Mono/Stereo/Left/Right Headphone Switch

[j-curve]

Shorting the left and right channels together is not the most elegant way to achieve mono.
Here's another approach:-



SW1 is a 6P4T shorting (make before break) rotary switch, quite a rare bird. With 30 connecting tabs in two layers, figuring out which is which is also a bit of fun. Once that's done though, the soldering is perhaps easier than the schematic would suggest, because you can basically run two wires around the switch, crossing them over just three times.
SW2 is a standard DPDT with centre-off position. It should be set to match the impedance of the headphones.

How It Works
The 27 ohm resistors prevent overloading of the source due to the low differential input impedance in mono mode.
SW2 selects a pair of matching resistors which go in parallel with the headphones, except in "left" or "right" modes where both of the headphone transducers are connected to the selected channel and the two matching resistors are connected to the other side. In this way, the input impedance is kept almost constant for both channels in all modes (important when sharing a source). Also, the output levels will be consistent across the modes, essential when listening for subtle differences in the signals.

Specifications
Input Impedance (in stereo/left/right modes)
(with 32 ohm headphones) 43 ohms
(with 80 ohm headphones) 67 ohms
(with 300 ohm headphones) 177 ohms
Input Impedance (mono mode, differential signal)
(with 32 ohm headphones) 37 ohms
(with 80 ohm headphones) 43 ohms
(with 300 ohm headphones) 50 ohms
Output Impedance
(with 8 ohm source): 17 to 31 ohms, depending on setting of SW2
(with 32 ohm source): 21 to 49 ohms
Insertion Loss: 2dB to 8dB, depending on SW2
The insertion loss is greater with low impedance headphones, which can be handy because they tend to have higher voltage sensitivity. Works a bit like a hissbuster.