I have just recently purchased a crown XLS1500 amplifier to be used with my Onkyo 875 receiver to power my front M80s. I didn't realize that the amp could only work with a 120 volt power supply. I only have 240 volt where I live. I then decided to try a cheap step down transformer to step 240v down to 120v. Suprising that this crown XLS1500 amp only draws max 250 watts yet claims to put out over 500 watts/channel into 4 ohms. Anyway that is a separate story.
With everything connected up I am getting a low frequency hum from the fronts. When I disconnect the rca input to the crown the hum disappears. It is only present when the onkyo receiver is on and the pre-amp out is connected to the crown amp.
Is there any way I can eliminate this hum or reduce it.
Should I use the XLR inputs on the Crown. To do this I would need to get an RCA to XLR adaptor. Is it possible this hum is caused by the step down transformer? I doubt it since the hum is not always present. Any ideas?

Is everything plugged into one outlet? It sounds like a ground hum which often pops up when interconnected devices are plugged into different outlets.

Try a cheater plug (no ground prong) to see if that makes the hum go away.

Yes everything is plugged into the one outlet. I even disconnected all devices and left only the receiver and amplifier connected but still the same problem.
Is the use of this cheater plug dangerous?

Are all your outlets on the same circuit to the breaker? Might be a ground loop hum. If you have a sub, what happens if you unplug the interconnect?

No, though it's better used as a diagnostic tool than a permanent solution.

Do you have a coax cable (TV, Cable) going into a device that's plugged into the receiver? If so, try disconnecting the coax cable.

Yes the outlets are on the same circuit breaker. Disconnecting the pre-amp out to the sub has no effect on the hum.

To get the true benefit of XLR connectors you're receiver/processor would have to have XLR outs... XLR's are a 3 wire system, RCA is a 2 wire system... To get rid of the hum, you need a true 3 wire system.

I had the same problem, i think it's the wiring in my house.. I am using all XLR connectors. However, my processor has XLR outputs....

By using a step down transformer, are isolating that outlet.
My money is on a ground loop problem. My house also have a ground loop... So, i have to deal with multiple electrical issues.....

I disconnected all inputs to the receiver but no difference however I just disconnected the hdmi out from the receiver and the hum is significantly reduced. Still annoying but much better. But I need to use this HDMI output.

you may not have a good earth ground.... that is one other possibility.

Where do you live? I would wager that this has something to do with the step down transformer.

Guys, if he's got 240V, I doubt they even have cheater plugs...