I've been working on a very simple circuit to mix audio from 2 audio sources passively. I've looked into both active and passive circuits and for the sake of simplicity, I'm working on a passive circuit. I'd rather not have to deal with adding a power source if I don't have to. It's 2 inputs and having to boost the signal a little on both inputs is not a big deal.

I have this circuit right now:

If I were to choose a lower resistor value for the 100ohm resisters, say 20ohm, should I be worried about damage to either source if my headphones are not plugged in and the sources are still driving the line or even if the headphones are still plugged in? Should I expect to see more power usage with the lower resistor values?

 

We do not know if your signal sources are able to drive 100 ohms or 20 ohms.
We cannot determine damage since you never said the signal levels.

 

What are the two signal sources? Are they capable of driving headphones directly?

ak

 

Ah, I forgot to add that. I'll be using an iPhone and an Xbox controller as sources. I don't know the exact signal values, but looking through this review of the iPhone 5's audio capabilities: https://www.kenrockwell.com/apple/iphone-5/audio-quality.htm, the voltage across the headphones should be < 100mV. I'm extrapolating that a little bit from this section in the review: "Let's bring it down from deafening (113 dB) to merely too damn loud (93 dB SPL), with a 100 mV signal. In other words, I turned the iPhone 5's volume down from FULL to about halfway: "

If the voltage across the speaker is 100mV, then using a, possibly wrong, voltage divider calculation (V = 100*24/(100+24). 24 is the parallel resistance of 100ohm and 32ohm), I need ~516mV coming out of the iPhone and the controller using the 100ohm resistors. Using 20ohm resistors and the same calculation, I'll need ~300mV coming out of the sources. So I think the signal levels will be between 100-500mV out of the sources.

 

Headphones are usually 32 ohms for each ear. Since you are mixing two different signals then both are probably stereo and you are showing the mixing for only one earphone.
100 ohms in series with one 32 ohm earphone attenuates 32/(100 + 32)= 0.24 times which is -12.4dB.
20 ohms on series with 32 ohms attenuates 32/(20 + 32)= 0.61 times which is -4.2dB.

We do not know if the iPhone produces clipping distortion at FULL but its volume control is probably logarithmic then HALF would ne about -20dB.