I built an audio mixer, and tested it with two different pairs of headphones. When I test it with one pair of headphones (I'll call Headphones #1), there is no noise (except for a few "pop" sounds when it first connects). When I test it with the second pair of headphones, there is very audible noise/crackling.
The only difference in specs that I'm aware of between the two pairs of headphones is the impedance. Headphones #1 is about 30 ohms, and Headphones #2 is lower impedance at around 8 ohms.
Given the below circuit, in which V1 and V2 are input audio signals, and the output is connected to a headphone channel, can you see a reason why noise would occur with Headphones #2? Why would a lower impedance headphone be subjected to audible noise with this circuit, or could it be due to a different characteristic?
The 1k resistor at the output is a discharge resistor I put in to prevent popping.