Why is it that if I plug a something other then a turntable into the "phono" input on my receiver the sound quality gets very distorted. Is this normal?
Felix - The phono input is for a turntable only. It is a complicated process, but in a nutshell the signal gets boosted and equalized for a turntable. If something else is used - say a cd player - the boosting and eq 'curve' will make the sound horrible. It may also damage the input.
It's called input overload and it can happen on any input. A magnetic phono cartridge input, however, is meant for a signal in the millivolt level. What you are putting into that input is something on the order of a few volts peak. Along with the RIAA equalization of a phono pre amp, you will not get good sound from this combination and you could possibly damage something.
The output of a line level source such asa CD player or a tuner is in the order of around 2 volts.
The output of a turntable's cartridge is either in the order of around 2mV (i.e. 0.002 volts) for a moving magnet cartridge or 0.3mV (0.0003) for moving coil cartridges.
Therefore, when you plug your CD player into the Phono input of an amplifier, it's trying to put in a signal between 1000 and 10000 times too strong. It's a testament to good design that all you get is a distorted signal rather than it blowing up.
Just to confuse matters, most modern amplifiers don't have a phono stage to boost a turntable's signal up to line level and yet they often still mark their inputs as Phono, Aux etc. So most people have the opposite effect of it being very very quiet and distorted in another way for other reasons.