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Headphone impedance

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peter(at)sportingaero.com
Guest

Posted: Tue Nov 30, 2010 1:36 pm Post subject: Headphone impedance

Hi,



I have a question concerning headset impedance, and what will work. I am 

trying to build a helmet for use in an aerobatic airplane, the principal 

requirement is to be as thin as possible on top of my head. I have found 

a parachuting helmet that seems ideal - and is very comfortable. Now I 

want to add comms. I was going to use a Halo headset, but its a problem 

sticking the ear plugs in your ears before putting the helmet on. At the 

weekend I was at a motorcycle show and was talking with an intercom 

maker. He sold me a kit to add comms to a motorcycle helmet, including 

an electret boom mic and a pair of thin speakers that fix inside the 

helmet. He didn't know the impedance of the speakers, but suspects they 

are quite low. I said I needed 600 ohms - his reply was that it wouldn't 

be a problem. The audio system is quite simple - Becker AR4201 straight 

to the headset. The Becker manual says:

Receiver data ... Rated output: for speaker operation:

at 13.75 V nominal operating voltage >= 3 W at 4 ohms

at 10.0 V operating voltage (emergency mode) >=  1.5 W at 4 ohms

with headset connected:

at 13.75 V nominal operating voltage >= 100 mW at 600 ohms

at 10.0 V operating voltage (emergency mode) >=  30 mW at 600 ohms



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?) Are all electret mics the same?



Thanks, Peter

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Tue Nov 30, 2010 6:46 pm Post subject: Headphone impedance

Looks to me your receiver requires a 4 ohm impedance.  BTW you have to be

careful not to confuse impedance with resistance even though they are

related.  Most low impedance headphones are 4 Ohms but check it should be

written on the phones somewhere.  A mismatch of impedance can result in poor

clarity and in some instances problems with the audio output transistors of

the radio.



Noel



--

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

rjquillin(at)cox.net
Guest

Posted: Tue Nov 30, 2010 10:49 pm Post subject: Headphone impedance

At 13:31 11/30/2010, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Peter Pengilly <peter(at)sportingaero.com>



 Hi,



 I have a question concerning headset impedance, and what will work. I am trying to build a helmet for use in an aerobatic airplane, the principal requirement is to be as thin as possible on top of my head. I have found a parachuting helmet that seems ideal - and is very comfortable. Now I want to add comms. I was going to use a Halo headset, but its a problem sticking the ear plugs in your ears before putting the helmet on. At the weekend I was at a motorcycle show and was talking with an intercom maker. He sold me a kit to add comms to a motorcycle helmet, including an electret boom mic and a pair of thin speakers that fix inside the helmet. He didn't know the impedance of the speakers, but suspects they are quite low. I said I needed 600 ohms - his reply was that it wouldn't be a problem. The audio system is quite simple - Becker AR4201 straight to the headset. The Becker manual says:

 Receiver data ... Rated output: for speaker operation:

 at 13.75 V nominal operating voltage >= 3 W at 4 ohms

 at 10.0 V operating voltage (emergency mode) >=  1.5 W at 4 ohms

 with headset connected:

 at 13.75 V nominal operating voltage >= 100 mW at 600 ohms

 at 10.0 V operating voltage (emergency mode) >=  30 mW at 600 ohms 	


  From your post, it is unclear if you plan on connecting your helmet speakers to the speaker output, or the headphone output.  It would be inappropriate to connect them to the HP output, as they would excessively load the receiver if connected to an output designed to feed a 600 ohm load.



 Were you to connect them to the speaker output, loading would not be an issue, but you may find level control to be difficult as the speakers would be much closer to your ear than cabin mounted transducer(s).  Some series resistance would likely be helpful and desired when connected to the speaker output, but used as helmet headphones.



 Additionally, if the radio provides speaker muting during xmit, you would have no 'sidetone'.  Something else to consider.



 [quote]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?) Are all electret mics the same?



 Thanks, Peter[b]

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Dec 01, 2010 10:54 am Post subject: 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.

 [img]cid:.0[/img]



 

 

   Bob . . .

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rjquillin

Joined: 13 May 2007
Posts: 123
Location: KSEE

Posted: Wed Dec 01, 2010 11:24 am Post subject: Headphone impedance

Bob,



  From the original post I interpreted, perhaps incorrectly, that these would be connected to the HP output.

  	  Quote: 	
		  Do I connect both speakers to the tels output 	


 and based my reply on that assumption.  And as assumptions are fraught with issues, asked:

  	  Quote: 	
		  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:

 [quote]From: "Robert L. Nuckolls, III" <nuckolls.bob(at)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.[b]

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Dec 01, 2010 7:33 pm Post subject: Headphone impedance

At 01:20 PM 12/1/2010, you wrote:

 	  Quote: 	
		  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 . . .

rjquillin(at)cox.net
Guest

Posted: Wed Dec 01, 2010 11:47 pm Post subject: 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:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>



 At 01:20 PM 12/1/2010, you wrote:

  	  Quote: 	
		  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 . . . 



 Month --

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nuckolls.bob(at)aeroelect
Guest

Posted: Thu Dec 02, 2010 10:43 am Post subject: Headphone impedance

At 01:43 AM 12/2/2010, you wrote:

  	  Quote: 	
		  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. 	


    Yeah . . . those recommendations were written

    based on magnetic interference studies done

    many moons ago when it was common practice

    to mount a speaker in the cabin overhead.

    Believe me, those were CHEAP speakers and

    not terribly well shielded magnetically.

    Further, there was no way you could get 1.3M

    separation from the whisky compass . . . but

    that's another problem.



  	  Quote: 	
		   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. 	


    It may well be that the speaker/headset configuration

    Peter is proposing won't drive from the headset

    output of the radio. In this case, some amplification

    is called for.



    I've been using a number of pre-packaged

    amplifiers off ebay . . . very reasonably

    priced. Here's an example that can be

    had for $6 postage paid.



  http://tinyurl.com/3a6oy7x 



    It needs to be boxed and power conditioned

    for attaching to a DC bus . . . and probably

    some gain setting resistors . . . but 

    a quick and dirty way to get substantial

    boost to a stereo headset using low

    impedance "speakers".



    Thanks for the manual. I've added it to my

    manufacturer's data library.



    Bob . . .



 

     [quote][b]

rjquillin(at)cox.net
Guest

Posted: Thu Dec 02, 2010 11:06 am Post subject: Headphone impedance

At 10:39 12/2/2010, you wrote:

  	  Quote: 	
		     Thanks for the manual. I've added it to my

    manufacturer's data library.



    Bob . . .

 	


 Always glad to add to the community library!



 tnx

 Ron    [quote][b]

peter(at)sportingaero.com
Guest

Posted: Thu Dec 02, 2010 2:02 pm Post subject: Headphone impedance

Thanks for all of the input!

     We've had 6" of snow over the last couple of days, so getting into     the shop is not so easy - measuring the resistance of the speakers     will have to wait for a few days. They are 30mm in diameter and 8mm     thick, but don't have any kind of identifying marks on them.

     I would like to use the standard headphone output from the radio as     its already wired in - and I didn't even wire up the speaker pin     from the radio, that would also make the set up transferable to     other aircraft. It also occurred to me that the standard radio will     drive at least 2 headsets in parallel - so a combined impedance of     300 ohms - but Ron got there first!

     

     To summarize what has been said,

     - Its likely that an aviation radio headset output would overdrive     these speakers (resulting in poor volume control and possibly poor     quality output), and may suffer some damage to the radio audio     amplifier (unlikely).

     - To use these speakers/phones I will probably need an audio     isolation amplifier (made as described in Bob's note) 

     - Alternatively an amp from ebay may suffice ( for $6 seems a no     brainer   ) -     but may require some power conditioning (interesting that specs say     4.5v, but legend on board says 12v), and also some gain setting     resistors [how do I calculate the value?]

     - To use the electret mic I will have to build a circuit per Bob's     sketch.

     

     I think there are are potentially 3 ways ahead.

     1. Buy amp above and start making mic circuit (perhaps gives the     ability to add an ipod input in the future)

     2. Cannibalise an old headset (at least all the components should     match)

     3. Use the Halo headset for the time being and just figure out     sticking the ear plugs in my ears with the helmet around my neck ...

     

     Thanks for your help, Peter

     

     PS I also found these to provide some eye protection 

     http://www.davida.co.uk/type.php?id=Visor_D4Vi9A

     

     On 02/12/2010 18:39, Robert L. Nuckolls, III wrote:     [quote]       At 01:43 AM 12/2/2010, you wrote:

        	  Quote: 	
		  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. 	
       

          Yeah . . . those recommendations were written

          based on magnetic interference studies done

          many moons ago when it was common practice

          to mount a speaker in the cabin overhead.

          Believe me, those were CHEAP speakers and

          not terribly well shielded magnetically.

          Further, there was no way you could get 1.3M

          separation from the whisky compass . . . but

          that's another problem.

       

        	  Quote: 	
		           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. 	
       

          It may well be that the speaker/headset configuration

          Peter is proposing won't drive from the headset

          output of the radio. In this case, some amplification

          is called for.

       

          I've been using a number of pre-packaged

          amplifiers off ebay . . . very reasonably

          priced. Here's an example that can be

          had for $6 postage paid.

       

                  http://tinyurl.com/3a6oy7x 

       

          It needs to be boxed and power conditioned

          for attaching to a DC bus . . . and probably

          some gain setting resistors . . . but 

          a quick and dirty way to get substantial

          boost to a stereo headset using low

          impedance "speakers".

       

          Thanks for the manual. I've added it to my

          manufacturer's data library.

       

          Bob . . .

       

       

        	  Quote: 	
		  



	
     [b]

peter(at)sportingaero.com
Guest

Posted: Thu Dec 02, 2010 3:17 pm Post subject: Headphone impedance

Also found user's guide for the ebay amp

     

     http://www.sure-electronics.net/download/AA-AB3213X_Ver1.0_EN.pdf

     

     catalogue here

     

     http://www.sureelectronics.net/Amplifier.pdf

     

     Peter

     

     On 02/12/2010 21:58, Peter Pengilly wrote:     [quote]              Thanks for all of the input!

       We've had 6" of snow over the last couple of days, so getting into       the shop is not so easy - measuring the resistance of the speakers       will have to wait for a few days. They are 30mm in diameter and       8mm thick, but don't have any kind of identifying marks on them.

       I would like to use the standard headphone output from the radio       as its already wired in - and I didn't even wire up the speaker       pin from the radio, that would also make the set up transferable       to other aircraft. It also occurred to me that the standard radio       will drive at least 2 headsets in parallel - so a combined       impedance of 300 ohms - but Ron got there first!

       

       To summarize what has been said,

       - Its likely that an aviation radio headset output would overdrive       these speakers (resulting in poor volume control and possibly poor       quality output), and may suffer some damage to the radio audio       amplifier (unlikely).

       - To use these speakers/phones I will probably need an audio       isolation amplifier (made as described in Bob's note) 

       - Alternatively an amp from ebay may suffice ( for $6 seems a no       brainer   ) -       but may require some power conditioning (interesting that specs       say 4.5v, but legend on board says 12v), and also some gain       setting resistors [how do I calculate the value?]

       - To use the electret mic I will have to build a circuit per Bob's       sketch.

       

       I think there are are potentially 3 ways ahead.

       1. Buy amp above and start making mic circuit (perhaps gives the       ability to add an ipod input in the future)

       2. Cannibalise an old headset (at least all the components should       match)

       3. Use the Halo headset for the time being and just figure out       sticking the ear plugs in my ears with the helmet around my neck       ...

       

       Thanks for your help, Peter

       

       PS I also found these to provide some eye protection 

       http://www.davida.co.uk/type.php?id=Visor_D4Vi9A

       

       On 02/12/2010 18:39, Robert L. Nuckolls, III wrote:        	  Quote: 	
		   At 01:43 AM 12/2/2010, you wrote:

          	  Quote: 	
		  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. 	
         

            Yeah . . . those recommendations were written

            based on magnetic interference studies done

            many moons ago when it was common practice

            to mount a speaker in the cabin overhead.

            Believe me, those were CHEAP speakers and

            not terribly well shielded magnetically.

            Further, there was no way you could get 1.3M

            separation from the whisky compass . . . but

            that's another problem.

         

          	  Quote: 	
		   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. 	
         

            It may well be that the speaker/headset configuration

            Peter is proposing won't drive from the headset

            output of the radio. In this case, some amplification

            is called for.

         

            I've been using a number of pre-packaged

            amplifiers off ebay . . . very reasonably

            priced. Here's an example that can be

            had for $6 postage paid.

         

          http://tinyurl.com/3a6oy7x 

         

            It needs to be boxed and power conditioned

            for attaching to a DC bus . . . and probably

            some gain setting resistors . . . but 

            a quick and dirty way to get substantial

            boost to a stereo headset using low

            impedance "speakers".

         

            Thanks for the manual. I've added it to my

            manufacturer's data library.

         

            Bob . . .

         

         

          	  Quote: 	
		  



	
       



	
     [b]

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Dec 02, 2010 5:02 pm Post subject: Headphone impedance

At 05:11 PM 12/2/2010, you wrote:

  	  Quote: 	
		  Also found user's guide for the ebay amp



  http://www.sure-electronics.net/download/AA-AB3213X_Ver1.0_EN.pdf

 

 catalogue here



  http://www.sureelectronics.net/Amplifier.pdf

 	


   Thanks for the links. I've used several devices

   from this firm. They're an excellent value and

   perform as advertised.



 

   Bob . . .    [quote][b]

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Dec 02, 2010 5:19 pm Post subject: Headphone impedance

At 03:58 PM 12/2/2010, you wrote:

Thanks for all of the input!



I would like to use the standard headphone output from the radio as 

its already wired in - and I didn't even wire up the speaker pin from 

the radio, that would also make the set up transferable to other aircraft.



   The only way you can make this 100% transportable is

   to add the necessary electronics to make your headset

   EMULATE the standard headset. I.e., 600 ohms of load.

   Back in the days I was burning my fingers and

   sniffing the smoke over my first soldering iron,

   the 'standard' headset was 2000 ohms impedance.

   These integrated well with popular crystal and vacuum

   tube radios.



   2K headsets are no longer made in volume and expensive.

   However, there are PLENTY of low impedance headsets

   for as little as $1 a pair that work with modern

   entertainment devices. A few years ago, I designed

   a small battery powered amplifier with a 2K input

   impedance and adjusted the gain such that similar

   signals produced the same sounds from the headsets.

   I.e., the modern el-cheapo headsets could be used

   on homebrew projects with similar performance as

   headsets of yesteryear.



  - Its likely that an aviation radio headset output would overdrive 

these speakers



   . . . probably UNDER-DRIVE. The electronics within

   the radio simply may not be capable of delivering

   the necessary energy into the lower impedance

   devices.



   (resulting in poor volume control and possibly poor quality 

output), and may suffer some damage to the radio audio amplifier (unlikely).



   . . . mostly. The probable effect is that the received

   audio will be heard but an severely reduced volume.



- To use these speakers/phones I will probably need an audio 

isolation amplifier (made as described in Bob's note)



   Quite likely. Further, if you're really interested

   in moving across airplanes with this, then your

   "adapter" may want to be a fat lump in the cordage

   for the speaker/microphone set.



- Alternatively an amp from ebay may suffice ( for $6 seems a no 

brainer   ) - but may require some power conditioning (interesting 

that specs say 4.5v, but legend on board says 12v).



   The major concern is for conducted noise and transient

   voltages. You'll want to use a three-terminal regulator

   to "step" down the 14v bus to say 9 volts. The regulator

   will provide the isolation from transients and very good

   filtering of bus noises.



  and also some gain setting resistors [how do I calculate the value?]



   Cut and try during the brass-board phase.



- To use the electret mic I will have to build a circuit per Bob's sketch.



   This too will need to be brass-boarded and checked

   on the bench. Do you have a harness to run the radio

   on the workbench?



I think there are are potentially 3 ways ahead.



1. Buy amp above and start making mic circuit (perhaps gives the 

ability to add an ipod input in the future)



   Recommended.



2. Cannibalise an old headset (at least all the components should match)



   Risky for mechanical issues. If you have mic and

   headset components with a good track record and they

   fit the helmet . . . then 80% of the battle is won.



3. Use the Halo headset for the time being and just figure out 

sticking the ear plugs in my ears with the helmet around my neck ...



   Ugh!



   Bob . . .





   Bob . . .

peter(at)sportingaero.com
Guest

Posted: Fri Dec 03, 2010 10:08 am Post subject: Headphone impedance

Hi Bob,



Thanks very much for all of that - it will certainly keep me busy over 

the next few weeks. This is about on the raggedy edge of my electronics 

capability so should be an interesting learning curve! Yes, installing a 

box in the fly lead between helmet and radio plugs is clearly the way to 

go. The radio connector is a Dsub, so no problems making a test harness.



Peter



On 03/12/2010 01:16, Robert L. Nuckolls, III wrote:

 	  Quote: 	
		   

 <nuckolls.bob(at)aeroelectric.com>



 At 03:58 PM 12/2/2010, you wrote:

 Thanks for all of the input!



 I would like to use the standard headphone output from the radio as 

 its already wired in - and I didn't even wire up the speaker pin from 

 the radio, that would also make the set up transferable to other 

 aircraft.



   The only way you can make this 100% transportable is

   to add the necessary electronics to make your headset

   EMULATE the standard headset. I.e., 600 ohms of load.

   Back in the days I was burning my fingers and

   sniffing the smoke over my first soldering iron,

   the 'standard' headset was 2000 ohms impedance.

   These integrated well with popular crystal and vacuum

   tube radios.



   2K headsets are no longer made in volume and expensive.

   However, there are PLENTY of low impedance headsets

   for as little as $1 a pair that work with modern

   entertainment devices. A few years ago, I designed

   a small battery powered amplifier with a 2K input

   impedance and adjusted the gain such that similar

   signals produced the same sounds from the headsets.

   I.e., the modern el-cheapo headsets could be used

   on homebrew projects with similar performance as

   headsets of yesteryear.



  - Its likely that an aviation radio headset output would overdrive 

 these speakers



   . . . probably UNDER-DRIVE. The electronics within

   the radio simply may not be capable of delivering

   the necessary energy into the lower impedance

   devices.



   (resulting in poor volume control and possibly poor quality output), 

 and may suffer some damage to the radio audio amplifier (unlikely).



   . . . mostly. The probable effect is that the received

   audio will be heard but an severely reduced volume.



 - To use these speakers/phones I will probably need an audio isolation 

 amplifier (made as described in Bob's note)



   Quite likely. Further, if you're really interested

   in moving across airplanes with this, then your

   "adapter" may want to be a fat lump in the cordage

   for the speaker/microphone set.



 - Alternatively an amp from ebay may suffice ( for $6 seems a no 

 brainer   ) - but may require some power conditioning (interesting 

 that specs say 4.5v, but legend on board says 12v).



   The major concern is for conducted noise and transient

   voltages. You'll want to use a three-terminal regulator

   to "step" down the 14v bus to say 9 volts. The regulator

   will provide the isolation from transients and very good

   filtering of bus noises.



  and also some gain setting resistors [how do I calculate the value?]



   Cut and try during the brass-board phase.



 - To use the electret mic I will have to build a circuit per Bob's 

 sketch.



   This too will need to be brass-boarded and checked

   on the bench. Do you have a harness to run the radio

   on the workbench?



 I think there are are potentially 3 ways ahead.



 1. Buy amp above and start making mic circuit (perhaps gives the 

 ability to add an ipod input in the future)



   Recommended.



 2. Cannibalise an old headset (at least all the components should match)



   Risky for mechanical issues. If you have mic and

   headset components with a good track record and they

   fit the helmet . . . then 80% of the battle is won.



 3. Use the Halo headset for the time being and just figure out 

 sticking the ear plugs in my ears with the helmet around my neck ...



   Ugh!



   Bob . . .





   Bob . . .

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