Humm is frequently caused by ground loops. This can happen in almost any system. There are a few ways to find out. My recommendation would be to unplug everything except your amps (this includes interconnects and power cords). If the humm continues with only the amp connected to the speakers, you can be pretty sure you have an amp problem and it needs repair. If not, then start by plugging in the pre-amp, connect the interconnects and listen. If there is still no humm move on the CD player. Once the humm appears this component is likely to have created ground loop between another component. You can use a cheater plug to lift the ground and see if this stops the problem. Since using cheater plugs is hardly advisable long term there are a few other things that sometimes help, which are as simple as changing where you plug the component in. In some cases I have found no other way to eliminate ground loop hums other than lifting the ground.
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