# Is this passive headphone mixer design safe?

I am looking for a cheap solution to allow me to connect a single set of headphones to two computers I am using. I don't need volume mixing or selection, I just want to merge the two signals.

I found this instructable, that has a very simple design. He essentially connects five jacks ground pins together, and then joins the left and right channels of each input jack to the corresponding pin on the output jack via a 1k resistor.

It almost seems to simple to me. I do not want to blow up the output jacks of my computers by applying an odd signal to them. Is this design safe to use? Can anybody explain what the purpose of the resistors is, and why it works or won't work?

With 1K ohm resistors it is probably safe, but it will sound marginal to terrible. I wouldn't do a passive mixer for anything with inductive or capacative loads (headphones, speakers, etc.).

If your purpose is to hear something, anything, and not blow up your stuff then this will be fine. If your purpose is to actually enjoy music then don't bother.

Headphones (and speakers) need a low-impedance amplifier driving them. If you have a high impedance (or use something like a 1K resistor) then you are essentially causing the frequency response of the headphones to go from hopefully flat to something not even close.

Think of it like this...

Impedance is basically a frequency-dependent resistance. Your headphones have an impedance that can change a lot depending on frequency. However, they are also designed so that (hopefully) the frequency response is mostly flat for a given voltage input, despite the wacky impedance graph.

Now you put a 1K ohm resistor in series with the headphone. That resistor, plus the impedance of the headphone, form a voltage divider. But the dividing ratio changes over frequency because the impedance changes over frequency. That causes what was a flat frequency response to become very much not flat.

• Can you recommend a circuit that will sound a bit better? Commented Apr 13, 2012 at 19:22
• @CMP You could do a "passive mixer" that feeds a headphone amp. Doing that would keep things simple, but removes the inductive load that has the wacky impedance graph. Otherwise, I don't have an example off the top of my head.
– user3624
Commented Apr 13, 2012 at 19:24

To "blow up" a component it is necessary to either A) apply sufficient current or voltage to it or B) short it out while it is under voltage. Clearly the amp does neither of these things.

Now, the question is, are we sure that we will have a voltage sufficient to provide a sound level equivalent to the level coming out of the PC? I cannot answer that just by looking at the circuit, and don't really have time to simulate ATM. By all means give it a shot, and if you're unsure about your simulation send it along and I'll try to see if it's accurate or not.

All that aside, there is a major problem here - no insulation whatsoever. Basically this is for sitting on your desk and not taking abuse of any kind. If I was doing this, even if I expected it to sit in one place its entire natural life I would do some heatshrink.