[Elecraft] Koss SB-45 Headset, update on comfort

[Jim Brown]

On Wed,1/6/2016 11:02 AM, Augie "Gus" Hansen wrote:
> Do you know why Elecraft calls the power provided to an electret 
> element BIAS? Unlike the large form condenser microphone elements that 
> need both polarizing potential and preamp power, usually provided via 
> a "phantom power" circuit, an electret element is permanently 
> polarized. The power applied via the phantom circuit is actually 
> providing only the drain voltage for the FET preamp in the electret 
> microphone capsule, not any kind of biasing voltage. 

That's exactly the reason, Gus -- electret elements have VERY high 
output Z, so they have a built-in FET follow circuit to get the source Z 
down to the level that it can drive the load of an audio stage. The 
circuit it's driving provides a positive voltage to operate that FET, 
through a resistor, typically around 5K, that serves as the load Z for 
the FET follower.

And, BTW -- pro electret mics, whose outputs are balanced, also include 
preamps with output Z designed to drive pro mic input stages. The output 
Z of the preamps built into those pro mics is typically in the range of 
100 - 400 ohms.

73, Jim K9YC