[Elecraft] K2: connecting a condenser mic

[David Woolley (E.L)]

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.
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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.
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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!
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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.
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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
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> 
> 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.
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> 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.
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> 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.
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> 
> 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
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