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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
