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The dimmers were installed by the builder about 20 years ago.

If the problem is not solved by removing the dimmers, I will get a dedicated 20AMP circuit for audio installed. One electrician quoted $700 not including the handyman charges to patch up the drywalls.

If your equipment can run on 220V you might consider that since it would not cost anymore than a 120V circuit. I am currently running some equipment on a 120V circuit and some on a 220V circuit.

If your system is plugged into two different circuits, on different circuit breakers, it's possible you could have a ground level difference at the outlets. I have had this problem and have fixed it by tying the grounds together at the outlets. The latest instance I had to deal with I ended up making two four inch long extension cords with a 12 gauge wire between the ground pins on the two plugs, each plugged into one of the circuits in use. No more noise.

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Tom,

I installed a balanced isolation transformer which did not help much. 60Hz hum is still coming thru. But, when I switch off the dimmers, level of hum goes down.[/quote]

A balanced transformer will not get rid of all the line noise. I have never found them to be the most effective way to minimize line noise. Having all the equipment running on one line and using a high quality line filter will give you the most bang for the buck when working with equipment sensitive to line noise.

Exactly what equipment are we talking about here? Brand and model and are each grounded or ungrounded?

It may not be entering your gear via power supplies. That type of noise is radiated. When isolation transformers don't make much difference, that's a clue.

A balanced transformer will not get rid of all the line noise. I have never found them to be the most effective way to minimize line noise. Having all the equipment running on one line and using a high quality line filter will give you the most bang for the buck when working with equipment sensitive to line noise.[/quote]

Ok. I will order the filter you suggested. Any recommendations for the chassis.
Ideally, I want to house two filters in one chassis for the two wall outlets. The two balanced transformers will be connected to them.

If you are truly obsessive there are Topaz Ultra Isolation Transformers -

https://www.surplussales.com/class/indu ... on_5k.html

You might be right.

Weird things are going on. Most of the noise is coming from 2 amps which are connected to the same power strip.

Even after unplugging the power cord from the amps, the tracer is still picking up the hum from the speaker cables connected to them. With the dimmers off, most of the noise in these speaker cables is gone. Looks like the speaker cables might be acting like antenna.

To test it further, switched on all the dimmers. The tracer is picking up noise in the middle of the room. If I bring the unconnected (on both ends) speaker cable close to the tracer, the volume of hum goes down significantly. If I switch off the dimmers, most of the hum is gone.

In the basement, I have 3 dimmers, 2 of them are 3-way and 1 is 4-way. When the switch on, tracer is picking up noise at the dimmer, but not at other end where a non-dimming 3-way switch is installed.

These 20-year old dimmers are infact radiating hum.

After getting rid of the dimmers, I will be getting double shield power cables and interconnects. Might have to do the same for speaker cables also.