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I got two of these 1000W balanced isolation transformers. Filtration is not the best in them, but they do provide good isolation. Best part is, they are only $200.

https://www.antekinc.com/bp-1000-balance-isiolation-power-supply/

I suspect your speakers or crossover components are acting like antennas.

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I have too much stuff - https://www.pleasebuymystuff.com

I dont have a passive crossover. Using a 5-way active analog crossover. Hum was there even before I installed this crossover.
Speakers are compression drivers.

Both David M and I have excellent results using a braided speaker cable. It should provide superb resistance to RF interference. I bought my kit from GR Research, David bought his parts from either eBay or Amazon. He can fill you in. You won't be disappointed in the sound and it may eliminate your RF issue.
PM me if you'd like more info.

Stuart

16 or 24 strand

BTW, the speaker cables that are picking up the interference are neither twisted nor shielded. They just have Teflon insulation and cotton as dielectric. The DIY twisted cable I made with WE16GA cable is working fine.

You might consider the Mogami coaxial speaker cables -

https://www.mogamicable.com/category/bu ... ure_sound/

While the speaker cables, no matter how long and positioned, will not pick up noise that would be reproduced by the speaker. What could happen is that induced RF into the speaker cables could adversely affect your amplifiers and cause them to act badly. If they are no-feedback tube amps, less likely. If they use feedback, either tube or ss, you may have a problem there. You might try to isolate the pick-up location by starting with the amp(s) and removing the interconnects with only the amps on to see if you have the buzz. A shorted input may be good to try as well as an open input. If quiet, then work your way through the system with one component at a time. Good Luck!

That looks like the Belden CAT5 braided speaker cables I made 20+ years ago. I will post a picture when I find them.

Seriously, those will be difficult to use with compression drivers, they are too think.

I have two 30ft twisted WE16 cables. I will slice it into smaller sections and start using for now.

https://jeffsplace.positive-feedback.com/the-vintage-beat-western-electric-we16ga-speaker-cable/

David,

The tracer probe is showing 60Hz noise in the speaker cables even though the 300B amp is unplugged. what would that indicate?

Yes, I removed all power chords. Going to connect one amp at a time and see how it goes. Hopefully, I will replace the dimmers tomorrow.

Shashi