How To Use An Electret Capsule With P48 Power

Balanced Signals and Phantom Powering For WM61A Microphone Capsules

Sep 19, 2009

Cheap electret capsules like the WM61A can perform well with a battery box. This article describes how to power them with P48 phantom power balanced inputs instead.

Low cost electret capsules such as the Panasonic WM61A can deliver very good results if they are mounted correctly and in the right environment. However ,the capsule is inherently an unbalanced device, designed to work with unbalanced inputs.

Power for the microphone is often derived either from the recorder via plug-in-power, or form a separate battery box for greater headroom. However, it is possible to power the capsules via 48V phantom power if a suitable interface circuit is used. This also has the advantage of converting the single-ended microphone output signal into a balanced signal with a lower output impedance.

This is a good match for professional microphone inputs on field recorders such as the Sound Devices 702 or 722 and mixer inputs. In addition, the P48 phantom power standard is well defined and has ample power for the capsule and buffer circuit, in contrast to the poorly defined plug-in-power system. Powering the microphone from the recorder saves the trouble of having to worry about the separate 9V battery in a battery box.

P48 Phantom Power Electret Capsule Interface Schematic

The schematic in Fig 1 consists of two functional parts. R3, R6, C4, C1 and D1 form a Zener stabilised 12V power supply which is used to supply the capsule via two resistors, R1 in the drain leg of the capsule as usually done, R2 in the source leg of the capsule. Though at different DC offsets, the AC signals developed across these two resistors are in antiphase, and this is the basis of the balanced output signal. That is buffered by the PNP transistors Q1 and Q2 working as emitter followers and applied to the signal cables as a true balanced signal.

The value of the resistors should be selected such that there is approximately 4V across them at the nominal current drain of the capsule, which will then also have about 4V across it, bringing the total voltage to 12V which is the power supply voltage.

The phantom power interface schematic is redrawn from one used by Scott Helmke of the micbuilders Yahoo group for his "Alice microphone", and is very loosely derived from the schematic of the Schoeps CMC5 microphones.

Electret Phantom Power Interface/Buffer Implementation

Typically, this circuit should go at the microphone end of the cable to get the benefit of the balanced output signal and the lower impedance of the buffer. For short runs of less than 3m it may be convenient to construct the schematic on stripboard in standard leaded parts and keep it at the recorder end due to its size, but it can be miniaturised in surface mount by using BC856B transistors and other SMD parts, in which case the circuit can fit inside a typical microphone casing.

Interfacing an electret capsule with phantom power is the best solution to powering these devices - there is ample power for the capsule to be powered without the loss of headroom that so often happens with the low voltages of plug-in-power, and the hassle of ensuring that the separate battery of a battery box has not gone flat is not present either.

The copyright of the article How To Use An Electret Capsule With P48 Power in Recording Engineering is owned by Richard Mudhar. Permission to republish How To Use An Electret Capsule With P48 Power in print or online must be granted by the author in writing.