Originally Posted by lebeux
So, my left monitor is emitting constant RF noise. It does it without being connected to anything, it does it when plugged into every plug in the house, it does it when every other electric appliance in the house is unplugged, it does it when plugged in using my other non-noisy monitor's cord.
The noise is constant and does not change when I turn the volume knob. It comes out of the tweeter. It's typical radio-like crackling broadband white noise, loud enough to be heard over music playing through the speaker at a reasonable volume.
Is my tweeter busted? Did I overlook some kind of possible interference?
As the noise is constant and unaffected by playing music, it's not a busted tweeter - you would expect a busted tweeter to add non-linear distortion.
Are the monitor's active? Either way - you say the noise doesn't change as you alter the volume, so if they are active monitors, the noise is entering the signal chain after the amplification. This does not rule out poor isolation of the amp's power supply from its output.
The next thing to try is physical perturbations (i.e. shake the fucker) - if this changes the nature of the noise, then a soldered connection has probably come loose. Whilst this would be trivial to fix, this probably isn't the case as the noise would usually be intermittent.
Taking all that into account, it is probably a less than perfect ground connection (possibly even a complete lift or component failure) in a filter network in the crossover or output side of the amp - but it all does depend on the crossover and gain structure your monitors use (I'm assuming you have an internal or external gain stage before the crossover, which goes directly to the drivers. If your monitors have gain stages after the crossover then the problem could be there.)
If you're comfortable checking components with a multimeter, soldering on PCBs (including flat components - unlikely, but possible), and voiding your warranty if the problem turns out to be more subtle, you should be able to fix it yourself.