The Phantom Menace, August 2002UNDERSTANDING THE POWER WITHIN 5/17/2004 8:00 AM Eastern
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).
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.
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.
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
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.