Electret microphones only need the power for the FET buffer
stage, as the capacitor diaphragm polarising voltage
is built in at manufacture.
Fig.1 Battery powered electret
microphone schematic (part values may vary)
diagram of a typical electret microphone is shown in Fig.1 - in
practice a DC blocking capacitor would be added at the output but we
will imagine this forms part of the recorder input for the moment.
a small modification, the battery and load resistor can be taken inside
the recorder, eliminating the need for the switch but still retaining
the two-wire unbalanced connection path, as shown in Fig.2
Fig.2 moving the
load resistor into the recorder in the innovation that became known as
So far so good. Unfortunately the
RL and indeed the operating voltage vary between different recorders.
Unlike 48V phantom powering there is no defined standard way of doing
this, so the mic operating conditions are in an unspecified
condition. HiMD recorders such as the Sony MZ-NH700 have the
supply in this
] running at
a meagre 2.3V. Thus even if the capsule tolerated
a mere 0.5V across it the current available would only be 0.27 mA and
absolute maximum is 0.33mA (substantiated by short-circuit current
measurement). There are some PIP mics this HiMD will not work with
correctly because of this. My old Kenwood DMC-F5R used a voltage of 5V
to supply the PiP and did not have problems powering any PIP mic I had.
constructors should also beware. The popular Panasonic WM-61A capsule
at 2V operation[2
], and loses 3dB
of sensitivity at 1.5V. It requires 0.5mA, and drawing 0.5mA form the
MZNH700 is just not possible. The capsule will run out of
spec and starved of current. A better way to use this capsule is to use
a battery box, along the lines of Fig.3
Fig.3 Battery box with
You don't need
a switch for the battery if you remember to unplug the PIP mic when it
is not being used. Purists may want to put a capacitor across the
battery, and wildife sound recordists may want to reduce the value of
the 1u output capacitor to 0.1uF which will form a 200Hz rolloff
working into the assumed 6.8kΩ load of the following recorder PIP input. A battery voltage of 6-9V will
still achieve at least 2V across the capsule at 0.5mA.
to mic powering
- Sony MZ-NH700 Service
manual, p33, RL as a parallel combination of 10kΩ and 22kΩ (effectively
6.8k). Voltage measured open circuit.
- Panasonic WM-61A datasheet .