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Old 3rd August 2011, 11:25 AM   #1
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Default Removing noise from power grid

I have bought a Skytec Pro 240 amplifier and I am having trouble with noises from the power grid. The main problem is the elevator here in the building, which produces unpleasant noise when someone is using it.

There are four 1000 uF filtration capacitors in this amplifier. Would it be enough to install 10000 uF capacitors instead or do I need to do something else with the power supply? I am not an expert, any help would be appreciated.
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Old 3rd August 2011, 11:48 AM   #2
DF96 is offline DF96  England
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Just slapping in more capacitance can create more problems, and might not solve the issue. Is the noise entering via the mains lead or other cables such as speaker leads? That is, is it conducted or radiated interference? Do other items have the same problem?
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Old 3rd August 2011, 12:05 PM   #3
marce is offline marce  United Kingdom
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Sounds like the Skytec's EMC immunity isn't the required level. These days when designing products you have to expect the worse caqse EMC problems.
In the UK, BT Vision have a exemption from certain aspects of the CE requirements beacuase it is based on EMI, adding high frequency mush to the mains, which all other equipement will have to filter out.
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Old 3rd August 2011, 12:09 PM   #4
AndrewT is offline AndrewT  Scotland
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The mains noise could be attenuated by fitting a filter at the mains input socket of the equipment. Does your equipment have a mains filter?

These are sold as ready made assemblies that bolt to the chassis and provide an IEC socket on the outside face for connecting to the mains.

Is the interference air borne?
Is the signal input filtered?
Again an RF filter can be added. This is a simple DIY modification.

Is the output filtered?
Not quite so easy to do DIY, but certainly possible and not costly.
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Old 3rd August 2011, 02:39 PM   #5
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Thanks for the replies! The interference is definitely conducted, it is caused by the elevator running. I opened the door and I could confirm that the buzzing starts just as the elevator starts moving and stops when it stops. It is also dependent on the output level - when I turn the amplifier up, the interference sounds get louder as well. The walls here are reinforced concrete so I think it is quite unlikely for airborne interference to get through those walls all the way from the top of the elevator shaft.
The only airborne interference sources were mobile phones and I removed them from the room.

There is actually a lot of room in the amplifier chassis, I could install some filtering device inside, without the need of another box.

I did not get any documentation with this amplifier, how can I verify if it has a mains filter? The only thing that seems to be able to filter anything are those four capacitors I mentioned, but that is only my uneducated guess.

What do you suggest?
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Old 3rd August 2011, 02:55 PM   #6
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What Andrew talks about looks like this
Click the image to open in full size.
No need for an extra box...
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Old 3rd August 2011, 03:19 PM   #7
DF96 is offline DF96  England
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RF goes through concrete, so radiated interference is certainly possible. As it gets worse at higher volume settings, it is getting in at the beginning. Does it still occur wth no inputs plugged in, or only on some inputs?

The capacitors are for smoothing, not filtering interference. They may have some limited filtering function too, but this is a side-effect of smoothing.
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Old 3rd August 2011, 05:20 PM   #8
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Thanks for the suggestions!
I tried it without input connected and the noise was not there. I had this connected to an integrated sound card (a temporary measure) so I tried connecting it to a portable mp3 player. The noise was not there. However, when I connected it to the integrated sound card again, the noise returned.
So the problem seems not to be in the amplifier or cables, it is in that integrated soundcard!

However, I plan to use this with a different computer and a stand-alone, higher-quality sound card. Do you think that the interference will be present even on the new PC with a better, stand-alone soundcard? The new PC will also have a better power supply so maybe if the interference is conducted, the problem will not be there. But if it is airborne, the new PC might have the same problem.

Is there any way I can verify whether the interference is conducted or airborne? Would installing an RF filter on the amplifier input help in this case? Does the filter affect sound quality?
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Old 3rd August 2011, 06:29 PM   #9
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PC powersupply....

Many older amps have smallish cermic caps right behind the input RCA's normally lower values like 8p or so, but I have seen magnitudes higher advised, give whatever you have in the parts drawer a shot, it can't break anything.
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Old 3rd August 2011, 06:39 PM   #10
DF96 is offline DF96  England
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PC power supplies often omit mains filtering components. This saves the manufacturer a few pence. The locations for them are left in place on the PCB, so if anyone complains he can say they were omitted by accident.

Does the noise occur in a battery powered AM radio? If so, it is radiated. On the other hand, it could be poor filtering and bad grounding in the PC power supply - they are not usually designed for high sound quality.
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