We often use our iPhones to listen to new songs, or to play music during breaks, playing though the instrument input of a guitar or bass amp. However, since the amp is mono, we get only one side of the stereo output. Depending on the song, it could be a large difference. Yes, I'm aware that SOME smaller amps have a CD or Stereo input, but none of ours do. I was just about to make or buy a special cable or adapter to combine the L and R channels. I did additional research at the last minute, and found that iPhones have a Mono switch built-in! Go to Settings > General > Accessibility > and you will find the switch to set the output to Mono. Just don't forget to set it back later. Just FYI - Mono provides enhanced intelligibility for people with hearing issues. Hope that this helps someone!