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Old 18th April 2008, 04:00 AM   #1
s4s4s4 is offline s4s4s4  Canada
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Default transformer hum please help


I have tried different combos of amps and plasma tv's with the same problem.

When the plasma is on my amp starts to hum then gradually becomes silent and then hums again. This cycle is continous and takes about 5-10 seconds for each cycle.

I assume the problem is with my power, but nothing else is on the circuit and I don't know where to begin to solve the problem.

Thanks, Mark.
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Old 25th April 2008, 01:24 AM   #2
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Have you tried a ground loop isolator?

You know, a 10R sandblock (speaker resistor), two fat diodes, and a 100nf ceramic cap?

Well, just in case, you put all of the above parallel, flip one of the diodes opposite of the other, connect earth ground on one end and amplifier neutral (chassis ground) on the other end.
I really hope that I described this correctly.

Do you have a cable tv box or sattelite tv box? Those can make a ground loop that's so loud it will buzz any amp in the house. My DirecTV does that.

Do you have a powered subwoofer?

Do you have the TV connected to the amplifier? If so, you might want to lift the amplifier's input ground by from 4R to 10R.

In the attached picture, the center resistor, at the far left side, is lifting the input ground for the lm1875 25w amplifier. This ground isn't the path of least resistance for the attached source device's power circuit.

P.S. For bridge rectifier, howabout a really quiet and super-slow KBPC2504 instead of super-fast?
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DIY Models & AppsClipNipperLM1875TDA7293Powerful Parallel TDA7293 kit ♦ My post has opinion.
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Old 25th April 2008, 01:17 PM   #3
star882 is offline star882  United States
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Have you tried using a digital amplifier with optical inputs?
"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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Old 4th May 2008, 11:26 AM   #4
gareth is offline gareth  Wales
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Have you tried some seperate earthed shielding between the two? Or moving the amp as far as is possible away from your plasma to see if the problem remains?
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Old 4th May 2008, 08:30 PM   #5
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Howabout a 330pF ceramic cap across the input impedance?
Another way to say that is, a 330pF as a load (from input + to input - ).

You can also add a 1 megaohm resistor at the RCA terminals (also a load).

These can help in case the television is throwing out some electromag in the RF band.

Also, check your source selector switch, because its possible that its only a 2-pole, which would leave the TV's input ground connected full-time, no matter which source is selected. For this reason, I favor a 3 pole (2 hot, 1 ground) or a 4 pole (2 hot, 2 ground) source selector switch.
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