Summing two amplifier outputs with just a resistor

[perfknee]

I am looking for a very small and lightweight way of combining the Right and Left channels from a 1/4" mixer headphone output into a single "can" or headphone. This is because I only hear out of one ear and want to not miss any sound that was mixed into the other ear's signal.

I've seen passive mixing networks involving a few resistors on a circuit board, which could potentially be put inline with my headphone cord. But I just recently disassembled a commercial product which does this, and they only use a single resistor to mix the R+L into mono. The Stanton DJ 300 headphones simply short the tip and ring of the connector together, and bypass the single 100 Ohm headphone driver with a 1 Meg Ohm resistor.

I've always been told that this sort of design would cause unstability in the headphone amplifiers, and indeed I have seen some weirdness with my headphone feed which could very well be attributed to this. But it's very attractive simply because there's no extra hardware except the resistor, and so I'm considering trying to convert a higher quality pair of headphones to single-sided using this strategy if it is kosher.

I thought I'd ask the amplifier experts here what they think... is this truly a bad idea, and if so do you have a suggestion for the simplest passive mixing network that would do a good job of mixing R+L into a single headphone?

[Nico Ras]

No its not. You can simply sum the two amps with 100 ohm resistors into one can.

[DF96]

I'm not sure what the 1M resistor does, unless the headphones don't provide a DC path themselves. Do the headphones have an on/off switch? In that case, the 1M resistor will maintain DC bias on any output caps in the source, although the source ought to do this for itself anyway.

Simply shorting both channels together will be fine provided the source has limiting resistors present. Most outputs will have these.

[perfknee]

Actually maybe it isn't a 1M Ohm resistor across the headphone driver; it's actually two very small glassy passive components (colored orange and with black bands toward the middle junction), like this:

driver + ---------< I>------Soldered together-------<I >---------- driver -

and I measure about 500k Ohms across each one. I don't think they are diodes because they give the same resistance when I reverse the probes, and I don't think they are capacitors because caps read zero resistance. I don't know what else it could be.

Anyway, to answer the asked question there is no switch on these headphones.

I am liking the idea of just adding a couple of resistors to the insides of my MDR-V6 headphones in order to convert them to mono. I guess the only downside would be reduced sensitivity of the headphones, but I can live with that since I've got a powerful headphone amp.

Just to be clear, this is the way it would be wired for a 100 Ohm driver right:?

Tip----------100 Ohm Resistor----------Driver +

Ring--------100 Ohm Resistor-----------Driver +

Sleeve--------------------------------------Driver -

I might actually end up putting both 70 Ohm drivers in parallel and then using ~35 ohm resistors instead.

[DF96]

Could be back-to-back Zener diodes to limit the voltage.

[perfknee]

Aha... I hadn't thought about zener diodes... yes I suppose they would measure the same both directions.

I think you're right that it's not a resistor at all but rather two diodes set up opposing each other like this:

Transient voltage suppression diode - Wikipedia, the free encyclopedia

So pardon my ignorance, but I'd like to re-ask the original question: Is this a reasonable way of summing two headphone signals, just shorting them together and adding two such diodes? It seems like it would have the advantage of not reducing signal level like resistors would, but I'm not clear on how it would really isolate the amplifiers so I can't judge if it's a good solution.

I'm curious to hear opinions about whether diodes or resistors would be the best way to convert my MDR-V6 headphones to mono.

[DF96]

The diodes, if that is what they are, are not there to convert stereo to mono but to protect your hearing from excessive volume.

[perfknee]

Wow this gets more and more interesting. I never would have guessed that a manufacturer would spend even a few cents on components that protect my hearing, without touting that in their marketing.

Of course I suppose that it not only protects one's hearing, but also the headphone driver, and so it could very well reduce warranty claims from druggy DJs who turn their phones up to 11. Man, I've got to watch Spinal tap again :-)

Anyway, if those are diodes for level-protection only then these Stanton DJ300 headphones are effectively summing by just shorting the two amps together, exactly what we're told not to do. Maybe that's why they're so rare.

[perfknee]

Thinking about this a bit more, it would seem that headphone amps would need to be designed to survive shorting of the two channels, since that could conceivably happen while the TRS plug is being inserted unless they have a jack that's designed to prevent that. So maybe shorting the two channels together is not the end of the world.

[qusp]

yep, they'll definitely be diodes, its actually not that unusual, i recable headphones quite a lot for my work and ive seen quite a bit of it. predominantly in studio type headphones that may perhaps be plugged into the wrong output; beyers are one example and yes your sonys, also some akgs