Posted by
David Woolley (E.L) on
Jan 30, 2010; 10:23am
URL: http://elecraft.85.s1.nabble.com/K2-connecting-a-condenser-mic-tp4480033p4484771.html
If it really is a simple condenser microphone, it will not work with
only 5, or even 12 volts of bias.
It is almost certainly an electret microphone, which is a composite
device, consisting of an electret transducer and a MOSFET pre-amplifier.
The element is biased by the permanent charge on the electret, and the
power supply is actually for the pre-amplifier.
-----
400k is too low for the element itself and too high for the FET, in a
two wire configuration. I suspect a two wire configuration and that the
reading is being distorted by reverse polarity or too low a measurement
voltage.
-----
Most, if not all, amateur use electret microphones are two wire devices,
even though those for PC sounds use stereo plugs; the ring and tip are
actually connected together, and the plug is just a trick to ensure that
the bias is not applied to a dynamic microphone, which will use a mono plug.
Applying 5 volts DC to one of these without either a resistor or AF
choke, will create an AC short on the output, and, depending on how good
the power supply bypassing is, you may get no output at all!
-------
A two wire electret will not behave like a resistor. It will have the
approximately square law characteristic of an FET drain, so you cannot
use simple potential divider calculations.
-------
The risk of a short would be due to a cable fault
Non-interleaved top posting by list policy, not desire. --------
indicate where to interleave.
Brian Machesney wrote:
> I have a condenser mic that is not on the list of "known" mics in the docs
> or on the Elecraft web site. I'm trying to decide whether to simply short
> the +5V to the AF when connecting my condenser mic to the KSB2, or to place
---------
>
> A DMM shows the DC resistance of the element to be 400 Kohms! Not really
> surprising, I guess, since a condenser mic is electrically similar to a
> capacitor.
---------
> The manufacturer specs the mic element at 4.5Kohms and 1.5V to 9.0V bias.
> Applying the KSB2's +5V directly to the mic element's 4.5K ohms should
> produce 1mA drain, no sweat for the KSB2, and right in the middle of the
> manufacturer's applied DC voltage spec.
---------
> resistor would apply nearly 11V to the mic, if the mic element and the
> series act as a pure voltage divider. The 2.2K ohm resistor would produce 8V
> at the mic.
--------
>
> The KSB2 schematic shows a 2.2uF electrolytic cap between the MIC AF and the
> rest of the KSB2, so I wouldn't think there's any risk of a short.
>
--
David Woolley
"we do not overly restrict the subject matter on the list, and we
encourage postings on a wide range of amateur radio related topics"
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