Stereo To Mono with SINGLE Powered Speaker

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If you are looking for the best way, then you should use a small mixer with 2 ore more mono channels. Plug your I-pod with a cable from the minijack to two quarterinch plugs in separate channels and leave the channels pan on middle. That way the mixer is summing your L+R to mono.
Simpler way ist to make a adapter which puts the two grounds of the red and white cable together and also the two hot wires. Plays, but I wouldnt recommend it. But sometimes I do it as well.
Avoid straight joining 2 outputs.
Join grounds and solder to ground of the male plug which goes into the powered speaker, but join 2 hot wires coming from IPod into the single hot plug pin by first adding a 1K resistor in series with each of them.
You connect the joint of said 2 resistors to the plug hot pin.
They are called "mixing resistors" for a reason.
A.k.a. "passive mixer".
This would be the correct way to avoid L + R current interacting and putting load on the line out of the connected device.
I'm sorry I forgot this elegant way of building a adapter. The components fit in a phone plug.

It's nice in theory - but in practice the headphone output can simply be joined together, they either have internal resistors, or if direct from a chip, the chip is designed to allow it.

So by all means stick two 1K resistors in as a passive mixer, but it won't sound any better, nor will it protect anything.
I posted a detailed instructional post, but it looks like it got passed over. Here we go again,... TRS (Tip, Ring, Sleeve) which looks like a stereo jack (left, right, ground) is not a stereo connection. It is a "balanced" connection = Signal Positive, Signal Negative, and Ground. This is a "low noise" method of moving an audio signal from one place to another. You cannot jack the stereo from a portable audio device into any TRS connection as you suggested.

You need on one of the following: A mixer (purchased, active opamp, or passive (with resistors)); as suggested by others, in order to combine the Left and the Right Signals together and then on to your powered speaker. If you can't build one yourself, buy a cheap one, plug your stereo connection into your mixer via a stereo 1/8" plug to a left and right mono 1/4" adapter breakout cable; and then plug the left mono cable end into one mixer channel strip and the right into another. Then you can use either the "balanced out" Left or Right mixer output to your powered speaker.

I would also suggest that you read about the PA Fundamentals; ie Balanced and Un-Balanced cabling, mixers, and how they function, and so forth. This knowledge will serve you well.

If this second post happens to make it up on the board, I could draw you a "passive" and an "active" method that would allow you to combine the stereo signal into a mono signal that would allow you to accomplish your goal with a degree of success.
Sorry about the duplicate posts. It was totally an error.

Thanks for all the replies. I understand balanced and unbalanced. This is just a strange predicament. Hooking up ONE (mono) speaker when you have a Stereo signal.

What happens if I send a stereo signal (Left audio, right audio, shield) to the speaker on a TRS cable? Is it better to use a TS cable, a TRS cable or it doesn't matter either way?

One more thing. Unrelated to above. WHEN USING TWO SPEAKERS. The speaker owner's manual labels this as a TRS input. Is it ok to use a STEREO-Y adapter (3.5mm stereo to dual RCA) and hook them up using RCA to TS adapters with TS cables to the TWO speakers? Does it matter whether I use TS or TRS cables in this instance?

Thanks again.

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