Build Your 48V Phantom Power Supply Circuit Diagram

Description

48 V ‘phantom’ power supply has become the prevailing standard for professional condenser microphones. The supply voltage—or bias—is applied across both conductors of the balanced, screened cable using two 6k8 resistors (see reference [1])—absolute value isn't crucial, as a variation of ±20% is permitted, but they must be matched to an accuracy of 0.4% or better [2]. Many microphones incorporate an output transformer, drawing power from a center-tap on its secondary winding.

If the currents flowing through the two wires of the balanced line, which move in opposite directions through the secondary winding’s core, are not identical, the magnetic fluxes they induce do not completely cancel out, and undesirable magnetization occurs, leading to distortion and a diminished microphone’s dynamic range. With an output current of 0.4 A, the power supply described in this article can ‘provide’ at least 40 microphones. The mains voltage is applied to a 30 VA transformer which generates 24 Vrms.

TL783KC Regulator

The TL783KC regulator IC1, detailed extensively at [3], is an adjustable regulator housed in a TO220 package. It offers remarkably low ripple and noise on its output voltage. This regulator includes a MOS series pass transistor and accepts an input voltage up to 125 V, rendering it well-suited for this application. Diodes D3 and D5, respectively, protect the PSU from transients at switch-off and reversed polarity.

Circuit diagram

Its secondary feeds a voltage-doubling rectifier formed by diodes D1 and D2 and capacitors C3 and C4. Capacitors C1 and C2 mitigate the switching noise produced by the rectifier diodes. This voltage-doubling rectifier yields approximately 72 V DC, affording a suitable margin to accommodate ±10% fluctuations in the mains voltage.

Voltage regulation is handled by the TL783KC regulator IC1, as detailed at [3]. This is an adjustable regulator in a TO220 package offering excellent residual ripple and low noise on its output voltage. The TL783KC regulator comprises 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, respectively, shield the PSU from transients at switch-off and reversed polarity. The output voltage is configured via resistors R1 and R2 following the formula:

Vout = Vref × (1 + R2 / R1)

where reference voltage Vref = 1.27 V. These resistors should ideally have a tolerance of 1%, and R2 is likely to dissipate 0.5 W. Resistor R3 establishes a minimum load, crucial for maintaining the PSU’s off-load voltage at 48 V, and is also utilized to power LED D4. If the LED is not employed, R3 must without fail be connected to ground. Last but not least, regulator IC1 needs to be mounted on a heatsink with a thermal resistance of less than 1.5 °C/W using the standard insulating kit: top-hat insulating washer, mica washer, and heat sink compound—ensure sufficient quantity, but not excessive application!