Careful when connecting headphone outputs to line inputs.

That line level is basically the same as headphone level at medium to high volume is widely known. I guess we all have at least once connected our phones or mp3 players to line inputs of amplifiers or something without a problem. However, this isn't always a good idea, especially when it comes to cheap and/or very old devices with built-in loudspeaker(s). Here they don't use a seperate output stage for the headphone out... They just connect it to the speaker stages with series resistor. For a headphone driver with a low impedance like 32 ohms or so this obviously works ok (not perfect though because the speakers/transducers are reactive not resistive). But when connecting a line input with a high impedance (usually 10k or higher) the series resistors lose virtually all their attenuation (voltage drop is always relative, Ohm's law) and you're left with full loudspeaker level... Turning the volume down to very low might make it play without distortion but with tons of noise and hum which makes it unlistenable anyways - what really helps is a 32R parallel resistor between GND and the outputs!

published 6 years ago