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Condenser mic uses dc-coupled impedance converter

08 Nov 2012  | Dimitri Danyuk

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The condenser microphone's diaphragm is the movable plate of a capacitor. With a polarized capacitor, the vibration of the diaphragm in relation to the back plate produces an ac audio-output voltage. The condenser capsule has a capacitance of 10 to 60 pF; thus, you should connect it to an impedance converter with extremely high input impedance for a flat frequency response.

The conventional impedance converter is a JFET source follower with an additional amplifying and power-decoupling circuit. You supply power to the impedance-converter circuitry using the same microphone-cable conductors that carry the audio signal. The balanced audio pair at the XLR connector's pins 2 and 3 both carry the same positive dc voltage, or phantom power, relative to Pin 1's ground. The amplifying/decoupling circuit contains an audio transformer or a couple of capacitors to separate the dc power from the audio signal.

High-value dc-blocking capacitors can generate measurable and audible distortion (reference 1). Microphone circuits rarely use the highest-quality capacitors because of space limitations. You can design the impedance converter without dc-blocking capacitors.

Figure 1 shows the self-balanced impedance converter. The self-polarized electret condenser-microphone capsule, X1, connects to the high-impedance gate of JFET Q1. Q2, an ac-current source, loads source follower Q1. Q2, thanks to C2, has high impedance but allows a fixed dc voltage on the Q1 source.

The circuit sources phantom power at 48V dc through RPH1 and RPH2 at the mixing-console end of the microphone cable. Q2's emitter drives—and RPH1 loads—emitter follower Q3. The signal from Q3's emitter bootstraps the drain of Q1, reducing the ac voltage across the gate-to-drain capacitance and resulting in lower input capacitance at the gate of Q1. RPH2 supplies current for shunt-regulator-voltage sources D2 and Q4. R4 and C4 attenuate zener-diode noise. Integrator IC1 compares the dc voltages on the XLR connector's pins 2 and 3 and, through Q2 and Q3, maintains a difference equal to the op amp's input offset voltage. Thus, if the microphone input at the mixer console is transformer-coupled, both ends of its winding are at the same voltage. No dc will flow through the winding and saturate the core. IC1 should have a common-mode-input-voltage range equal to that of the positive-supply rail. You can accomplish this task using, for example, an op amp with a P-channel JFET input stage. Tables 1, table 2 and figure 2 show typical performance parameters for the impedance converter in figure 1.

Reference
1. Gaskell, Robert-Eric, "Capacitor 'Sound' in Microphone Preamplifier DC Blocking and HPF Applications: Comparing Measurements to Listening Tests," Audio Engineering Society, Presentation 130, Paper 8350, May 2011, pg 1.

About the author
Dimitri Danyuk is from Miami, Florida, United States.

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