Condenser microphone uses dc-coupled impedance converter

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.

The circuit sources phantom power at 48V dc through R_PH1 and R_PH2 at the mixing-console end of the microphone cable. Q₂’s emitter drives—and R_PH1 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. R_PH2 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 and 2 and Figure 2 show typical performance parameters for the impedance converter in Figure 1.