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Old 21st August 2012, 03:13 AM   #1
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Lightbulb Combining sources, need to make 3.5mm audio signal go one way...

Hi All,

TL;DR:
What's the most minimalistic (preferably passive) solution which would allow me to combine mono and stereo TRS audio sources without combining L & R channels?

EDIT: ADDED IMAGES IN POST BELOW



Full description:

I'm combining an 4P4C / RJ9 and PC audio source to a single headset (PC and Plantronics S-11, Razer Carcharias), using a RJ9 to 3.5mm TRRS adapter and a Y-cable (along with Mic/TRS-to-TRRS from PC to splitter and splitter to headset). It works perfectly when I connect my smartphone instead of the work phone.

The problem is that the source (S11 unit) for the headset is mono only with the signal on the right channel. The left doesn't go directly to ground, but is at least indirectly connected to ground (can hear at a tiny fraction of volume from PC), and right channel of PC audio is also slightly muddled (just get right channel direct to headset).

What I'm thinking of is a simple (can it be done without IC's?) circuit which allows me to combine the mono channel in a one-way manner to the two stereo channels (to draw an analogy, if it were DC and not line-level audio, I'd just use a diode from the mono channel to each of the stereo channels; only know the basics - that doesn't include working with signals).

Any ideas?

Thanks in advance.

Last edited by Andreuha; 21st August 2012 at 06:16 PM.
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Old 21st August 2012, 10:26 AM   #2
DF96 is offline DF96  England
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I think this could be done with a hybrid transformer. Telephone networks use (used?) them to separate forward and return speech circuits sharing a single cable. Some RF circuits use a similar method.

Probably not worth it for your application. Why not use a switch?
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Old 21st August 2012, 11:00 AM   #3
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I'm unclear on what this stuff is... but you are plugging a 3.5mm stereo plug into a mono jack and want it to play through both sides??

if so all you need is a specific adapter used only for the stereo headset when it is plugged into the mono source. it would be wired so that its mono plug goes back to a stereo jack that is of course wired so the tip and ring are both connected together. You can not then use a mono plug in that now mono wired stereo jack (the adapter). One might use a pair or single series resistor(s), something like 1/2 of the nominal headset impedance, between the plug and jack (in the adapter) for safety just in case a mono plug got stuck in there, preventing a dead short to ground.

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Old 21st August 2012, 02:59 PM   #4
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This might be a better explanation of the setup in question.
Click the image to open in full size.


Right now I just disconnect the extension cable from the headset unit so as not to interfere with PC audio.

It may sound very anal, and it probably is, but if I can do this I'll achieve maximum de-clutter.
Click the image to open in full size.
(Note the headset cable is routed along underside of desk. The working end is just long enough to comfortably move around in my chair, or swing it under the table and hang the headset around the rear edge of the desk)


@DF96, this isn't a normal telephone - it's an IP phone connected over Ethernet.

Last edited by Andreuha; 21st August 2012 at 03:18 PM.
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Old 23rd August 2012, 03:40 AM   #5
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In the schematic shown "Left" will have no output signal, it is on ground.

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Old 23rd August 2012, 03:37 PM   #6
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Yes, that's right (the headset which the unit comes with only has one speaker, Plantronics' engineers probably didn't design it to be used with atypical headsets). So when I combine with a PC audio output the left channel is completely gone. I can just cut the wire on the left channel and splice the downstream end into the wire for the right channel; but I'd really like to preserve stereo separation.


Any ideas?
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