A typical representation of the left and right amplifiers to a pair of headphones or earbuds looks like the picture below.
To convert a stereo signal to a monaural signal a process called summing or mixing must take place. Obviously one could use a simple audio mixing console and feed the left channel into one input and the right channel in another. This will work but is more complicated than it needs to be.
A passive method would be to use two resistors of equal value to mix the signals into one as shown below.
Using an off the shelf summing cable like the Hosa CMP-103, the tip and ring are shorted together on the 1/8 inch end and connects with the tip of the 1/4 inch plug end. This will cause the left and right channels to be shorted together possibly damaging the headphone amplifier stages as shown below.
Using a simple 1/8 inch to 1/4 inch mono cable will send only the left channel to the output and short the right channel output to ground possibly damaging the amplifier.
The solution would be to internally add protection resistors to the headphone amplifier circuit as shown below.
Unfortunately the output stage protection resistors as shown below would not transfer the full output of the headphones.
The cable described in the previous page correctly sums the audio to a mono signal which will protect any audio source while providing proper levels to a mixer or amplifier system as shown below.
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