/The impedance matcher that I purchased was to allow two
/different manufacturers headsets to work on the same
/radio/intercom system without transmitting problems. Now
/I understand that on newer systems, the impedance matching
/circuit is built into the radio, but on the old ones it is
/not, and therefore requires two headsets if equal impedance.
/I also assume that we are talking about microphone impedance
/as the gizmo plugs into the mike input, and the mike jack
/plugs into the gizmo.
Aha!!!! now I understand. I thought we were talking about a headset
system. I'll bet it has to do with the voltage drop across dissimilar
microphones. A bit of history . . . .
Like all things having to do with aviation, certifciation and
regulation has plowed us into technological ruts while the rest
of the world enjoys new and improved products on a cycle of every
few years.
Microphone circuits in aircraft radios are carry-overs from the
very FIRST aircraft radios that used carbon granule microphones
not unlike those used in telephones for about the past 100 years.
These microphones looked like a variable resistor who's value
wobbled around at a audio rate. To convert this resitance wobble
into a voltage wobble, a small amount of current had to be impressed
across the microphone. In the very early days, an audio transformer
allowed a few volts d.c. to be applied to the microphone while running
the current through the primary. The secondary drove the grid of the
first audio stage vacuum tube. Later, transistorized "matching"
circuits were readily applied to accomodate the carbon microphone's
power requirements . . . in spite of the fact that the carbon microphone
went the way of the dinosaur a long time ago.
Modern noise canceling microphones are electret elements and quite
suited to interface with a more contemporary audio systems but for
the sake of "standarization", manufacturer's are obliged to make
modern microphones look electrically like antique microphones
so they will function well with aircraft radios.
The problem you are working with is most certainly driven by the
fact that there are no standards for uniformity from one manufacturer's
microphone to another for votlage dropped across the microhone's
electronics when biased up with the somewhat standard audio input
circuits for most transmitters. So, when you parallel dissimilar
microphones, the one that draws the most current and/or operates
at a lower voltage "steals" bias current away from the other microphone.
I believe what you have is a (1) battery to provide an independent source
of microphone bias. (2) a resistor to allow d.c. energy to be coupled to
the microphone circuit without seriously loading the audio output.
(3) a capacitor to couple just the audio component of the microphone signal
over to the radio without upsetting the bias to the microphone . . .
IRRESPECTIVE of the characteristics of other microphones that share
the input.
/The funny thing is, that it actually seems to work, but
/I must admit, I don't really know why. To be honest, having
/spent all that money, I feel a little reluctant to prise the
/circuit board off of the double sided tape to see if there
/is anything underneath the board, for fear of tearing the
/tracks and ending up with an overpriced battery, box and
/capacitor that no longer works.
Yes, this circuit does make sense and yes, you could have built it for
a very little cost. I rather suspect there is nothing on the other
side of the etched circuit board you mentioned. I'll guess that the
resistor is on the order of 100 ohms (about 50 ma of bias to the microphone
assuming half of the 9 volt battery is dropped across the resistor).
Any electrolytic capacitor on the order of 10uF or more would suffice
for audio coupling.
Bob . . .
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