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Headphone power requirements

 
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peter(at)sportingaero.com
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PostPosted: Fri Nov 16, 2012 1:43 pm    Post subject: Headphone power requirements Reply with quote

Hi,

I'm looking for some advice about headphone power requirements as for some time I have been trying to find an in-ear type of headphone to use under my helmet. I have a Halo headset, but if I ever have to jump out something will hurt (but that's better than being dead). I want some in-ear head phones with a straight connector that will dis-connect if I ever jump - the mic side is already sorted using a motorcycle helmet microphone.

I have found an in-ear phone company not too far away who claim between 25 and 40db of attenuation and who also say they provide equipment for aviation use (http://www.ultimateear.com/pdf/SoundEar_data_sheet.pdf). However when I asked if they could provide a set compatible with an output suitable for a nominally 600 ohm headset they were a confused and asked for the radio specs.

I emailed a Becker AR4201 spec sheet that lists the headphone output as,
"at 13.75V nominal operating voltage >= 100mW at 600 ohm ... dropping to 30mW at 10.0V"

Their comment is that their component supplier says,

“14V nominal drive is much higher than any of our receivers are supposed to be driven at. Typical drive voltage is 0.1V which is 12dB below 10% THD. 14 V is 140 times this or about 30dB above 10% THD.”

I interpret the Becker spec to mean that when the bus voltage is 13.75V, the output power is at least 100mW into 600 ohm. And that means the headphones can expect to see 7.7v? Is that correct?

I was expecting the company to offer some kind of matching/attenuating box - but I guess this is beyond their capability ...

Are we talking the same language here?

If Vern Little is reading this and has any of his small amplifiers in a D-sub left on the shelf then I would really like one!

I really do not know much about audio systems, if anyone can provide some advice I would be very grateful. There must be a simple way forward (surely ...).

Regards, Peter



[quote][b]


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nuckolls.bob(at)aeroelect
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PostPosted: Fri Nov 16, 2012 4:45 pm    Post subject: Headphone power requirements Reply with quote

I emailed a Becker AR4201 spec sheet that lists the headphone output as,
"at 13.75V nominal operating voltage >= 100mW at 600 ohm ... dropping to 30mW at 10.0V"

Their comment is that their component supplier says,

“14V nominal drive is much higher than any of our receivers are supposed to be driven at. Typical drive voltage is 0.1V which is 12dB below 10% THD. 14 V is 140 times this or about 30dB above 10% THD.”

These folks didn't understand what they read.

Their .1 v (presumed RMS) for 12 db (1/4 voltage)
below that which produces 10% total harmonic distortion
(diaphragm rattle). All this says nothing about what
you hear. I suspect that the 100 mv into one of those
devices would be painful for a normal hearing person.

Earbuds I have on hand measure about 20 ohms. Making
them to some higher resistance presents some interesting
trade offs. Higher resistance takes more turns of finer
wire (hard). Lower resistance is fewer turns but lower
sensitivity . . . but increased ability to deliver power
from a device operating on a single AA cell.

So if I were to take a WAG, the device you're considering
is about 20 ohms resistance. 100 mv rms into 20 ohms
is 1/2 milliwatt! That much power coupled right to the
ear canal is probably really loud.

I interpret the Becker spec to mean that when the bus voltage is 13.75V, the output power is at least 100mW into 600 ohm. And that means the headphones can expect to see 7.7v? Is that correct?

Yes. The 'problem' is that we don't know what the real
output impedance is for the headset audio port. It's
PROBABLY pretty low . . . a hard connection to some
form of audio amplifier chip. A chip powered at
14v cannot deliver more that 14v pk-pk, or 5 volts
rms. So that 100 mw figure MIGHT be a peak value.
One never knows for sure without measuring.

In any case, your problem is no so much one of
impedance matching for maximum transfer of power
but one of attenuation that (1) avoids over load
of Becker audio output chips and (2) keeps the
vast majority of that 100 mW from being coupled
to your grey matter.

I was expecting the company to offer some kind of matching/attenuating box - but I guess this is beyond their capability ...

Are we talking the same language here?

No . . . in fact, we're not sure what language Becker
is using either.

If Vern Little is reading this and has any of his small amplifiers in a D-sub left on the shelf then I would really like one!

I really do not know much about audio systems, if anyone can provide some advice I would be very grateful. There must be a simple way forward (surely ...).

I'm 99% sure that you can experiment with a simple
series resistor between the Becker output and the
ear piece coil.

Starting with a 620 ohm resistor loads the Becker at
640 ohms. No risk. 7.7 VRMS into 620 gives you 12
mA RMS into the network. 12 ma across your 20
ohm ear piece coil produces 250 millivolts . . .
about 2.5 x that which the company claims some
'optimum' level of performance.

So backing your volume control off probably gets
things into the right church. Further fiddling
with the resistor might be necessary to get into
the right pew.


Bob . . . [quote][b]


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peter(at)sportingaero.com
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PostPosted: Mon Nov 26, 2012 12:59 pm    Post subject: Headphone power requirements Reply with quote

Hi Bob,

Over the weekend I managed to get everything together to hook up an old Terra TX760D to a set of 'ipod' headphones. It all worked!!

The audio quality wasn't very good, but that is probably a function of my power supply, breadboard wiring and poor antenna grounding, but the headphones produced a reasonable noise. They had reasonable volume, from quiet to too loud using the radio volume control. I used a 680 ohm 2W resistor and paralleled the L & R headphone - their resistance was 30 ohms each channel on the multimeter.

Now I will build something into a project box with reasonable shielding and try with my helmet in the aeroplane.

Thanks very much for your suggestion.

Peter

On 17/11/2012 00:45, Robert L. Nuckolls, III wrote:

[quote] I emailed a Becker AR4201 spec sheet that lists the headphone output as,
"at 13.75V nominal operating voltage >= 100mW at 600 ohm ... dropping to 30mW at 10.0V"

Their comment is that their component supplier says,

“14V nominal drive is much higher than any of our receivers are supposed to be driven at. Typical drive voltage is 0.1V which is 12dB below 10% THD. 14 V is 140 times this or about 30dB above 10% THD.”

These folks didn't understand what they read.

Their .1 v (presumed RMS) for 12 db (1/4 voltage)
below that which produces 10% total harmonic distortion
(diaphragm rattle). All this says nothing about what
you hear. I suspect that the 100 mv into one of those
devices would be painful for a normal hearing person.

Earbuds I have on hand measure about 20 ohms. Making
them to some higher resistance presents some interesting
trade offs. Higher resistance takes more turns of finer
wire (hard). Lower resistance is fewer turns but lower
sensitivity . . . but increased ability to deliver power
from a device operating on a single AA cell.

So if I were to take a WAG, the device you're considering
is about 20 ohms resistance. 100 mv rms into 20 ohms
is 1/2 milliwatt! That much power coupled right to the
ear canal is probably really loud.

I interpret the Becker spec to mean that when the bus voltage is 13.75V, the output power is at least 100mW into 600 ohm. And that means the headphones can expect to see 7.7v? Is that correct?

Yes. The 'problem' is that we don't know what the real
output impedance is for the headset audio port. It's
PROBABLY pretty low . . . a hard connection to some
form of audio amplifier chip. A chip powered at
14v cannot deliver more that 14v pk-pk, or 5 volts
rms. So that 100 mw figure MIGHT be a peak value.
One never knows for sure without measuring.

In any case, your problem is no so much one of
impedance matching for maximum transfer of power
but one of attenuation that (1) avoids over load
of Becker audio output chips and (2) keeps the
vast majority of that 100 mW from being coupled
to your grey matter.

I was expecting the company to offer some kind of matching/attenuating box - but I guess this is beyond their capability ...

Are we talking the same language here?

No . . . in fact, we're not sure what language Becker
is using either.

If Vern Little is reading this and has any of his small amplifiers in a D-sub left on the shelf then I would really like one!

I really do not know much about audio systems, if anyone can provide some advice I would be very grateful. There must be a simple way forward (surely ...).

I'm 99% sure that you can experiment with a simple
series resistor between the Becker output and the
ear piece coil.

Starting with a 620 ohm resistor loads the Becker at
640 ohms. No risk. 7.7 VRMS into 620 gives you 12
mA RMS into the network. 12 ma across your 20
ohm ear piece coil produces 250 millivolts . . .
about 2.5 x that which the company claims some
'optimum' level of performance.

So backing your volume control off probably gets
things into the right church. Further fiddling
with the resistor might be necessary to get into
the right pew.


Bob . . .
Quote:

[b]


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nuckolls.bob(at)aeroelect
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PostPosted: Mon Nov 26, 2012 3:11 pm    Post subject: Headphone power requirements Reply with quote

At 02:57 PM 11/26/2012, you wrote:
Quote:
Hi Bob,

Over the weekend I managed to get everything together to hook up an old Terra TX760D to a set of 'ipod' headphones. It all worked!!

The audio quality wasn't very good, but that is probably a function of my power supply, breadboard wiring and poor antenna grounding, but the headphones produced a reasonable noise. They had reasonable volume, from quiet to too loud using the radio volume control. I used a 680 ohm 2W resistor and paralleled the L & R headphone - their resistance was 30 ohms each channel on the multimeter.

Paralleled????? The resistor would go in SERIES with each
headset driver. That 30 ohm load might account for a
less than optimal audio quality . . . you may be overloading
the audio output amplifier in the radio.

Quote:
Now I will build something into a project box with reasonable shielding and try with my helmet in the aeroplane.

It might not warrant a box. Power levels you're
working with are a handful of milliwatts. I think
I'd consider cutting into the headset cord, splicing
1/4w resistors tack soldered in series with each
'ear piece' and covering in heat shrink. Stagger
the resistors along the cord so that you can heat-shrink
each one and then put another shrink over the whole.


Bob . . . [quote][b]


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retasker(at)optonline.net
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PostPosted: Mon Nov 26, 2012 4:01 pm    Post subject: Headphone power requirements Reply with quote

I suspect what he is saying is that he paralleled the L&R of the headphone and inserted the 680 ohm resistor in series
with that.

Dick Tasker

Robert L. Nuckolls, III wrote:
Quote:
At 02:57 PM 11/26/2012, you wrote:
> Hi Bob,
>
> Over the weekend I managed to get everything together to hook up an old Terra TX760D to a set of 'ipod' headphones.
> It all worked!!
>
> The audio quality wasn't very good, but that is probably a function of my power supply, breadboard wiring and poor
> antenna grounding, but the headphones produced a reasonable noise. They had reasonable volume, from quiet to too loud
> using the radio volume control. I used a 680 ohm 2W resistor and paralleled the L & R headphone - their resistance
> was 30 ohms each channel on the multimeter.

Paralleled????? The resistor would go in SERIES with each
headset driver. That 30 ohm load might account for a
less than optimal audio quality . . . you may be overloading
the audio output amplifier in the radio.

> Now I will build something into a project box with reasonable shielding and try with my helmet in the aeroplane.

It might not warrant a box. Power levels you're
working with are a handful of milliwatts. I think
I'd consider cutting into the headset cord, splicing
1/4w resistors tack soldered in series with each
'ear piece' and covering in heat shrink. Stagger
the resistors along the cord so that you can heat-shrink
each one and then put another shrink over the whole.

Bob . . .

*
*


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nuckolls.bob(at)aeroelect
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PostPosted: Tue Nov 27, 2012 3:54 am    Post subject: Headphone power requirements Reply with quote

At 05:59 PM 11/26/2012, you wrote:
Quote:

<retasker(at)optonline.net>

I suspect what he is saying is that he paralleled the L&R of the
headphone and inserted the 680 ohm resistor in series with that.

Aha! But of course . . . yet another example of
how poorly studied words can be the foundation
for a equally poor assessment of fact. Also
a good example of why it's a good thing
to have more than one pair of eyes doing the
reading.

Good call sir!
Bob . . .


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