Building adapters

Many times over the years I have wanted to make an adapter from one
jack to another. Inevitably I end up wondering if I need to add a
resister or capacitor to the usually obvious connections. Can someone
explain how I should be thinking about this and how to figure out what
discrete component I may or may not need? When I look up information
about each end of the connection, all I usually find is an impedance
rating for each. (I haven't looked impedances up yet for my example
below.)

The current project I have is to be able to plug my (amplified! I'm
hearing impaired) cell phone headset (1/8" stereo jack: one conductor
for the microphone, one for the mono headphones, and one for ground)
into my computer, which takes the (typical) 1/8" stereo jack normally
used for stereo speakers and a 1/8" mono jack for a microphone input.

Using the following numbering scheme...

Headset jack: (1) sheath/base = ground, (2) middle conductor =
microphone, (3) end conductor = mono headphone. Stereo computer
speaker jack: (4) sheath/base = ground, (5) middle conductor = Left
speaker, (6) end conductor = Right speaker. Mono computer microphone
jack: (7) sheath/base = ground, and (8) end conductor = microphone.

My oversimplistic approach would be to connect (1) to (4) & (7), (2)
to (8), and (3) to (5) & (6).

I might have the positive conductors switched on the stereo
connections (I haven't checked for which is which), but I think this
example makes clear what my simplistic approach is, and may be useful
for someone's response. My thoughts always run toward "impedance
matching" (whatever that is), but I don't really know enough to run
with it from there.

Thanks for any help anyone can give!

Greg

 

[Dan Coby]

greg1@goldfinches.org wrote:

Many times over the years I have wanted to make an adapter from one
jack to another. Inevitably I end up wondering if I need to add a
resister or capacitor to the usually obvious connections. Can someone
explain how I should be thinking about this and how to figure out what
discrete component I may or may not need? When I look up information
about each end of the connection, all I usually find is an impedance
rating for each. (I haven't looked impedances up yet for my example
below.)

The current project I have is to be able to plug my (amplified! I'm
hearing impaired) cell phone headset (1/8" stereo jack: one conductor
for the microphone, one for the mono headphones, and one for ground)
into my computer, which takes the (typical) 1/8" stereo jack normally
used for stereo speakers and a 1/8" mono jack for a microphone input.

Using the following numbering scheme...

Headset jack: (1) sheath/base = ground, (2) middle conductor =
microphone, (3) end conductor = mono headphone. Stereo computer
speaker jack: (4) sheath/base = ground, (5) middle conductor = Left
speaker, (6) end conductor = Right speaker. Mono computer microphone
jack: (7) sheath/base = ground, and (8) end conductor = microphone.

My oversimplistic approach would be to connect (1) to (4) & (7), (2)
to (8), and (3) to (5) & (6).

I might have the positive conductors switched on the stereo
connections (I haven't checked for which is which), but I think this
example makes clear what my simplistic approach is, and may be useful
for someone's response. My thoughts always run toward "impedance
matching" (whatever that is), but I don't really know enough to run
with it from there.

Thanks for any help anyone can give!

Greg

The major complication with your approach is that you are connecting
the left and right channels from your computer together. The computer's
sound card may not be happy with that situation.

I am deaf in my right ear. As a result, stereo headphones can be a
problem. To use a set of Bose noise canceling headphones, which has
a high impedance input, with my Ipod nano, I created an adapter. The
left an right channels from the Ipod are connected to a pair of 1 kohm
resistors. The other ends of the resistors are connected together and
the junction point are connected to both the left and right channels of
the head phones. The resistors form a simple mixer to merge the left
and right stereo channels in a mono output.

Since you said that your headset is amplified, it may accept the fairly
high impedance of a similar resistor mixer. You may need to go to a
lower value for the resistors. The details depend upon the specifics
of your headset.


Dan