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Old 26th May 2004, 11:25 AM   #1
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Question my amp makes a loud hum

I have a eti480 amp it works fine on the computer but when I plug it into the head phone or rca plug in the tv ( new panasonic flat screen) it makes a very loud hum. can anyone tell me whats up
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Old 26th May 2004, 12:20 PM   #2
hjelm is offline hjelm  Sweden
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Guessing here but your mains is earthed?
Then i think you have a ground loop between the tv and amplifier, via the earth in the wall.
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Old 26th May 2004, 04:07 PM   #3
DRC is offline DRC  United Kingdom
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A lot of TV's dont have an earth (double insulated) so my guess that what you can hear is the frame sync pulses leaking throught to the audio. This is typically at mains frequency (50 or 60Hz) but is NOT phase locked. I have come across this (in fact it happens without fail ) with CRT televisions and assumed it to be due to scan coils but you may be getting it with a LCD too

Can you hear this with a decent pair of 'phones plugged in instead ??

Also, do you have any kind of cable tv / broadband connected to the tv ??
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Old 26th May 2004, 09:40 PM   #4
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no I cant hear hum with head phones or with pc speakers only the amp . I have tried eathing the amp case and the center tap but made no diff. hum is about 50hz same as power and runs at full wattage I need earmuffs to stay in the room
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Old 27th May 2004, 06:08 AM   #5
hjelm is offline hjelm  Sweden
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If the TV doesn't have earth unconnect all other cales but the power to see if it helps. My guess is the antenna cable is connected to the same ground as the RCA or headphone.
Doesn't help you much but if it is the antenna then it is possible to build a hp filter to block the 50Hz.
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Old 27th May 2004, 10:50 AM   #6
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I found the problem it had something to do with the rca block on the back of the amp and the input wires on one of the amps was on wroung way round its all good now it only has a small hum when it pluged into the tv now about 1w can only hear it when tv is on mute
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