Grounding, Phantom Power, Etc - Mackie SR24•4 Owner's Manual

APPENDIX D: PHANTOM POWERING,
GROUNDING AND OTHER ARCANE MYSTERIES
Phantom Powering and Microphones
History
Condenser (capacitor) microphones differ
from dynamic and ribbon microphones because
they are not self–generating. That is, they cannot
generate electricity in response to an impinging
sound wave. A condenser microphone modifies
an external source of electricity to reflect the
effects of a sound wave striking its diaphragm.
Dynamic and ribbon microphones use
magnetism to generate electricity in response to
a sound wave: they are self–generating. Further-
more, both of these types of microphones are
inherently low impedance devices. It is possible
to connect a dynamic microphone element
directly to a balanced, low-impedance mixer
input. Many commercially made dynamic
microphones do just that.
On the other hand, a condenser microphone is
an inherently high-impedance device. How high?
Verrrrrrry high. On the order of a billion ohms
(1 Gigaohm). This is high enough that the inherent
capacitance of a foot of shielded cable would
audibly reduce the output of the microphone. All
condenser microphones have an impedance
converter, in the form of a vacuum tube or field-
effect transistor (FET), built into the microphone,
and located extremely close to the microphone
element. The impedance converter and the micro-
phone element itself require an external power
source.
1
What is it, exactly?
The obvious external power source for any
modern microphone is a battery. About the only
electronic advantage that a battery has is that its
output is pure DC. The only other advantage (to
the battery company) is that you have to keep on
buying them.
Tube microphones require several different
voltages for operation. This invariably means a
multi-conductor cable and non-standard (not
XLR) connectors. A tube microphone will always
have an associated external power supply.
In the late 1960's, Neumann (you know, the
folks that brought you the U47 and U87
microphones) converted its microphones to solid-
state, adopting a system of remote powering that
they called, and trademarked, Phantom Powering.
Because of the trademark, some manufacturers
use terms like Simplex Powering, etc. Over the
years, the trademark has become genericized and
now refers to any device that is powered accord-
ing to DIN standard 45 596 (or maybe it' s DIN
standard 45 595, we're not exactly sure...).
So, why "Phantom" Powering? Because (like
the Phantom in the old comic strip) it' s there
when you need it, and invisible when you don't.
This technology is not new, it actually predates
rocket science. Like many other things in audio,
it was brought to you by the telephone company,
who used it to get an extra circuit from a pair of
wires. In effect, so does your phantom powered
microphone.
What is important is: phantom powering is a
compatible system. Your dynamic/ribbon micro-
phones as well as your condenser microphones
work side-by-side, from the same microphone
inputs, without further thought on your part.
Technically speaking, phantom powering refers
to a system where the audio signal is applied to
the balanced line in differential-mode, and the
DC power is applied common-mode. The audio
travels via pins 2 and 3, and the power travels
between pins 2 and 3 simultaneously and pin 1.
Microphones that do not require power simply
ignore the DC present between pin 2, pin 3, and
pin 1. If you measure with a voltmeter between
pin 2 and pin 3, you will read 0 Volts DC. This is
what your dynamic microphone sees. Measuring
between pin 2 and pin 1, or between pin 3 and
pin 1, you will read the phantom power voltage,
usually 48V, without a microphone connected.
The dynamic microphone, as well as your balanced
mixer input, ignores this voltage.
Lately, the term phantom power has been
perverted to refer to any remote powering
1 To be strictly correct, electret condenser microphones are a
bit different as the microphone element does not require a
power source for operation (it is more-or-less permanently
self-polarized). Regardless, the impedance converter still re-
quires an external source of power.
Appendices
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