Ask Eddie: Transducers Front and Back

Aug 1, 2011 9:00 AM, By Eddie Ciletti

THE HOW AND WHY OF MIC DIFFERENCES

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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 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.

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.

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.

Email Eddie, visit tangible-technology.com or drop by the blog.






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