Today's Message Index:
----------------------
1. 10:08 AM - Re: Z13/8 all electric airplane (Robert L. Nuckolls, III)
2. 10:26 AM - Re: Microphone gain tuning (D L Josephson)
3. 06:41 PM - Re: Re: Microphone gain tuning (Robert L. Nuckolls, III)
4. 07:15 PM - Back when they were still carbon dust . . . (Robert L. Nuckolls, III)
5. 08:42 PM - S700-2-10 master switch (Sigma Eta Aero)
Message 1
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| Subject: | Re: Z13/8 all electric airplane |
At 07:38 PM 5/16/2013, you wrote:
>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?
>
Either one.
Bob . . .
Message 2
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| Subject: | Re: Microphone gain tuning |
On 5/16/13 11:57 PM, AeroElectric-List Digest Server wrote:
>> >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.
Bob, if that's your experience with the actual circuits in common use,
then by all means it should be fine. The circuit I saw was a simple
emitter follower and not particularly high impedance, asking for more
audio current than it was happy with would be a problem. 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.
And yes, the dynamic noise-canceling mics, usually the M101 or the M87,
are excellent. It takes 40 to 60 dB of gain to bring one of these to
carbon mic level however.
And about the noise canceling element, covering either front or back
will make it into an omnidirectional microphone with a really odd
frequency response.
--
David Josephson
Message 3
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| Subject: | Re: Microphone gain tuning |
At 12:24 PM 5/17/2013, you wrote:
>
>On 5/16/13 11:57 PM, AeroElectric-List Digest Server wrote:
>>> >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.
>Bob, if that's your experience with the actual circuits in common
>use, then by all means it should be fine. The circuit I saw was a
>simple emitter follower and not particularly high impedance, asking
>for more audio current than it was happy with would be a problem.
>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.
Hmmmm . . . I think the examples I encountered were single
transistor devices. This goes back a bit. I think I was
working with some of first Electro-Voice hand-held electret
mics and matching them to head sets of a brand I don't
even remember.
Emitter follower? Oh yeah, PNP upside down so that they'll
bias up with the radio's output.
I guess the only work around with passive components would
involve some series inductance to maintain the DC path
and paralleled with a series resistor/capacitor network
to set attenuation in the AC path.
A bit messier but perhaps doable. But yes, in these days
of every guy and his dog making/selling accessories, interface
conventions are not very 'standard'. Good thing we're working
with OBAM aircraft.
>And yes, the dynamic noise-canceling mics, usually the M101 or the
>M87, are excellent. It takes 40 to 60 dB of gain to bring one of
>these to carbon mic level however.
It seems that the trade offs for mechanical accommodation
for noise cancelling compromises sensitivity. Fortunately
the electronics are simple and cheap so I say "you take
care of the noise, I'll take care of the gain."
>And about the noise canceling element, covering either front or back
>will make it into an omnidirectional microphone with a really odd
>frequency response.
Very few of our fellow travelers are aware of the significance
of the various noise cancelling techniques. Hmmmm . . . that
might make a good topic for a Kitplanes article. I've been
doing some writing for them. Would you like to do one on intricacies
of the aviation microphone? I'll be Paul Dye would be pleased
to have it offered.
Bob . . .
Message 4
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| Subject: | Back when they were still carbon dust . . . |
> 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
Emacs!
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 . . .
Message 5
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| Subject: | S700-2-10 master switch |
I have an Aeroelectric recommended electrical system for a Rotax 912S with
a S700-2-10 master switch that has about 350 hours on it. -I've recently
been seeing the overvoltage protection light coming on periodically so toda
y I went through the troubleshooting checklist for the Rotax alternator and
regulator/rectifier. -When I got to testing the master switch the OVP wo
uld kick on an off when the switch was pulled and wiggled so I'm pretty sur
e the switch is bad. - Is that pretty common and should I be using anothe
r switch that's more robust? -Or are there multiple manufacturers of this
switch, some with better quality than others? --
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