Today's Message Index:
----------------------
1. 10:54 AM - Re: Headphone impedance (Robert L. Nuckolls, III)
2. 11:24 AM - Re: Headphone impedance (Ron Quillin)
3. 07:33 PM - Re: Headphone impedance (Robert L. Nuckolls, III)
4. 11:36 PM - Next generation connectors (Robert L. Nuckolls, III)
5. 11:47 PM - Re: Headphone impedance (Ron Quillin)
Message 1
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| Subject: | Re: Headphone impedance |
So the question is, how are these speakers that I am about to stick
into my helmet going to react when the radio output is fed to them?
If their impedance is low I'm guessing that will impact the volume,
so I imagine I will have to turn the radio volume up? It a noisy
airplane and is already up fairly high. Can I add some resistors to
help out? Do I connect both speakers to the tels output (doesn't that
cut the already low impedance in half?)
They're going to be fine . . . in fact, perhaps TOO fine.
1 mW of audio into an efficient headphone is quite audible.
The power delivered to the popular ear-buds is on the order
of hundreds of microwatts.
Check the chapter on audio systems for the significance
of "output impedance" as a listed characteristic of an
accessory. You'll have PLENTY of output capability from
the speaker connections for this radio. In face, you'll
probably want to add some resistance in series with the
headset leads such that a comfortable listening level
is achieved at 1/3 to 1/2 of volume control travel.
Another reader suggested that use of the radio's speaker
output would probalby kill the sidetone feature for
being able to monitor your transmitted signal.
You COULD use an audio isolation amplifier like:
http://www.aeroelectric.com/Catalog/AEC/9009/
Using such a device between your radio's headset
output and the low impedance headphones keeps
the sidetone feature working and takes care of
any integration problems that might be encountered
by direct connection.
Are all electret mics the same?
The microphones are all built on the same technology.
The ELECTRONICS that converts a delta-charge into
some value of delta-voltage is super simple. Bias
the microphone element up through a resistor and take
the audio off the junction between microphone element
and resistor. Aircraft radios are unique amongst voice
communications technologies. WAAaayyy back when, the first
radio-telephone transmitters were fitted with the most
popular microphone technology of the time that was
shared with the telephone.
Like the electret mic, the carbon mike needs to be powered
by it's associated circuitry. The carbon microphone
is a relatively low impedance, variable resistance, high
output device VERY suited to the telephone which takes
operating power from the wires that come into the back
of building.
The electret is a high impedance, variable voltage, low
output device. It also needs to be powered. But to emulate
the legacy carbon microphone, it needs some electronics.
An exemplar circuit is shown below.
Emacs!
Bob . . .
Message 2
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| Subject: | Re: Headphone impedance |
Bob,
From the original post I interpreted, perhaps incorrectly, that
these would be connected to the HP output.
>Do I connect both speakers to the tels output
and based my reply on that assumption. And as assumptions are
fraught with issues, asked:
> From your post, it is unclear if you plan on connecting your helmet
> speakers to the speaker output, or the headphone output?
to clarify the issue.
So, lacking knowledge of the Becker output topology, I'll still stand
by my comments.
Connecting a four, or eight ohm, load to a 600 ohm source could be problematic.
Additionally, if the drivers he is proposing to use are four ohm, and
connected in parallel, they would present a two ohm load to the
output circuit...
At 10:45 12/1/2010, you wrote:
>From: "Robert L. Nuckolls, III" <nuckolls.bob@aeroelectric.com>
>
>So the question is, how are these speakers that I am about to stick
>into my helmet going to react when the radio output is fed to them?
>If their impedance is low I'm guessing that will impact the volume,
>so I imagine I will have to turn the radio volume up? It a noisy
>airplane and is already up fairly high. Can I add some resistors to
>help out? Do I connect both speakers to the tels output (doesn't
>that cut the already low impedance in half?)
>
> They're going to be fine . . . in fact, perhaps TOO fine.
> 1 mW of audio into an efficient headphone is quite audible.
> The power delivered to the popular ear-buds is on the order
> of hundreds of microwatts.
>
> Check the chapter on audio systems for the significance
> of "output impedance" as a listed characteristic of an
> accessory. You'll have PLENTY of output capability from
> the speaker connections for this radio. In face, you'll
> probably want to add some resistance in series with the
> headset leads such that a comfortable listening level
> is achieved at 1/3 to 1/2 of volume control travel.
>
> Another reader suggested that use of the radio's speaker
> output would probalby kill the sidetone feature for
> being able to monitor your transmitted signal.
Message 3
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| Subject: | Re: Headphone impedance |
At 01:20 PM 12/1/2010, you wrote:
>Bob,
>
>So, lacking knowledge of the Becker output topology, I'll still
>stand by my comments.
>Connecting a four, or eight ohm, load to a 600 ohm source could be
>problematic.
>Additionally, if the drivers he is proposing to use are four ohm,
>and connected in parallel, they would present a two ohm load to the
>output circuit...
It's certainly problematic. At the same time
I don't think it represents a hazard to
system components. I think all the specialty
chips designed to drive audio loads (line,
spkr, phones, etc) are inherently overload
protected.
It's unfortunate that the folks who write
black box specifications don't seem to have
a grasp of the notion of "output impedance".
What most installation manuals need to say
is something like . . .
Headphones Output: 1 V rms minimum into
100 ohms. Now, I pulled those numbers out
of the air but the intent is to illustrated
the radio's ability to drive perhaps as many
as 6 pairs of headsets in parallel and still
offer an output voltage equal to or greater
than ear-spltting levels with poor efficiency
headphones. Nobody's output impedance is really
600 ohms. See the first 3 pages of chapter 18.
But it's also true that the radio's headset
audio output device might go into a self-
preserving current limit when presented with the low
impedance loads of the proposed helmet speakers.
So it takes a bit more current to drive these
speakers masquerading as headphones. This COULD
come from the radio's own speaker output or
by using a device like an isolation amplifier.
The headphone/speakers are probably 8 ohm or
greater. I've run across small speakers with
impedances of 20 to 30 ohms. Wiring them in
series would offer at least a 16 ohm load.
It's entirely possible that the headset output
RATED to deliver energy to a 600 ohm load will
work okay with the new configuration.
I think the risks to hardware are low. A
simple experiment will determine the facts.
Peter, have you looked at these speakers
with an ohmmeter?
Bob . . .
Message 4
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| Subject: | Next generation connectors |
>
>Comments/Questions: What is your opinion of sealed connectors verses
>standard Molex? I work in the automotive industry (Honda)and these
>connectors seem to be much more durable and less subject to
>vibration and corrosion. I am building The RV-8 and Would like your thoughts.
The Amp Mate-n-Lock nylon connectors were first
integrated onto single-engine Cessna models about
1968. My relatively short tenure in the electronics
and aviation arts suggested that airplanes were
somehow worthy of technologies having extra-ordinary
quality and robustness. I was understandably
skeptical of the cheap, white plastic connectors.
This was decades before I came to understand failure
tolerant design. I was also relatively ignorant of the
causes/effects in the history of aviation failures.
40 years later we observe that the Mate-n-Lock
connector has probably exceeded anyone's expectations.
Of course any connector is located such that there
is risk for drip or splash gives us pause to consider
special circumstances. But for the most part, the
the perceived quality or robustness of any given
connector has little influence on risks and only
a small influence on cost of ownership.
Some of the modern automotive under-the-hood connectors
for are pretty impressive. I wouldn't discourage
anyone from considering them if they have access to
tools and a source of products.
Bob . . .
Message 5
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| Subject: | Re: Headphone impedance |
Some worthy observations you've made Bob.
I did some digging, and have attached an IM for the unit.
Of interest in section "C" and "D" on pdf page 21:
C. Speaker connection
A 4 to 8 Ohm speaker can be connected to audio output P1- 1AF-asym.
CAUTION : The magnetic field of a speaker influences the magnetic compass.
When choosing the mounting point, a minimum distance of 1.3 m
must be maintained between the speaker and the magnetic compass.
D. Headphone connection
Up to two headphones with an impedance of 600 Ohm can be connected to
the audio output P1 - 2,3 AF-HI/LO.
At 19:29 12/1/2010, you wrote:
><nuckolls.bob@aeroelectric.com>
>
>At 01:20 PM 12/1/2010, you wrote:
>>Bob,
>>
>>So, lacking knowledge of the Becker output topology, I'll still
>>stand by my comments.
>>Connecting a four, or eight ohm, load to a 600 ohm source could be
>>problematic.
>>Additionally, if the drivers he is proposing to use are four ohm,
>>and connected in parallel, they would present a two ohm load to the
>>output circuit...
>
> It's certainly problematic. At the same time
> I don't think it represents a hazard to
> system components. I think all the specialty
> chips designed to drive audio loads (line,
> spkr, phones, etc) are inherently overload
> protected.
>
> It's unfortunate that the folks who write
> black box specifications don't seem to have
> a grasp of the notion of "output impedance".
>
> What most installation manuals need to say
> is something like . . .
>
> Headphones Output: 1 V rms minimum into
> 100 ohms. Now, I pulled those numbers out
> of the air but the intent is to illustrated
> the radio's ability to drive perhaps as many
> as 6 pairs of headsets in parallel and still
> offer an output voltage equal to or greater
> than ear-spltting levels with poor efficiency
> headphones. Nobody's output impedance is really
> 600 ohms. See the first 3 pages of chapter 18.
>
> But it's also true that the radio's headset
> audio output device might go into a self-
> preserving current limit when presented with the low
> impedance loads of the proposed helmet speakers.
>
> So it takes a bit more current to drive these
> speakers masquerading as headphones. This COULD
> come from the radio's own speaker output or
> by using a device like an isolation amplifier.
>
> The headphone/speakers are probably 8 ohm or
> greater. I've run across small speakers with
> impedances of 20 to 30 ohms. Wiring them in
> series would offer at least a 16 ohm load.
> It's entirely possible that the headset output
> RATED to deliver energy to a 600 ohm load will
> work okay with the new configuration.
>
> I think the risks to hardware are low. A
> simple experiment will determine the facts.
>
> Peter, have you looked at these speakers
> with an ohmmeter?
>
>
> Bob . . .
>
>
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