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Old 7th October 2003, 11:51 AM   #1
Tensop is offline Tensop  Lebanon
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Default Amplifier Noise, who wants to help :)

Hi guys, im in posession of an old rotel amplifier which is picking up noise from my local power point.

The noise(when at full volume) on other mains points is very low, and only audible within a metre of the speaker.
However on my current mains point the noise is quite high. when the amplifier is anywhere above the 25% point of the volume control(with a 20hz frequency and a multimetre i measured this to be 6v AC) it puts out an audible noise, and at full volume it is putting out enough to register .8v AC!, dc offset however remains low(6mV left, 5.5 right). the noise is not being born inside the amplifier nor being transmitted to it by rf noise(internal shielding is excessive, the main amplifier module and pcb are covered with 1.5mm thick steel plates, and all wiring from the control amplifier to the main amplifier are shielded and twisted pair. my current circuit that i am on(2 rooms and a bathroom) has these devices on it:

2x computer power supplies
2x monitors
2x vcrs
1x TV
2x amplifiers(one is switchmode, in the other room)
1x air conditioner(not on at the time)
1x heater
3x mobile phone chargers(all smps)
and various other little plug packs, as you can see this circuit will naturally have alot of noise with these kind of devices on it.

from what i've looked at, the amplifier inside goes:

mains in > voltage selector > fuse > traffo > bridges+ main caps > amplifier module, preamp. due to the age of the amplifier(12 years old) i beleive the capacitors may be showing their age. I am considering replacing the capacitors(electros) and the resistors.
is there anything i can add to supress noise(ferrite chokes, cable beads, .1uF caps across the bridge, replace the bridge diodes)?

Thankyou for your help in advance,

regards,
tensop
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Old 7th October 2003, 12:16 PM   #2
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Before you do anything, check which appartus who is the bandite. Turn off everything and then turn on one thing at the time.

A guess is the "switching" things.
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Old 7th October 2003, 12:22 PM   #3
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Hi,

When you say "other mains points", do you mean within the same building, or at someone else's place ?

What sort of noise do you have ? - is it 50Hz hum, 100Hz buzz, or some other noise ?

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Old 7th October 2003, 12:25 PM   #4
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Default WMD, weapons of mass distortion

I had to trouble shoot a similar problem in a system once. There was UPS rectifier noise (it was a 7 KVA on line UPS) clearly audible over the speakers. Clearly I had something picking up this noise and transmitting it to that amp. Late one night I started simplifying the signal path, bypassing equipment and unplugging it one piece at a time.

Eventually I was left with a Bryston B-4 feeding a big pair of Tannoy monitors and still had the noise. I floated the ground on both the amp chassis and signal paths in all the possible permutations to no avail. I was on a dedicated circuit from the UPS and the noise was still there. I'm not sure why I tried it at this point or why i didn’t try it earlier, but I unplugged the three (name omitted for embarrassment) fancy surge suppressing, noise filtering, plug strips and plugged the amp directly into one of three duplex outlets coming from the UPS. DEAD SILENCE.

I chucked the fancy hundred dollar plug strips in the trash and used some $5 Waber plug strips to replace them. The amp stayed in the duplex.

It seems my problem was all about the line filters in the plug strips returning noise from the ground to the AC line. Now that’s a quality design. Weapons of Mass Distortion.

My suggestions:
- Dedicated circuit for the stereo. Pricy but in the total scope probably worth it.

- Isolate, Isolate, Isolate... Unplug everything in the house except the amp, and then add them back one at a time. The fridge can stand 5 min. w/out power in the name of clean audio. Eventually you should be able to find the mystery source that’s mucking up the works. My issue, unfortunately, took a few months to find. I couldn’t see the obvious; it should have been a 5-minute fix.

That’s all I seem to know.

-Dave
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Old 7th October 2003, 12:27 PM   #5
moamps is offline moamps  Croatia
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Is this noise generated without any input cable (any source)connected to amp?
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Old 7th October 2003, 12:28 PM   #6
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Quote:
Before you do anything, check which appartus who is the bandite. Turn off everything and then turn on one thing at the time.
I agree w/ Peranders, but don’t just switch it off, physically remove it from the AC line. Line filters can still return ground path noise to the mains when the equipment is off.

-Dave
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Old 7th October 2003, 12:41 PM   #7
Tensop is offline Tensop  Lebanon
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Ok, ive tried disconnecting everything in the house except the essentials:

fridge
security alarm mains connection
cordless phone+answering machine

the noise was reduced slightly(the high end noise disapeared) yet i still have low end(100hz,50hz and something around 800hz(really, really do need an oscilloscope)
but this was still not as good as simply moving the amplifier to another mains point.
also i forgot to mention that the washing machine(same circuit) produces a very audible click.

Is it possible to run a power lead to the amplifier from another area of the house, or will this violate safety regulations/cause looping?
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Old 7th October 2003, 01:02 PM   #8
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OK, but first short circuit the input of the amp. Do you still have hum? Turn up the volume. Hum now? Maybe you have dry caps causing less smoothing or none for that matter? Check how much ripple you've got. If you have 40-50 volts you should not have more than a volt or two if you measure AC with you voltmeter.
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Old 7th October 2003, 01:25 PM   #9
Tensop is offline Tensop  Lebanon
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shorting the input created more noise.

i also have no accurate way of measuring ripple sorry to say
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Old 7th October 2003, 02:00 PM   #10
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Assuming that "moving the amplifier to another mains point" means another power circuit in your house, then there is the possibility of resistance causing a voltage drop on this particular circuit. I would suggest checking your switchboard (taking care of course!) for any fuses / circuit breakers getting warm or hot - a loose connection here will produce heat.

Failing that, a seperate circuit from the switchboard would be good idea, but would have to be installed by electrician.


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