The Phantom Menace, August 2002

UNDERSTANDING THE POWER WITHIN
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As August heats up, so does the Minnesota State Fair—a mix of
old and new, farm animals and related technology, outdoor concerts and
unusual foods on sticks. It's what's on the end of some other sticks,
however, that has my pantaloons in a bundle. Yes, this month's focus is
on the studio microphone, and I will be geeking on the
much-misunderstood subject of phantom power as it applies to both
condenser and ribbon mics.

THE MENACE STRIKES BACK
A customer with an Amek Angela console calls about a few nagging
problems, the most annoying of which is a buzz in aux-6. (The console
is easily 12 years old.) I had the master module brought to the shop
for a D&C (where I added a miniheadphone jack) and then scheduled a
house call.

The client uses aux-5 and aux-6 for the Cue system; the former was
clean while the latter was snot. I had expected the problem to be in
the headphone system and was surprised to learn otherwise—after a
minimal amount of troubleshooting. The master module still needed some
attention, so I addressed each issue while monitoring through the new
(and very local) headphone jack.

LOOKING FOR CLUES
Two supplies powered the console: one for the audio, phantom and logic,
and the other for the LED metering. With the light meter supply off,
the buzz was gone (Clue One). The bottom panels were pulled to inspect
the various power connections to the motherboard. Once confirming the
location of the aux-summing buses, I used a screwdriver (with clip lead
to ground) for further interrogation while monitoring via headphones.
The six aux-summing buses are physically in order on the momma board;
shorting bus six made the buzz go away (Clue Two). There was one
additional circuit-board trace next to aux-6, and a quick check of the
schematic revealed it to be the phantom power bus. A 'scope probe on
the 48-volt rail revealed an oscillation (Clue Three) that disappeared
when the light meter supply was off, yet phantom power was still
present. Odd…

The rear-panel access to the light meters and their power
connections revealed, via 'scope, that one of the power rails was
indeed oscillating. Here's where science comes in: High frequencies
travel on the outside of a wire, a phenomenon known as “the skin
effect.” The power distribution cabling for both supplies travels
as one bundle in the console; the close proximity exists for a long
enough distance so that the oscillation induced itself into the wire
carrying 48 volts to the phantom bus. The short-term fix was a
.1µF ceramic cap from the phantom bus to chassis ground. With
aux-6 now clean, I accepted the supply for an overhaul at the
customer's earliest convenience.

Note: Phantom power is a common-mode DC signal. Even if the
oscillation (which was beyond the range of human hearing) made it to
the microphone, it would have been (mostly) canceled by way of the mic
preamp's Common Mode Rejection Ratio (CMRR, the relationship between
signal amplification and noise rejection).

Figure 1: Portion of Amek/TAC power supply showing one of three regulators. Capacitors C9 and C10 (shown)—as well as C4, C5, C13 and C14 (not shown)—should be replaced.

I wrestled with this light meter supply many moons ago, as it is the
very same one Amek used in the TAC Scorpion. And so, like all geeks who
never throw anything away, I had the one schematic that the Angela
customer didn't have; a partial view is shown in Fig. 1. The power
supply consists of two LM-338 positive regulators wired in series to
create bipolar 18 volts. C9 on the schematic is specified as 10µF
(electrolytic), while C10 is .1µF (tantalum) but on the circuit
board, both are generic 10µF electrolytic caps. (On the positive
regulator, not shown, look for C4 and C5. If this supply is used with
the Scorpion, check the phantom supply caps, C13 and C14, as well.)

All of these caps were replaced with 10µF versions of
Panasonic's FC Series, (105° centigrade, switching grade). I
paralleled each with a .3µF K Series (a tantalum-grade
electrolytic) just for overkill. Three of the old caps tested at 60% of
their rated value, and one was no longer a cap. Combine this with the
current being drawn over a long cable acting as antenna, and it's not
surprising that this supply oscillated.

Figure 2: Phantom power distribution in pure hardware

PHANTOM POWER: DON'T BE AFRAID
Questions about phantom power generate much e-mail. To clear up some of
the mystery, let's see how it applies in a basic way—to condenser
and dynamic microphones. Figure 2 shows the essential hardware; 48
volts feeds a pair of 68k-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.

As mentioned earlier in this piece, phantom power is distributed as
a common-mode DC signal, riding piggyback on top of the audio signal.
This clever solution was backward-compatible with existing cables and
microphones. The pair of sine waves represents the differential signal
(one is 180° out-of-phase with the other); noise is represented by
the red spikes, both of which are in phase. When all signals get to the
preamp, the differential input amplifier does just that, it
“looks for the difference.” Subtracting pin-3 from pin-2
translates into a double negative, otherwise known as
“addition” for the intended audio but
“subtraction” for the noise (aka, cancellation). Table 1
shows how CMRR can be different at various frequencies.

TINY RIBBON, BIG SOUND
I spoke with David Royer of Royer Labs (www.royerlabs.com)
and Wes Dooley of Audio Engineering Associates (www.wesdooley.com),
both of whom manufacture ribbon microphones in the good old U.S. of A.
Each does his best to educate users on the do's and don'ts of ribbon
technology, offering mic placement tips, accessories and a generous
warranty policy. (Ribbons are more vulnerable to plosives than dynamic
mics.) David now has a phantom-powered ribbon mic that kills two birds
with one stone by increasing the output level and protecting the ribbon
from miswired cables.

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 to share at the upcoming AES show in October. One tip to use an
RCA 44 safely on kick drum blew me away (without blowing away the
ribbon): Simply lay the 44 on its back against a pillow in the bottom
of the drum so that the air goes across the face. I can't wait to try
that!

The “ribbon” is a narrow strip of aluminum foil,
hammered out in the same old-world-style tradition as gold, 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 as long as the cables are correctly wired. If you
have a transformerless mic preamp, then turn the phantom power off
before connecting the mic so that you allow time for the blocking caps
to discharge, just in case they do so at an uneven rate.

AVOID A DREADFUL MISCARRIAGE
Under normal circumstances, 48 volts are 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 a
transformer. Because 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 — as would happen with a miswired
cable—48 volts would be applied either across the mic's
transformer or across the capsule itself. Turning a dynamic mic into a
tweeter is not a good thing. I've seen bad cable trash a perfectly good
Sennheiser MD-409 (that is not transformer isolated).

The output impedance of most microphones is 200 ohms, 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 usable level. An input
transformer's DC resistance (10 ohms to 40 ohms, typical) is
considerably lower than its AC impedance (300 ohms to 12k-ohms,
typical). Connecting a miswired 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.

CONDENSATION
Condenser mics are thirsty for power; tube mics, in particular, require
their own supplies, most of which are bulky and 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 52 volts, thanks to a “switching” or
switch-mode power supply that converts phantom power into the necessary
capsule-polarizing voltages. Pretty clever, eh? Two condenser mic
manufacturers publish their current requirements as 3 mA. As you can
see, not all of the mics shown have the same voltage tolerance or
current demands.

LAWR AND AWDAH (LAW AND ORDER AS PRONOUNCED BY SOME NEW
YORKERS)

Just a quick detour to our old buddy Ohm's Law with a little test to
see how much current might be supplied by the phantom power
distribution circuit (and misapplied to a mic's output transformer
under miswired conditions).

Using Fig. 2 as reference, short pin-2 and pin-3 to ground so that
the two 68k-ohm resistors combine in parallel to become 34 k-ohms.
Apply Ohm's Law: A=V/R (Amps equal Voltage divided by Resistance); the
maximum current that can be delivered by two resistors to ground is 14
milliAmps (mA) = 48 volts/3,400 ohms, or 7 mA 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 winding; it
just appears as a momentary spike.

TAPE HEAD CLEANING TIPS
I must spend two paragraphs on head cleaning, having recently received
two Tascam DA-45 DAT recorders with trashed heads. Just about every
mastering engineer has a Tascam DA-45 to play 24-bit DATs, and while
these machines may not see heavy use, keeping the heads clean is key to
low errors in 24-bit mode. Despite the myths and
“mythters,” if clogged heads are the cause of digital fuzz,
then cleaning tapes will do the job (provided you don't go ballistic
and do 10 cleanings each time there's a problem). I prefer
head-cleaning cloth for both its efficacy and its safety. I use
Twillwipes; visit www.chemtronics.com for a dealer near you. Cleaning
tapes are safer than what's been destroying heads lately—namely,
chamois-on-a-stick, which I neither use nor recommend.

Apply 99% isopropyl alcohol or denatured alcohol to the cloth. Place
the cloth against the side of the head drum with one finger while
rotating the drum — counterclockwise only, slowly—with the
other finger. When the cloth is correctly applied, it will be possible
to feel the head chip as it passes under your finger. This added
feedback confirms that manual cleaning will be worth your effort. It is
far less dangerous, because the wide contact area reduces surface
pressure and the possibility of snagging and breaking off the head
chip. Follow the wet cloth with a dry cloth. Check each cloth for dirt;
alternate wet and dry until satisfied. Allow time for the alcohol to
dry before reinserting a tape.