IMA: When NOT to Apply External Bias
9 May 2008 – In a recent Reflector
post
Don Wilhelm (W3FPR) wrote …
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“Caution: you must know your …
microphone element type –
a dynamic element can be
damaged
if bias is applied to it.”
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Background
The two types of microphones commonly used with amateur radio equipment are
electret and
dynamic.
These terms refer to the principle on which their transducer (element) operates.
The output of an electret transducer is inherently low,
so electret microphones always incorporate a built-in pre-amplifier.
In contrast, the output of a dynamic transducer is normally sufficient
for use with modern solid-state transceivers, without the need for a pre-amplifier.
As a result, pre-amplified dynamic microphones are rarely encountered.
Theory
Regardless of its type, if a microphone has an internal pre-amplifier, then it needs to be powered somehow.
In amateur radio applications, this power is provided in one of two ways:
Externally
Power is applied to the mic AF output line through an external
biasing resistor.
This method is typical of Elecraft®, Heil, and Icom
electret microphones.
If the optimal value of this resistor is not specified in your microphone’s documentation,
then you should request this information directly from the manufacturer.
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Internally
The pre-amplifier is biased internally
either from a battery or from +5V provided by the transceiver.
This method is common to Kenwood and Yaesu electret microphone.
They put the bias resistor inside the microphone housing, so you should not apply external bias.
(Often these microphones also have a DC blocking capacitor in the mic AF output
line to protect your radio.)
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Practice
Dynamic microphones rarely have a built-in pre-amplifier,
and most that do are biased internally.
However, a very small number of pre-amplified Icom dynamic microphones
do require external biasing.
If your dynamic microphone does not have circuitry to protect its transducer,
then mistakenly applying an external bias will damage it!
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The best way to ensure that you are properly configuring the IMA jumpers is to “know” your microphone(s).
Yes, the PDF
User’s Guide
will get you started.
However, much of its content is derived from information that has not yet been verified by hands-on testing.
So your safest recourse is to know the element type, pin-out, and bias requirements of your microphone(s).
In addition, many other microphones (and headsets) beyond those listed in the
User’s Guide
can be used with the IMA, provided that you know how to.