Hum, hmm... sort of.

Being a broke college student, I run an Onkyo P-304 Preamp, M-504 amplifier, monster cable mkII 400 rca's, DIY speakers and crossovers (which never cease to amuse/amaze me), and Grado SR 225 headphones. As my calculus professor would say, "nothing special".

The turntable is a Yamaha YP-450 that I got from a friend who's wife was freaking out about clutter in the basement, as they are due in a few months. It needed a lot of work, but after a week of digging for parts and working at it, I finally brought it back to life. I also installed a Grado Blue cartridge / stylus (I wasn't sure if it even worked at this point, so that $80 was a HUGE risk!) and aligned it with a protractor.

My issue: When I play the turntable through the preamp/amp/speakers, I get a barely audible hum when I increase the volume significantly. It is not audible during playback, however it does increase from ever so slight to barely audible when I engage the motor. This does not occur when I play it through my cans only. Also, the hum is not audible with the CD input, spare turntable input to 'line 1' (internally RIAA EQ'd plastic cheapo for playing lower quality wax on), or the tuner.

What would cause the hum through the amplifier, but would not cause the hum through only the preamp / headphones?

My only guess would be that the hum may be inherent to or a result of the preamp's output circuitry, as the internal headphone amplifier is not the same circuit as the amplifier output.

Any insight would be much appreciated!

Thank yinz (Pittsburghese for all of you) very much, this site has been a HUGE help!
I would think that the headphone out would use the preamp circuits in the same way that the external amp would--ie a 'power' amp of some sort. Plus for sure it's using the turntable output from the pre or else you would get terrible sound. And all of the sources going through the amp also go through the preamp and only the phono hums, you have what looks like a logical issue (which is why you are asking). I wonder if the phono preamp circuit gain might be sufficient enough (through the line stages) of driving the headphones but not when driving though the amp and so when you crank up the volume of the speakers for the phone you aren't hearing that. Good luck!
Thanks man. At this point, I'm not about to rip apart the P304 that I paid $100 for. It seems to work perfectly otherwise, and the hum isn't audible unless you're digging for such noises and really strain to hear it. Like mostly everyone on here, however, I'd like the sound to be as 'ideal' as possible.

Is there anywhere that would provide wiring diagrams for an Onkyo P304?