[ARC5] The U.K. Aircraft Dynamic Mics (was "T-1154...)
David Stinson
arc5 at ix.netcom.com
Mon Jun 25 18:30:10 EDT 2012
----- Original Message -----
From: "Mike Hanz" <aaf-radio-1 at aafradio.org>
Subject: Re: [Milsurplus] T-1154 Transmitter
> I wish the U.S. experiments with the dynamic microphones had been
> more
> comprehensive - the British used them with great success because of
> their significantly improved intelligibility compared with carbon
> mics.
> It wasn't for lack of trying - the mid-1930s U.S. aircraft sets had
> a
> dedicated interphone amplifier for dynamic mics. The final decision
> seems to have been economic in nature - you save a stage of
> amplification with the carbon mics. .....
Speaking of mics in U.K. sets:
I've found no detailed documentation on the microphones
used in U.K. aircraft.
Can someone enlighten me about the types and specifications
of dynamic mics used in WWII U.K. aircraft?
Are they voltage- or current-driven designs?
They must have had a high output level, given what
it takes to use the "Dynamic" setting on an ART-13.
And also, the specs on the carbon mics used?
Getting audio into the TR9D has been interesting.
The documents I have are "circuit concept" overviews,
not actual schematics. When using a dynamic mic,
the audio from the secondary of the microphone transformer
is coupled to the first audio grid through 300 *pico* Farads.
And if that seems odd- when using a carbon mic,
you remove a jumper which puts *30 picoFarads*
(yep; 30) in series with the 300.
That's 27 picoFarads coupling audio to the amplifiers.
The audio is very "tinny," as one would expect,
but there is a bigger surprise:
With a "standard" telephone mic element, which I use on
every other carbon-mic rig, and the low 2-volt mic supply,
the input is still severely over-driven, even with the 27 pFd
of coupling. I'm guessing it was designed this way
for a low-output throat mic?
Would the cut-off of low frequencies by hi-reactance
coupling be an attempt to deal with
the low-freq noise of the engine?
This all seems strange because the audio input of the
RAAF AT5 transmitter is "conventional"
and works well with a telephone element.
Was the TR9D's carbon mic specific to that set?
I don't consider I've "met the challenge" of a set unless
I've made contacts with it on the air.
For the TR9D, I put 400 ohms in series with
the mic element and bypassed the 27 picoFarad
coupling with 4 mFd.
This brought the level down and made the audio
understandable, but it still sounds... crummy.
More tinkering to do,
if I could get an actual circuit diagram.
Back to work.
73 Dave AB5S
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