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Re: Phantom Power. Question from The Ghost Who Walks :-)

Subject: Re: Phantom Power. Question from The Ghost Who Walks :-)
From: "Richard L. Hess" richardlhess
Date: Sun Jan 3, 2010 12:06 pm ((PST))
Hello, Greg,

Alice Springs is one of the far-flung places that
I might want to visit someday, but I probably
will not with the hassle that air travel has
become. We're planning a driving trip to Labrador this summer.

You have received a lot of good answers,  but I'd
like to tie together a few things to be clear.

(1) FORGET using the phone voltage for phantom
power--you'd be better off using a mains-powered
unit and you're just as tied down. You can't use
it raw for all the reasons stated.

(2) Fortunately, your AT815b microphone will work
from 9-52 V phantom and it appears that the
improved specifications for "phantom" apply over
the whole range. As you noted, the dynamic range
improves when you switch from 1.5 V to 9 V
powering the microphone. While it might be nice
to come up with 48 V phantom power for other mics
(DPA, Neumann, many Sennheiser, some Audio
Technica [AT4041, for example] and some AKG mics
require 48 V phantom power), since the mic you
wish to power will work with 12 V phantom power,
I would suggest setting up for that. While you
can set up for 9 V phantom power, 12 V is easier
and lasts longer. Using a single 9 V battery is
sub-optimum because the voltage of an alkaline
battery falls fairly fast and 9 V is the lower
margin of the powering range. You have more
"headroom" at 12 V and the 12 V configuration is a standard.

Please note that the reason the dynamic range is
improving is NOT because the noise floor is
changing, but the higher operating voltage allows
a greater swing before clipping so it allows for
a higher undistorted output level. The noise only
appears to improve by perhaps 1 dB.

The self noise of this mic is 24 dB(A) which is
derived from 94 dB - 70 dB (read the notes).
Since the sensitivity is ~ -36 dBu then the mic
noise floor is ~ -106 dBu. BEWARE, some external
phantom power supplies have a noise level of -70 dBu !!!!

What I'm about to tell you will be quieter than your mic.

The easiest way to produce 12 V phantom power is
with 8 alkaline AA cells (or AAA cells) in a
holder, all in series. If you use NiMH cells,
you're better off with 10 cells. Battery holders are available.

I would place a 470 =B5F 25 V capacitor across the
battery output to be safe (+ of capacitor to + of
batteries, - of capacitor to - of batteries).

Arrange for a female XLR connector and a female 3.5 mm stereo connector.

Connect the sleeve of the 3.5 mm stereo connector to pin 1 of the XLR.

Connect the battery/capacitor negative terminal
to XLR Pin 1. Connect the battery/capacitor
positive (+) terminal to two 680 ohm 1%
resistors. Connect the end of each of the two
resistors to pin 2 and pin 3 of the XLR.

Connect the positive ends of two 100 =B5F 25 V
capacitors to pins 2 and 3 of the XLR connectors.

Connect the negative end of the capacitor going
to pin 3 to pin 1 of the XLR connector.

Connect the negative end of the capacitor going
to pin 2 of the the XLR connector to both tip and
ring of the 3.5 mm connector.

Connect one end of a 10,000 ohm resistor to the
joined tip and ring of the 3.5 mm connectors and the other end to the sleev=

All this can mount in a small, pocket-sized
aluminum box, or you can use flying cables and make it smaller.

This will work with any P12 (12 V phantom) mic
like your AT shotgun and will give you all the
headroom that the mic is capable of.

The two 100 =B5F capacitors isolate the phantom
voltage from the plug-in power voltage and the
stereo plug is used to feed the shotgun to both channels at once.

The 470 =B5F capacitor is just there to make sure
there is a very low impedance ground return at
audio frequencies even as the batteries age.
Remove the batteries from the unit if you are not
going to use it for a month to avoid the
capacitor's leakage from discharging the batteries sooner than expected.

 From what I've read, the LS10 will be
more-than-capable of capturing all of the dynamic
range of the AT mic you have (which is not
spectacularly quiet for nature work). For
example, my Sennheiser MKH 416s are about 8 dB
quieter--and about 3x the cost (I lucked into mine used).

Good luck and have fun with it.

While the power adapter for the mic will be
larger than your recorder, it will power the mic
(with AA cells) for perhaps 1000 hours or more.
You will probably only need to replace the cells
once or twice a year if you use it a lot, every two years if you use it les=

You have received good advice about lightning,
but if you're in an area prone to lighning, I'd
be careful about running long cables outside as
even a cloud-to-cloud discharge can induce a fair
voltage in a long cable and poof goes the
recorder and/or mic. Well-shielded and the quad
arrangement should reduce the risk, but it is still there.



At 12:31 AM 2010-01-03, greg.winterflood wrote:
>I've recently acquired an Olympus LS-10 Linear
>PCM Recorder. I used to use a Marantz PMD 222
>single channel tape recorder and an
>Audio-Technica AT815b shotgun mike. I've noticed
>talk about Phantom Power on the list, but don't
>quite understand it. I have checked the
>specifications of my mike. It takes Phantom
>Power. I've opened the mike and found that after
>~6 years of dis-use the leak proof 1.5V battery
>in the mike base was intact. Phew!

Richard L. Hess                   
Aurora, Ontario, Canada
Detailed contact information: =

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