Originally Posted by Roger Dressler
By all means reduce the amp's level controls and compensate with the MRX gain trims (can go as high as +10 or more), that can make a big difference.
Perhaps the ground lift would work better on the MRX than the amp. It's not possible to predict. But now that we see there's a "chassis ground" component to the hum issue, you can explore further to see if other methods get you better results.
Over on the ATI AT52xNC thread, SteveH posted this about chasing down hiss in a home theater. I thought it showed some pretty useful insight, so I'm sharing it on this thread to see if it's of any help.
Re: "hiss". the experiments to understand the source of the hiss is easy but time consuming. It's what I call "dividing the world in 1/2" through a series of experiments. The statement: "So-and-so amp (or preamp) didn't do it before." I've heard that one about 500 times now and is rarely ever relevant. The blame is usually assigned to the last item in the mix and people want to jump to that conclusion. But in reality, a new interaction between the grounds take place.
Steps to Isolate:
1. Disconnect EVERY cable to the amp but the speaker wires. By definition, every wire also means the 12 volt trigger. Also, no subs right now. Reason: the goal is to remove ground loops. If the hiss is gone, that pretty much exonerates the amp from generating the "hiss". If you still have a "hiss", then you should be pointing at the amp. Normally if the amp is the culprit, then the hiss is coming from a specific channel. So put our ear to each speaker and note accordingly. The amp itself is rarely ever the problem; maybe one out of a hundred? So I'm going to take a leap and say the ATI is going to be dead quiet.
2. If there is no hiss in step one, reconnect up the balanced or RCA cables to the preamp. On the preamp, disconnect every input and output wire. That means the no HDMI in, no HDMI to the TV, no composite video or component, and no subs. Just the analog wires from the prepro to the amp. If you have another amp to run ATMOS, don't connect to it as we are trying to figure out if the amp in question is the problem. Now turn on the preamp. Do you hear a hiss? If so, is it volume dependent? If you hear hiss and it is not volume dependent, chances are you are dealing with a ground loop OR a high noise floor on the preamp. If the hiss increases with volume, you have a noisy preamp problem OR (rarely) a noisy dimmer switch on the same circuit. Next, make sure the preamp and amp is plugged in the EXACT same outlet. Being on the same circuit/breaker is not good enough. Use an extension cord if needed. Is there now a hiss? If it disappears, it is a ground loop. If not, your preamp is noisy. If there is no hiss, leave the amp and preamp connected like it is in this step, make sure the preamp and amp are still plugged into the SAME OUTLET and proceed to step three.
3. If you are at step three, we are searching for the source of the ground loop. It could be that your sub is plugged into another circuit. Or your TV. Or even if you are on the same circuit, plugging in an outlet 15 feet away allows for a signal to ride on the small drop in resistance. At this point, you have to write down everything you do in an effort to figure out when there the hiss is interjected. It's outside the scope of this thread so I won't go though the next 10 steps. There are a dozen or more ways to make the ground loop disappear. Some of them include Jensen transformers on the cable input. Or tying chassis ground to every device in a daisy chains (and then tying it to ground). Another way is to run a dedicated line. Of doing a short ground jump from a couple of circuits behind the equipment. Others lift grounds.
Hoping this helps you run that hiss to ground.