Combine 2 headphone outs

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Sight unseen, I would think any of the simple opamp headphone amp circuits would work to sum the signals. The opamp's very high input impedance makes a very good summing junction. It's not what you asked for, but that's where I would start.

Mixing mono to one channel is easy at headphone level, as you said you can put two small value resistor like 100 ohms or even less to prevent both the mono and the one channel stereo amps outputs to fight each other. This way you don't loose stereo.

There might be a way to inject mono at both channels at the same time and preserve stereo. For this you have to disconnect headphones ground and reconnect it to the mono output (assuming the mono output is low impedance, that is, direct from its amp output). This low impedance is important as it will provide a ground to the stereo output. When a mono signal "appears", it will drive both headsets.

Jack

Thanks Jack. Can I just check a few things? - If I'm just adding it to one channel, will that reduce the volume of the channel, such that I need to put a resistor on the other channel? If so, how do I work out the value of that resistor, other than trial and error

- Again, if I'm just adding to one channel, I take it I join the mono ground to the single channel stereo ground?

- How do I know if I'm using too small a resistor (if I try

Hi Mike, thanks. I've looked at a few small amps, but all I want is a cheap (

Thanks Ken. That's very helpful. I've just decided I would like to add ANOTHER mono source. This time a mono signal from a handsfree kit of a mobile phone. I would cut off the earbud and feed in the wire, just like the 2-way. How can I adapt your suggestion to accomodate that?

I don't mind which signal goes to either ear - or both, so long as the MP3 goes to both. I would prefer not to lose stereo, but it's not a big problem.

Thanks. David

Thanks very much Ken.

Can I ask for a bit of clarification as I just did a bit of electronics as a kid. I don't understand the term 'wiper'. I know veriable resistors have 3 pins, and I presume the wiper is one of those?

Also. I'm actually thinking of adding another mono source. This time a mono earpiece from a hands free kit on a mobile phone. How would this affect that lovely little box? I'm not fussed if the 2-way goes to one ear and the phone to the other or both to both, just as long as L& R can be balanced, and the MP3 signal gets to both, ideally with stereo preserved, but not a must. Thanks again. David

Brilliant. Thanks Ken. I really appreciate your quick help. One or two last points, just to check I get it right before doing this. With the pots that are in series with the RTs, does the wiper AND the other, non-input pin connect together, also with the same pins of the other in series pot. This wire then goes in to the wiper of the joining pot? i.e. one of the pins takes the input, and the other 2 pins connect to the wiper of the crossing pot?

You suggest 470ohm pots for the series and a 1K one for the joint one? (all linear)

I joint the grounds of all three channels directly?

Lastly, how should I adjust the series pots. Do I use them as a gain for the individual channels or what. Or, do I set them at 470 and then reduce ohms until I get ??problems??, but what would they be??

Thanks again. David.

I had in mind the DIY amps at

Hi David.

Given that the two outputs are of probably the same order (both would presumably go to similar types of headphones) the impedance match shouldn't be too much of an issue in practice, although I'd expect the RT output to be the lower impedance.

I'd use a variable resistor, say 470 Ohm, in series with the RT output and send that to the wiper of another variable resistor, same value or maybe 1k, with the two ends going to L and R from the MP3 player. The second pot will allow you to balance the input across the two channels (or essentially null out on one side).

A small box, a mono 3.5mm socket and a stereo one to suit your MP3 player headset and you're away! A bit dirty, but in practice it should be fine.

Cheers.

Ken

Does this help any? (I've just diddled with the art a bit. :-) )

Cheers! Rich

Thanks, yeah, that confirms it. Cheers. David

The wiper is the centre contact of the three. If you get hold of a multimeter and put it into resistance mode, you can measure between the two outside pins - the resistance will (or should!) stay constant. Measure between the centre (wiper) and either end and you'll see the R change as you rotate the knob.

To add another source just add another pot in series, joining to the wiper of the second (the one feeding your headphones). Or you could just say screw it and leave out the one across the two channels and just feed the RT to one channel and the other source tot he other channel, again via a pot so you can adjust the relative volumes.

RT---/\\/\\/\\------| |----- to L ^ | \\ |_________|----->/ | \\ RT---/\\/\\/\\------| / ^ | | |_________| |____ to R

I think that ASCII 'art' may be readable. Shows two RT inputs, but whatever....

Cheers.

Ken

HI. Just build the circuit and it works well. Only problem is the volume from one of the sources is a bit too quiet. It is the mobile phone handsfree output. What are my options for increasing the volume of that channel? ?? Mono amp. If so, do I feed it in before the pot that is in series with that channel?? David.