HOW MUCH PHANTOM
©2002 ~ 2013 by Eddie Ciletti
Appeared in MIX magazine
Updated 5 December 2013
There is an overall fear of Phantom
Power, which under normal circumstances is invisible to Dynamic
Mics, both coil and ribbon. Phantom Power is for professional condenser
mics equipped with an XLR connector and NOT for computer / gaming
headset mics equipped with a 3.5mm TRS plug. (Headset
mics are typically Electret and require only 1.5 volts.) Phantom
power is backward compatible with all Dynamic Mics.
When possible, it is a good idea to
have the master phantom power supply turned off until all mics are plugged
in. This is difficult to achieve under normal conditions, and
so it is perfectly fine to plug a ribbon mic's XLR cable into a mic panel
(or preamp) with phantom power on. That said, Phantom Power must
be off IF the mic lines appear in a patch bay.
Phantom Power is distributed as a common-mode
DC signal riding piggy-back on top of the balanced audio signal.
48 volts DC is applied to both pin-2 and pin-3, ground is on pin-1.
Plug in a mic cable and power is safely applied, but connecting a mic via
TRS patchbay applies 48 volts to tip (pin-2) while the ring is momentarily
connected to ground. While the full 48-volts is isolated from the
coil or ribbon via transformer, patching (or a mis-wired / broken cable)
can send a damaging spike across the coil or ribbon
Condenser mics require power for their
internal electronics and to polarize the capsule. Vacuum tube mics
require an external power supply. Early solid-state ('transistorized)
condenser mics could be internally powered via battery or via Phantom Power
without special cables and with no harm to existing dynamic microphones
- they don't even see it! Watch
this video about how a condenser mic is made.
In Figure-1, the pair of sine waves represent
the differential audio signals - one signal is 180-degrees out-of-phase
(reverse polarity) with the other. Both Phantom Power and Noise
(red spikes) are common mode - they have the same polirity on pin-2 and
pin-3. When all the signals get to the pre-amp, its differential
input amplifier does just that, it "looks for the difference," subtracting
pin-2 from pin-3 translates into a double negative - otherwise known as
"addition" for the intended audio, but "subtraction" for the noise (a.k.a.
cancellation). The relationship between signal amplification
and noise rejection is called the Common Mode Rejection Ratio (CMRR). Table-1
shows how CMRR can be different at various frequencies.
Figure-1 shows the essential hardware:
48-volts feeds a pair of 6k8-ohm resistors connected to pin-2 and pin-3,
the signal pins. The DC "return" path shares pin-1 with the earth / shield
connection. A local capacitor keeps the signal clean.
TABLE-1: Common Mode Rejection
Ratio (CMRR) specs for three frequencies as published by John Hardy for
his Twin Servo 990 mic preamp using the Jensen JT-16-B Input Transformer.
Click on these links,
if you want to know how much current is available to
- and what typical condenser mic current draw
is. For more on Ribbon Mics, check out "Tiny
Ribbon, Big Sound" sidebar.
AVOID A DREADFUL MIS-CARRIAGE
Under normal circumstances 48-volts is
applied to both pin-2 and pin-3 (with respect to ground) and not across
the coil or ribbon, both of which are typically isolated from the outside
world via transformer. Since there is no potential difference, Phantom
Power is invisible to dynamic and ribbon mics. However, if pin-1 and pin-2
(or pin-1 and pin-3) are reversed ó as would happen with a mis-wired cable
ó either 48-volts would be applied across the micís transformer or across
the capsule itself. Turning a dynamic mic into a tweeter is not a good
thing. Iíve seen a bad cable trash a perfectly good Sennheiser MD-409 (that
is not transformer isolated).
The output impedance of most
microphones is 200-ohms ó 50-ohms for ribbon mics; the ribbon itself is
less than an ohm and requires a step-up transformer to get the signal to
a useable level. An input transformerís DC Resistance (10-ohms to 40-ohms,
typical) is considerably lower than its AC Impedance (300-ohms to 1k2-ohms,
typical). Connecting a mis-wired cable with the phantom power ON will send
a momentary spike across the transformer to the coil or ribbon, stretching
the latter out of shape.
Condenser mics are thirsty for power; tube
mics in particular require their own supplies most of which are bulky,
inconvenient but beautiful. Solid-State Condenser mics were liberated by
Phantom Power, allowing some to be battery operated.
As you can see in Table-2, some
mics will operate on as little as 9 volts on up to 52volts, thanks to a
"switching" or switch-mode power supply that converts phantom power into
the necessary capsule polarizing voltages. Pretty clever, eh? Two condenser
mics publish their current requirements as 3mA. As you can see, not all
the mics shown have the same voltage tolerance or current demands.
as per DIN 45596
as per DIN/IEC
11 - 52V
3 mA typical
P 48, DIN 45 596, IEC 268-15
Table-2: Phantom power requirements of a few select condenser
Just a quick detour to our old buddy Ohmís
Law, with a little test to see how much current might be supplied by the
phantom power distribution circuit (and mis-applied to a micís output transformer
under mis-wired conditions).
Using Figure-1 as reference, short
pin-2 and pin-3 to ground so that the two 6k8-ohm resistors combine in
parallel to become 3k4 ohms. Apply Ohmís Law: A=V/R (Amps equal Voltage
divided by Resistance);
the maximum current that can be delivered by
two resistors to ground is 14milliAmps (mA) = 48-volts / 3400-ohms or 7mA
per resistor. The resistors plus the low DC Resistance of a transformer
(5-ohms to 50-ohms) makes almost no difference in the total current, which
is considerably high for this application, enough to power an LED! Remember
that DC does not travel across the transformer windings, it just appears
as a momentary spike.
So, when building a phantom power supply
to power multiple microphones, simply use 14mA per microphone as
the reference. No one microphone requires this much power, but it
will ensure that the power supply is operating with a comfortable margin
ó headroom ó when driving ALL microphones. A supply designed
for 24 condenser microphones should be capable of delivering 360mA without
a sweat ó that's less than 1/2 amp!
I spoke with both David Royer ó www.royerlabs.com
ó and Wes Dooley of Audio Engineering Associates ó www.wesdooley.com ó
both of whom manufacture ribbon microphones in the good old US of A. Each
does their best to educate users on the doís and doníts of ribbon technology,
offering mic placement tips, accessories and a generous warranty policy.
(Ribbons are more vulnerable to plosives than dynamic mics.)
Wes manufactures the AEA R44 to the original
specs offering replacement parts that are interchangeable with the original
RCA 44 ribbon mic. Like many retro manufacturers, Wes has taken the time
to talk to veteran designers and engineers, collecting some of their stories
and sharing them at every opportunity.
The "ribbon" is a narrow strip of aluminum
foil ó originally hammered out in the same old-world style tradition as
gold leaf ó gently locked into place with just enough tension to center
it within an extremely powerful magnetic gap. (The resonance is at the
lowest possible extreme of the audio band.) It is both delicate and articulate.
Ribbon microphones are perfectly capable of interfacing with phantom power
so long as the cables are correctly wired. IF you have a transformer-less
mic preamp, turn the phantom power off before connecting the mic, allowing
time for the blocking caps to discharge just in case they do so at an uneven
how a ribbon mic is made.