The A3 adaptor is highly recommended for use with the OKM; it is very small and fits between the OKM and the recording device. It comes with some useful features: a bass roll-off filter, a -20dB attenuation pad, and a battery tester.
The OKM usually gets its power from the plug-in power available on most modern mini-recorders, which is 1.5V - the A3 supplies the microphones with a 6V charge, increasing headroom significantly and removing the need for plug-in power. The interface can now be plugged into the microphone input or the line input (depending on sound pressure levels) which means a larger range of sound pressure levels can be recorded simply by changing the combination of input and the attenuator pad setting. Any unwanted low frequency background noise can be filtered away using the bass roll-off filter.
For more information please visit the Soundman website.