, especially the 9-52V ones,
are affected the most, and the C451's that I've done this
to have not complained.
If you have 2 boards, and one has individual phantom switches
and the other doesn't, let the one that has the global
switch provide phantom powering.
Rick Chinn
Uneeda Audio
 

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