Today's Message Index:
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1. 12:47 AM - Re: Microphone gain tuning (Holger Selover-Stephan)
2. 12:53 AM - Re: Microphone gain tuning (D L Josephson)
3. 07:44 AM - Re: Microphone gain tuning (Robert L. Nuckolls, III)
4. 05:39 PM - Z13/8 all electric airplane (Mauri Morin)
Message 1
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| Subject: | Re: Microphone gain tuning |
Hi Bob, Ken,
Many thanks for the very detailed explanation of the situation. A slap on the forehead
is in order: I have covered up the front-facing side of the microphone,
and thus might have disrupted the noise reduction mechanism. Interestingly,
I mentioned this to the vendor, but it wasn't commented on in their response.
Let me first run tests with the front side uncovered. Then try your resistor-capacitor
network solution, Bob. I really like the simplicity of the latter, just
what I was hoping to hear from you!
I'll report back with further findings. Once this is figured out yet another DIY
headset is going into production for the daughter. The pooch stays on Mutt Muffs
as he rarely contributes anything useful to the conversation.
Thanks again!
Holger
On May 15, 2013, at 9:25 AM, Robert L. Nuckolls, III wrote:
> You can place a resistor-capacitor network ACROSS the
> microphone's leads thusly.
Message 2
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| Subject: | Re: Microphone gain tuning |
Nearly all noise-canceling microphones in aviation headsets these days
(including the Acousticom mentioned) use a single diaphragm electret
capsule with front and back ports just as described in the Wikipedia
entry Bob quoted (I know, I wrote it...) There's a simple circuit,
usually one or two transistors, to interface with the aircraft radio's
carbon mic circuit, provide some gain, and power the FET in the electret
mic. You need to be able to adjust the gain of this circuit. There is a
version of this same mic, the Acousticom 5715, that is adjustable. It
would really be better to get the adjustable version rather than try to
kludge an attenuator.
A resistor and capacitor across the mic output could cause some
distortion, as the mic would then be loaded with a lower impedance than
just the radio input.
Trying to muffle the microphone with some thick material over it is
seldom successful as the frequency response and noise cancelling
properties will be altered significantly.
--
David Josephson
Message 3
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| Subject: | Re: Microphone gain tuning |
>
>
>A resistor and capacitor across the mic output could cause some
>distortion, as the mic would then be loaded with a lower impedance
>than just the radio input.
I've not found this to be the case. The artificial
lowering of radio input impedance has no non-linear
contributions. The amplifiers common to electret-
to-carbon interface are pretty high output impedance
to begin with and 'adjustment' of loading to bring
two microphones into agreement with each other is
not severe.
To be sure, having an adjustable mic cartridge is
the ideal condition, but when a non-adjustable
is what you have in hand, then a little 'jiggering'
of the gain is a low risk experiment.
>Trying to muffle the microphone with some thick material over it is
>seldom successful as the frequency response and noise cancelling
>properties will be altered significantly.
To be sure, effective noise cancelling design is not
a trivial task. Exploiting the designer's skills
is not well served by altering the environment that
the designer targeted. Even so, I've seen some
wide variation in the levels of noise-cancelling
success.
I think the best cartridges I've ever encountered were
the very low impedance, dynamic cartridges common to
military headsets. We were working with aviation
headsets and intercoms on the Cessna O2A and T41
programs back about 1968. I did a dynamic-to-carbon
interface module to connect the military mic to a
121.5 emergency transceiver on the T41. Those were
VERY good microphones but with an output in the 1-2
millivolt range. An interesting task.
Bob . . .
Message 4
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| Subject: | Z13/8 all electric airplane |
Bob,
I'm using your Z-13/8 diagram wiring my RV-8 and want to use a CB Switch
combo to replace the individual circuit breaker and switch for the AUX
ALT ON/OFF SWITCH. The circuit breaker has two terminals labeled line &
load. Which one do I hook to GND?
Mauri Morin
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