> I mean that the microphone is drawing current in an unbalanced way. As l=
ong as it draws exactly the same current from both legs, a potential noise =
component on the phantom power voltage will be rejected successfully by the=
(differential) preamplifier.
Eric,
Mics can be simple 2-wire unbalanced, floating 3-wire where the mic circuit=
is not connected to anything but the input + and - signals, or fully
balanced where the mic output is symmetrical about ground as with a centre=
tapped transformer or the equivalent circuit.
Passive moving coil mics are often fully floating, but anything needing
power also needs a power negative connection.
Phantom current power is supplied from a fairly clean medium-Z source
equally applied to + and - signal connections, fully balanced both ends, bu=
t
- here is the problem - the power return has to be through the grounded
screen. The mic capsule can't now be fully floating, but has to be balanced=
to eliminate power supply noise.
Normally any noise from the phantom power cancels out on the + and - inputs=
,
but if the mic is unbalanced, or has one leg referenced to ground, or has
different + and - impedances, then phantom power noise will no longer cance=
l
out.
Any mic which doesn't have fully balanced outputs will pick up any
(unbalanced) noise on the phantom powering. The noise requirement for
unbalanced power is of the same order of mic noise down near -120dB.
David Brinicombe
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