The square-ish, 4-lead component in the upper right quadrant of the picture below. The body is about 4mm x 4mm, chamfered along the top edge in this photo. It has 3 lines of printed text. The top line reads - ------ + (i.e., minus-sign, bar, plus-sign, with the polarity signs directly beneath the top leads). The second line starts with a stylized 'f' (pretty sure it's a Fairchild device), then AE21. The third line reads (I think) M8105 or M810S.

It is part of a general/business aviation pilot's communications headset. The circuit to the powered electret microphone runs through the device in question. The mic is wired to the two leads on the top side. On the flip side of the board, where the mic leads are soldered in, the side of the circuit labeled "+" on the device is labeled Mic+ on the board. The other top lead, labeled "-" on the device, is labeled "Mic-" on the board. The bottom two terminals are wired to a plug that connects to the aircraft's radio/intercom system. The mic bias is provided through this plug. I am not able to directly measure the voltage and polarity of the voltage across the mic but the spec is 8 - 13 volts. Confusingly, the bottom left lead (seemingly the "-" side of the device) is labeled "M Out" on the board, and the bottom right lead is labeled" M Gnd".

Any help is much appreciated.
Mac

 

The size, number of leads, + and - pin labels...
All point to a bridge rectifier.

See the following datasheet. This is not exact but will point you in a direction.

http://www.mouser.com/ds/2/308/MB4S-1125074.pdf

Check corners away from the + and - labels to see if they both have a ~ (tilde) label.

 

Aha! MB10S. Found this: http://www.mouser.com/ds/2/308/MB10S-1125051.pdf Mine does not have the tildes, but I think that's it.

Trying to figure out why one would put a bridge rectifier in a mic circuit. It wouldn't protect against ESD, would it? Maybe to enforce polarity of the power to the mic? I had understood that aviation electret mics are agnostic as to polarity but maybe this model is an exception.

Anyway, many thanks,
Mac

 

Aha! MB10S. Found this: http://www.mouser.com/ds/2/308/MB10S-1125051.pdf Mine does not have the tildes, but I think that's it.

Trying to figure out why one would put a bridge rectifier in a mic circuit. It wouldn't protect against ESD, would it? Maybe to enforce polarity of the power to the mic? I had understood that aviation electret mics are agnostic as to polarity but maybe this model is an exception.

Anyway, many thanks,
Mac

Maybe the power supply or an automatic gain control (peak detection) of some sort.

 

Maybe the power supply or an automatic gain control (peak detection) of some sort.

It is a bridge rectifier.

Maybe DF01S or DF10S.