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nuckolls.bob(at)aeroelect
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 Posted: Fri May 17, 2013 6:15 pm Post subject: Back when they were still carbon dust . . . |
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| Quote: | | The problem is not a nonlinear element, it's just that the circuit runs out of output current. There are two common input impedances for aircraft audio circuits, set by the value of the resistor feeding "bias" current to the mic at either about 150 or about 600 ohms. Some amplified dynamic mics have difficulty driving the 150 ohm variant without clipping. |
Just for grins, I dug out the service manual I have on
a Narco VT-1 to see how they biased up the microphone
for this venerable tube radio
[img]cid:7.1.0.9.0.20130517210346.01e80a40(at)aeroelectric.com.0[/img]
The mic bias comes off the cathode of V403 (Class-A
speaker/headphone amp and transmitter modulator). It
goes through a couple of rc filters before joining
to the mic input line at the junction of R404/R426.
The manual tells us there's 10v at the cathode of
V403 in transmit but not what to expect on the mic
line. Mic audio is routed up through R426 to the
RF cold side of a TRF transformer for the marker
beacon RF. This means that V402 doubles as detector
for Marker Beacon and MIC pre-amp! Clever. VHF
receive audio comes into to the junction of C406/R410.
So back in the days when they were still carbon, 10v
through 1500 ohms would bias the critters up. I wonder
how a modern pseudo-carbon mic would work with that
radio. I'll brass-board it up next time I have my
headsets out . . .
Bob . . .
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millner(at)me.com
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| Posted: Sat May 18, 2013 9:32 am Post subject: Back when they were still carbon dust . . . |
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