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The Rick Chinn
P48 power supply

The P48 box

Our friend Rick Chinn writes:  This electronic gadget is an adapter that I made to allow the Sony C-55 microphone to run from a P48 (phantom) powered input. The original C-55 slightly predated phantom powering, and it has a five-pin XLR connector. Pin 5 connects to pin 1, which turns the microphone on and off (in addition to the battery switch on the response control ring). Sony did make an external power supply for this, which just grabbed pin 4 on the XLR-5. The battery for this (as well as the Sony ECM-377 microphone) is nearly extinct, so the P48 box is the way to fly.

The Sony C-55 microphone predates the widespread adoption of phantom power for powering condenser microphones. This circuit extracts the phantom power voltage from the balanced line and delivers it to the microphone via the extra pins in the five-pin connector. The C-55P microphone has a three-pin conector and can be powered by P48 powering.

R1 and R2 pick off the phantom voltage of the balanced line coming from the microphone preamp. D1, a 9v1 zener, pulls the voltage down to 9.1 volts. C1 bypasses any noise caused by the zener and lowers the source impedance of the virtual battery created by the zener and the phantom power voltage. D2 ensures that the microphone’s battery, if installed, can’t “backwash” the zener in the adapter box. It diode ORs the virtual battery with the battery in the microphone.

C2 and R3 ground the circuit’s metal box for RF, but not for DC, to prevent potential ground loops caused by setting the box onto something conductive that is part of a different ground system.

The microphone’s output is transformer-balanced, making it inherently compatible with a phantom-powered microphone input. The remaining two pins of the XLR5 connector have the microphone’s battery on one pin and the ground side of the microphone’s power switch. This pin must be grounded for the microphone to work on battery, which also lets the battery back up the P48 supply. As long as P48 is present, no current is drawn from the battery. The microphone’s usual output cable, XLR5-F to XLR3-M, has pins 1 and 5 shorted at the XLR-5.

Rick Chinn ps