[Elecraft] Mics for the K-3

[David Woolley]

Joe Subich, W4TV wrote:
> There are two means of supplying bias for an electret (or preamplified) 
> microphone.  The first, used by Elecraft in the K3 and by Icom, is to 
> simply add DC voltage to the microphone "hot" lead.  The second, used 
> by the rest of the world is to use a "power adder" circuit to the mic 
> line and supply the bias externally.

I'm not clear what you mean by the Elecraft K3 method here; are you 
saying that there is a DC path from input pin of the first active 
element in the rig's microphone amplifier, or are you simply saying that 
the bias is injected at the plug end of the cable.  In the latter case, 
this is also the technique used by typical PC sound card/electret 
microphone combinations.  Although the bias is fed to the ring in the 
jack, the ring and tip are actually interconnected.  (I think the reason 
for this is so that the same socket will work with dynamic microphones, 
with a mono jack.)

I've checked the schematics, and there doesn't seem to be a DC path.

> 
> The power adder consists of a capacitor between the mic element and the 

In the case of the K2, that capacitor is internal to the rig, but does 
exist.  It's also the case for the K3, the capacitor is C17 on the third 
page of the KIO3 schematic.

(It may well make a difference at RF, where the inductance of the cable 
is signficant.)

> Yaesu/TenTec/Flex-radio) or even a battery.  The resistor is not there 
> for voltage dropping, it is there to limit the current to the mic 
> element to a safe value and to prevent the internal impedance of the 

As I understand the way that electret microphones work, you must not use 
a resistor value that effectively limits the current, as the FET would 
be operating in its cutoff region, which is not the ideal region in 
which to operate (although not as bad as bipolar transistors, which 
would have particularly bad second harmonic distortion).

> power supply, particularly any filter or bypass capacitor, from 
> loading the mic (shunting the output). 

That's generally true, provided the amplifier input impedance is 
relatively low.  If its high, the need to avoid cutting off the FET will 
limit the value of feed resistor and it may actually become the 
effective load on the microphone.

The ideal power feed would have very high AC impedance and very low DC 
resistance, but a choke would be much more expensive than a resistor, to 
the extent that it is not even really worth analyzing whether the 
circuit would be stable, and immune from mains hum.

-- 
David Woolley
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