I am trying to connect an old-fashioned 4-pin modular analog telephone
handset to be used with a 3-conductor universal 2.5mm cell phone
headset jack. I'm almost there but I got it wrong. Help me if you
can...

(You can see http://www.pokia.com to get an idea of what I'm trying to
do: yes, an old-fashioned handset for a cellphone.)

What I have done results in audible speaker volume, but the microphone
part of the headset results in very very faint to undetectable gain to
the listener. I also had reports of 'interference', which can be
anything from RFI (unlikely) to clipping, (or just a weak solder).



Please critique:

The standard 2.5mm cell phone headset jack has a common ground (for
both microphone and speaker) on the sleeve (outside/base ring), and
the positive for the microphone on the tip, and the positive for the
speaker on the middle (ring).

The 4-pin modular handset has two separate conductors each for the
microphone and speaker: 4 conductors, not a common ground.


Therefo Here's what I did: I simply shorted two of the conductors
on the modular (one coming from the speaker and the other from the
microphone, but otherwise chosen at random), and wired that to the
sleeve (of a 3-conductor 2.5mm plug), wiring the left over microphone
wire to the tip, and the left over speaker wire to the ring.


Do I need to have something more complex? Or should I have chosen a
different two conductors to short as a common ground (I didn't think
polarity was critical in analog handsets).

NOTES: Cell phone headsets usually have (powered) electret mikes, and
the typical impedance of the speakers between cell handsfree headsets
& analog handsets is different (I think analog handsets have lower
impedance). Also, I'm trying to do the direct opposite of:
http://www.siliconchip.com.au/cms/A_30528/article.html which
describes how to use a cell phone headset with an analog 4-conductor
modular phone line.

thanks...

"Santaduck" wrote in message
m...
I am trying to connect an old-fashioned 4-pin modular analog telephone
handset to be used with a 3-conductor universal 2.5mm cell phone
headset jack. I'm almost there but I got it wrong. Help me if you
can...

(You can see http://www.pokia.com to get an idea of what I'm trying to
do: yes, an old-fashioned handset for a cellphone.)

What I have done results in audible speaker volume, but the microphone
part of the headset results in very very faint to undetectable gain to
the listener. I also had reports of 'interference', which can be
anything from RFI (unlikely) to clipping, (or just a weak solder).



Please critique:

The standard 2.5mm cell phone headset jack has a common ground (for
both microphone and speaker) on the sleeve (outside/base ring), and
the positive for the microphone on the tip, and the positive for the
speaker on the middle (ring).

The 4-pin modular handset has two separate conductors each for the
microphone and speaker: 4 conductors, not a common ground.


Therefo Here's what I did: I simply shorted two of the conductors
on the modular (one coming from the speaker and the other from the
microphone, but otherwise chosen at random), and wired that to the
sleeve (of a 3-conductor 2.5mm plug), wiring the left over microphone
wire to the tip, and the left over speaker wire to the ring.


Do I need to have something more complex? Or should I have chosen a
different two conductors to short as a common ground (I didn't think
polarity was critical in analog handsets).

NOTES: Cell phone headsets usually have (powered) electret mikes, and
the typical impedance of the speakers between cell handsfree headsets
& analog handsets is different (I think analog handsets have lower
impedance). Also, I'm trying to do the direct opposite of:
http://www.siliconchip.com.au/cms/A_30528/article.html which
describes how to use a cell phone headset with an analog 4-conductor
modular phone line.

thanks...

"Santaduck" wrote ...
I am trying to connect an old-fashioned 4-pin modular analog telephone
handset to be used with a 3-conductor universal 2.5mm cell phone
headset jack. I'm almost there but I got it wrong. Help me if you
can...
......
Do I need to have something more complex? Or should I have chosen a
different two conductors to short as a common ground (I didn't think
polarity was critical in analog handsets).

NOTES: Cell phone headsets usually have (powered) electret mikes,

And telephone handsets of that era had carbon (or carbon equivalent)
microphones. Your problem would appear to be interfacing a carbon
microphone to the typical electret mic input. Circuits for replacing
carbon with electret (which is what is inside the typical carbon-
equivalent mic cartridge) have been around for decades, but not for
the other direction since it is (as you say) pretty "retro" and "niche".

I'd guess that carbon (and most likely carbon-equivalent) mics
need considerably more DC "bias" (probably both voltage AND
current) to operate properly than an electret mic. I believe there
are websites that discuss the theory and operation of carbon mics
if you were interested. Tomi Holander's website is a terrific
source of both theoretical and practical info on this kind of topic.
http://www.epanorama.net/index.php

Your best bet is likely replacing the mic cartridge with an electret
mic element (which are *very* inexpensive.) Only problem I forsee
is that the coiled handset cord is unshelded and may not be suitable
for an electret (or dynamic for that matter) mic. In the case of cell-
phone use, this may be more of a problem for RFI than for hum
and noise, etc. Some judicial use of RF-filtering caps and/or
inductors might be able to solve this, though.

and
the typical impedance of the speakers between cell handsfree headsets
& analog handsets is different (I think analog handsets have lower
impedance).

I don't think that is anywhere near as much an issue as the microphone
mismatch. Your own experiment appears to support this theory.

Your old handset is using a carbon microphone. The cell phone needs an
electret microphone. You may be able to find a way to substitute the
microphone. It would have to mechanically fit. This will probably take some
customization. Take care for the supply polarization to the electret
microphone.

In substituting parts like this, there is also the concern for RF
interference, and safety under some conditions. Take care that whatever you
are doing, especially with a cell phone, that you are in proper compliance.
All attachments for celliar phones must meet proper safety and RF radiation
performance specifications.



--

Jerry G. GLG Technologies GLG
==========================


"Santaduck" wrote in message
m...
I am trying to connect an old-fashioned 4-pin modular analog telephone
handset to be used with a 3-conductor universal 2.5mm cell phone
headset jack. I'm almost there but I got it wrong. Help me if you
can...

(You can see http://www.pokia.com to get an idea of what I'm trying to
do: yes, an old-fashioned handset for a cellphone.)

What I have done results in audible speaker volume, but the microphone
part of the headset results in very very faint to undetectable gain to
the listener. I also had reports of 'interference', which can be
anything from RFI (unlikely) to clipping, (or just a weak solder).



Please critique:

The standard 2.5mm cell phone headset jack has a common ground (for
both microphone and speaker) on the sleeve (outside/base ring), and
the positive for the microphone on the tip, and the positive for the
speaker on the middle (ring).

The 4-pin modular handset has two separate conductors each for the
microphone and speaker: 4 conductors, not a common ground.


Therefo Here's what I did: I simply shorted two of the conductors
on the modular (one coming from the speaker and the other from the
microphone, but otherwise chosen at random), and wired that to the
sleeve (of a 3-conductor 2.5mm plug), wiring the left over microphone
wire to the tip, and the left over speaker wire to the ring.


Do I need to have something more complex? Or should I have chosen a
different two conductors to short as a common ground (I didn't think
polarity was critical in analog handsets).

NOTES: Cell phone headsets usually have (powered) electret mikes, and
the typical impedance of the speakers between cell handsfree headsets
& analog handsets is different (I think analog handsets have lower
impedance). Also, I'm trying to do the direct opposite of:
http://www.siliconchip.com.au/cms/A_30528/article.html which
describes how to use a cell phone headset with an analog 4-conductor
modular phone line.

thanks...