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'[EE]: Telephone headset adapter'
2002\03\15@142258 by Chris Carbaugh

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Hello all,

I'm trying to adapt a headset to be used in place of a phone handset.
The phone in question is a rather old small business style key-phone.

Most off the shelf headsets use a 3 conductor wire, speaker pos, mic
pos, and common ground.

Standard handsets have two ground conductors, isolating the speaker from
the mic.

What is needed to adapt a 4 wire handset output jack to a 3 wire
headset?

If I try to share the ground, I of course short it, and get feedback.

Any thoughts?

Thanks,
Chris

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2002\03\15@232521 by Jim

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Does Radio Shack still sell any small audio
matching or isolation transformers?

One of those should be all you need to
'isolate' one side or the other's ground.

Jim


{Original Message removed}

2002\03\16@151131 by Dwayne Reid

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At 01:30 PM 3/15/02 -0500, Chris Carbaugh wrote:
>Hello all,
>
>I'm trying to adapt a headset to be used in place of a phone handset.
>The phone in question is a rather old small business style key-phone.
>
>Most off the shelf headsets use a 3 conductor wire, speaker pos, mic
>pos, and common ground.
>
>Standard handsets have two ground conductors, isolating the speaker from
>the mic.
>
>What is needed to adapt a 4 wire handset output jack to a 3 wire
>headset?

You have 2 issues here: isolating the common wire between ear and mic on
the headset *and* adapting the mic.  Of these, the mic is the greater problem.

Microphone capsules in standard old-fashioned telephone handsets are carbon
mics.  Their response to speech is a change in resistance.  The microphone
in your headset is a voltage output device.

A simple 2 or 3 transistor pre-amp is required to adapt your headset mic to
the telephone.  A 1:1 audio transformer can be used to isolate the ear piece.

I'm sure the info you need is on the net somewhere but I'm not sure what
search terms to use.  Try searching for the info - if you can't find it,
get back to me next week and I'll see what I have here.  Anything I might
have will be in paper form - a fax is probably the best I can do.  But see
what you can find yourself first.

dwayne


Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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