The making-sound.co.uk stereo battery box offers three advantages over using a portable recorder alone:
In many cases, it increases the maximum signal before clipping by biasing the capsules with a higher voltage than plug-in power would normally provide (8.4V versus the usual 2-3V).
It is surprising how distracting rumble can be. On many street recordings, I have found rumbly noises (car engines, generators, air conditioning etc.) to be far more noticable on the recordings than in real life. Wind noise can also be a problem when recording outdoors when standard wind protection (foam wind shields and furry wind muffs) is not enough. Filtering out these unwanted low frequencies is done by engaging the high-pass filter. Filtering out wind noise can also increase the maximum level before clipping by ensuring the wanted audio signal is not superimposed on an unwanted low-frequency signal.
Battery boxes are simple devices. Here is the schematic of the making-sound.co.uk battery box.
The capsules plug into the stereo minijack socket. They are biased by the 9V battery via a reverse protection diode D1 and one resistor for the left capsule and another for the right capsule (R1 and R2).
In addition to blocking the DC bias voltage from entering the recording device, C1 and C2 form a low-pass filter with R3 and R4 in parallel with the input impedence of the recording device. Large value capacitors C3 and C4 are normally connected in parallel to C1 and C2 to allow low frequencies to pass through. When C3 and C4 are disconnected by opening the switch, the signal passes through only C1 and C2 which block low frequencies due to their low values.
Power is effectively disconnected when you unplug the microphones.
The battery box is designed around a standard 9V battery clip to keep it as small as possible. The main PCB is the same size as the battery clip. A panel provides some shielding and protection to the components. The output is a stereo minijack on a flying lead. The high-pass filter switch is recessed to prevent accidental switching.
I have sold out of the latest batch of battery boxes. Thank you to all buyers. Please register your interest with me if you would like to purchase a battery box should I make any more in the future. If enough people would like to buy a battery box, I will order another batch of PCBs.