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Ask Eddie: Transducers Front and Back

THE HOW AND WHY OF MIC DIFFERENCES 8/01/2011 5:00 AM

GEEK: My friend Paul Wolff doesn’t like the moniker, but I embrace it. The positive connotation of “geek” describes someone who is happily hardwired for hardware and technology. I won’t share his negative connotation, in case you’re eating.

 

TRUTH: D.I.Y.ers are learning from hands-on experience that it takes trial and error to get things right. Audio is an endlessly fascinating subject, and driving this trend is curiosity and the availability of obscure bits of audio history found on eBay and Craigslist. This resurgence of D.I.Y. tinkering is generating many great questions.

RECAP: In last month’s issue, when “Ask Eddie” debuted, the topic was console upgrades and needing to understand what power supplies do. As I write this, one week into July, three topics have already been posted to my new Mix Blog. Two related e-mails came from Europe, one from Greece asking why conventional phantom-powered studio condenser mics are not compatible with electret condenser mics (without an adapter). A German query about an obscure Grundig GCM 3 condenser microphone required some clarification about how phantom power differs from an external power supply.

So this month it’s...

TRANSDUCERS!
Microphones convert acoustic energy into electrical energy and loudspeakers reverse the process. We know a speaker can be used as a sub-kick mic, and I have even used a pair of cheap dynamic mics as headphones (when I was in ninth grade). Dynamic mics and speakers have three things in common: a magnet, a coil of wire and a diaphragm.

When an electrical current flows through a wire, it radiates a magnetic field. The speaker’s “cone” is connected to a coil of wire that lives in a magnetic field. We all know that variations in audio signal current move the cone—opposites attract. You can connect a 9-volt battery to any raw speaker and, depending on how the battery polarity is applied to the speaker terminals, the cone will move out—or in—and stay there!

Conversely, when you scream into a mic, the diaphragm pushes the coil into a magnetic field so that an electrical signal is generated. The signal is naturally balanced—two signals of opposite polarity appear across the two “voice coil” wires, also known as a “differential” signal. The benefit is improved noise immunity (details to come). A transformer matches the low-impedance voice coil (in the 10Ω to 20Ω range) to the standard mic impedance (200 Ω).

In a ribbon mic, the coil of wire is replaced by a thin strip of “corrugated” aluminum foil, just a few microns thick, that doubles as the diaphragm. Sound pressure moves the ribbon within a concentrated magnetic field, generating an electric signal. A transformer matches the foil impedance, which is less than 1Ω, to the standard 200Ω impedance.

Condenser mics are a different animal. The diaphragm material typically is Mylar, measuring 3 to 6 microns thick, with a metalized coating that is so molecularly thin you can see through it. The diaphragm is suspended like a drumhead over the back plate (a precision-drilled brass disc). The metalized coating and the back plate are conductors separated by an insulator (air). Together, this creates a capacitor; condenser is the olde-skule name.

When a fixed DC polarizing voltage (typically 40 to 80 volts) is applied to the diaphragm, the back plate will be grounded (though it can be the other way round). Sound pressure moves the diaphragm and modulates the DC voltage in the process. If you’ve ever used a synth, think of sound pressure “pushing” the modulation wheel, adding vibrato to a fixed pitch.

The modulated DC signal is extremely vulnerable and requires a very high-impedance amplifier, either vacuum tube or Field Effect Transistor (FET). In nearly all cases (except the Grundig GCM-3), the amplifier is located in the mic body. Vacuum tube amplifiers require a separate power supply with multiple voltages, while FET amplifiers were initially battery-powered, either externally (like the Sony C38) or internally (like the earliest version of the Neumann U87).

Electret condenser mics are typically found in computers, cellphones, cameras, portable recording devices, communications headsets, boom and “invisible” mics for stage, video and film. The electret condenser mic can be very small, requires very little power (for its built-in amplifier) and supplies its own polarizing voltage via the electret material, which is the electrostatic version of a magnet, storing an electrical charge instead of magnetism.

 

SIGNAL DISTRIBUTION
As mentioned, moving-coil and ribbon mics deliver a balanced/differential signal by design. Because their native impedance is so low, an internal transformer matches the coil or ribbon impedance to the 200Ω standard. Condenser mics also need either a transformer or special circuitry to deliver a balanced signal. Check out the pair of sine waves on the left side of Fig. 1 and note that they are of opposite polarity (a balanced, differential signal). Superimposed on both audio signals are a pair of common-mode “noise” signals (in red) that have identical polarity (in phase). When the common-mode signals meet a balanced/differential audio input, they are rejected (or more precisely, subtracted) from the differential signal. The degree to which signal and noise can be differentiated is called the Common Mode Rejection Ratio, or CMRR.

 

Figure 1: The secret life behind the female XLR jack

Figure 2: An electret capsule assembly and typical schematic. Inside the capsule is a Field Effect Transistor amplifier. External to the capsule is the FET’s “load” resistor, across which the audio signal will appear.

Table: 3.5mm (eighth-inch) “stereo-mini” connector wiring for electret mics. Older electret capsules had three connections because the load resistor was in the capsule. But as the schematic in Fig. 2 shows, the load resistor is now external to the capsule, allowing stereo mics on the same connector.

Remember how early solid-state (transistorized) mics were battery-powered? Well, that was a “temporary fix” until a very clever solution came along. Some genius figured out that if common-mode noise can sneak in under the radar and be rejected, so could 48VDC, which powers the mic’s amplifier and supplies a polarizing voltage to the capsule. That’s right: Phantom power is injected as a common-mode DC signal, the beauty of which is that no modification to the existing signal distribution system is required. Unless mic cables are wired incorrectly, dynamic and ribbon mics are safe. Yeah!

Relative to a studio condenser mic, the electret circuitry is simpler, which is one reason why your cellphone and camcorder are not the size of a U47. The wiring of the typical eighth-inch (3.5mm) TRS electret microphone connector (see the table) is determined by whether the mic is mono or stereo. Similar microphones may also be terminated with Switchcraft and Hirose 4-pin connectors. The extra pins are used to program wireless beltpacks for a specific mic.

NOW YOU KNOW WHY…

  • Studio condenser mics and electret mics are not compatible without a moderately sophisticated adapter.
  • A power supply for a vacuum tube microphone is not “phantom” power.
  • Balanced/differential signal distribution is less susceptible to noise.

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