Primo EM172s are cheap, low noise and transparent-sounding electret microphone capsules. They are normally soldered onto a cable and plugged into the stereo mic input on a handheld recorder (see how here). Whilst some recorders provide very high quality results doing this (such as the Sony PCM-M10), it would be great to be able to use EM172s with professional XLR microphone inputs on mixing consoles, recorders and mic preamps. Such XLR inputs can be very low noise and have useful features such as a high-pass filter and a pad. Furthermore, if the mic capsule signal is properly buffered and balanced, XLR cables can be long (into the hundreds of meters) without any audible signal degredation.
The proposed electret phantom power supply (refered to as “EPPS” from now on) achieves the above. The capsules plug into the EPPS which in turn plugs into a phantom-powered microphone input. It needs to do the following:
Multiple capsules can be wired in parallel for lower noise. The EPPS can power up to four capsules per side (eight capsules total).
Although designed for the low-noise Primo EM172 capsules, this design should work for other similar capsules too, such as the (now discontinued) Panasonic WM-61A, or any other two-legged capsules.
Download the full schematic: ElectretPhantomSupply_Schematic.pdf
A quick look at the EM172 datasheet tells us that the capsule already includes an FET:
Powering such a device is therefore very easy - 3-10V via a resistor. 8.2V via 10k ohms works well (R1 in the schematic below). C1 blocks this DC voltage from entering the op-amp. Anything higher than 0.1uF for C1 works. R7 baises the input signal to half the op-amp power supply (4.1V via R7).
Buffering is done by a simple op-amp in voltage follower configuration (U1A above). The op-amp needs to have a low power consumption (I was aiming for less than 5mA for a dual op-amp), very low noise and be relatively inexpensive.
C6 and C7 block phantom power and need to have a voltage rating above 48V. 63V is a safe option. R11 and R12 set the output impedence to 56 ohms. R11 and C10, and R12 and C12 are low-pass filters that prevent any unwanted high frequencies from leaving the device.
The output is impedence balanced.
Phantom power is tapped off via 4x 10k resistors. D1 caps the voltage at 8.2V and resistor divider R9 and R10 provides the virtual ground for biasing the input signal before it enters the op-amps. C3 and C5 stabilize the +8.2V and +4.1V voltage rails.
The output is via a 5-pin header intended for a stereo XLR cable onto which mono XLR plugs are soldered.
Case grounding is applied via the minijack input socket.
I usually have a small number of electret phantom power supplies ready for those who would like to purchase one. The price is £95 plus shipping. I accept Paypal payments. If you are interested in purchasing an electret phantom power supply, please send me an email with the shipping address so I can provide a quote for shipping.