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Old 31st January 2006, 09:47 PM   #1
pjanda1 is offline pjanda1  United States
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Default Still have noise.

I've got a BrianGT NI3875 that is very minimalist and I have been having some noise issues. (FM, I can hear the announcers with my ear up to the 94db sensitive speaker) I asked about the same problems a few months ago, and a few potential solutions were presented. I didn't do much at the time, as I found moving stuff around helped.

As suggested I replaced the 1M law faking pot with a 25k log (also PEC carbon, now available from digikey.) The pot didn't make the slightest difference (Thorsten was right, the 1M pot was fine, but the 25K log does track better at low volumes.) He was also right, in that the problem is coming from my speaker cables.

Their position dramatically affects the noise level. However, it is a big pain to always find the right spot. Even then it isn't entirely quiet. I tried a zobel at both ends of the cable. It didn't do anything but sound worse. I've also tried some higher inductance cables (though not that high,) and they help some. Do I have any options beyond adding an inductor at the outputs? If I do, what type and value are best?

I've left the grounding scheme alone, though I will change it to Carlos' if it might help. I also haven't added RF filters at the board inputs, though since the problem comes from the outputs I don't see how it would help.

thanks for any advice,

Paul
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Old 31st January 2006, 09:53 PM   #2
cowanrg is offline cowanrg  United States
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maybe its not a problem with your amp, but a problem with noise on the ground, etc?

i have seen systems that had nasty hums, buzzes, etc... and it was because of some mis-wiring in the house or something. one of the common problems is improperly grounded cable tv coax. if you have a cable tv coax hooked up in that system, try unplugging it.
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Old 31st January 2006, 10:03 PM   #3
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how can speakercables pick up radio? does it have something to do with the feedback/ground in the amp, because i dont see how a normal radio signal could drive an 8ohm load without any amplification.
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Old 31st January 2006, 10:05 PM   #4
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You could try a ferrite ring on the speaker cables.

:edit: Yes the feedback network is a possible point for noise injection.
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Old 31st January 2006, 10:31 PM   #5
cowanrg is offline cowanrg  United States
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Quote:
Originally posted by neutron7
how can speakercables pick up radio? does it have something to do with the feedback/ground in the amp, because i dont see how a normal radio signal could drive an 8ohm load without any amplification.
i cant possibly answer that, but ive seen it happen before. after months of trail and error, the customer unhooked their coax from their tv and the system was silent...
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Old 31st January 2006, 10:57 PM   #6
pjanda1 is offline pjanda1  United States
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I don't have any TV coming into the system. I'll try to find a cheater plug and lift the ground, but I don't see how moving the speaker cables would affect the noise so much if it were caused by a ground problem.

I think it is being amplified through the feedback loop. Ferrites are the obvious solution, but everyone says they sound horrible unless perfectly matched to the cable. I don't have any around to try. Any idea how to pick the right ferrite?

thanks for the suggestions,

Paul
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Old 31st January 2006, 11:03 PM   #7
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I hope that doesn't mean you're going to lift a safety earth? That is a very bad idea.

As for the ferrites, I used to live about 1/2 a mile from the main TV broadcast tower for SE England at Crystal Palace, and had real problems with RF breakthrough. I used cheap clip on ones, and noticed no sound degradation at all. Who is "Everyone"?
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Old 1st February 2006, 12:09 AM   #8
cowanrg is offline cowanrg  United States
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wasnt there a thread on this forum somewhere where a guy's speaker cables were acting like antennas and picking up tv and radio signals? he ended up using ferrites and it stopped it?

i think it was in the chip amp or class d forum. i remember it being like 10 pages or so and they guy couldnt figure out what to do. its probably in the class d forum, because i dont typically read stuff in the chip amp forum.
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Old 1st February 2006, 12:28 AM   #9
Eva is offline Eva  Spain
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It looks like the amplifier IC is oscillating. Note that oscillation is a requirement in order to pick up AM radio.

It may happen due to poor PCB layout, poor input filtering, too much capacitive loading of the speaker output, poor wiring where signal wires go together with speaker or power wires, etc...
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Old 1st February 2006, 12:59 AM   #10
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Pink Mouse,

Why is lifting the Earth Ground a big deal? I bought a descent subwoofer and it had a terrible hum that I couldnt get to go away. Finally, I called Martin Logan direct and the suggested the cheap plug converter to lift the saftey ground and bada bing...the hum was gone.

He said it wouldnt cause any problems, but its a $3000 sub, so if you have some inside knowledge... I want in!LOL

Dominick
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