48v phantom microphone power supply

48 V phantom` powering has become the standard for professional condenser microphones. The supply (or rather bias) voltage is applied over both wires of the balanced screened cable via two 6k8 resistors (see reference [1]) the absolute value is not critical, since a variation of ±20% is permitted, but they must be matched to an accuracy of 0.
48v phantom microphone power supply - schematic

4% or better [2]. Many microphones are fitted with an output transformer, and derive their power from a center-tap on the secondary. If the currents supplied by the two wires of the balanced line, which flow in opposite directions through the two halves of the secondary winding, are not identical, the magnetic fluxes they induce in the core of the transformer do not cancel out properly, and spurious magnetization occurs, causing distortion and a reduction in the microphone`s dynamic range. With an output current of 0. 4 A, the PSU described in this article can supply` at least 40 microphones. The mains voltage is applied to a 30 VA transformer which supplies 24 Vrms. Its secondary feeds a voltage doubling rectifier formed by diodes D1 and D2 and capacitors C3 and C4. Capacitors C1 and C2 suppress the switching noise produced by the rectifier diodes. This voltage-doubling rectifier provides around 72 V DC, and so offers an adequate margin to allow for ±10% fluctuations in the mains voltage. Voltage regulation is taken care of by TL783KC regulator IC1 on which an abundant amount of information may be found at [3]. Basically, this is an adjustable regulator in a TO220 package that offers excellent residual ripple and low noise on its output voltage. The TL783KC regulator includes a MOS series pass transistor and accepts an input voltage of up to 125 V, making it an ideal candidate for this application. Diodes D3 and D5...

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