[Elecraft] Bias and Dynamic Mics

[Jim Brown]

On Wed,11/4/2015 5:43 AM, Charlie T, K3ICH wrote:
> If you ever wondered why a bias voltage can cause
> distortion in a dynamic microphone element, just
> consider what happens when you connect a DC source
> to a speaker.  The voice coil move in one
> direction depending on the polarity.  Now think of
> a dynamic mic element doing the same thing.

This is one of those ideas that, while having a solid theoretical basis, 
doesn't hold up once you plug numbers into the problem. That's because 
the bias voltage is typically provided from a rather high impedance 
source, so the bias current is too low and the sound pressure level is 
too low to drive the diaphragm anywhere near its limits.

The simple fact is that it's virtually impossible to overload a dynamic 
mic. One major mic manufacturer advertised that their dynamic mics could 
reproduce a gunshot without distortion. It wasn't a lie -- almost any 
pro dynamic mic can do that. When you hear distortion from a dynamic 
mic, it's the mic producing enough output to overload the mic preamp, 
and the solution is to either reduce the preamp gain or add a pad 
(passive attenuator). Controls to do both are built into the mixers used 
for live sound, broadcast, and recording.

73, Jim K9YC