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Old 13th December 2007, 06:01 PM   #1
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Default strange hum problem

I have a hum issue. The attached image shows AMP-PSU Fig1 and fig2.

With fig1 hum is present with the amp input terminals umterminated. Short circuiting the input still produces hum. BUT!!! when the amp is connected to a source (pc/ cd player/ protable cd player powred with only batteries) the hum dissapears completely.

Trying fig2 does not remove hum but produces noise and instabilities.

The amps and PSU have been insterchanged for several other models but the problem persists. This issue is also happening with some commercial apparatus i purchased.

Would fig3 solve the problem or do i need to check all my mains wiring at home?

P:S:- i was living in another place before no issues were met am getting theses since i moved to a new place.


Thanks for the support
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Old 13th December 2007, 10:08 PM   #2
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Default Re: strange hum problem

Quote:
Originally posted by zeus_threat
I have a hum issue. The attached image shows AMP-PSU Fig1 and fig2.

With fig1 hum is present with the amp input terminals umterminated. Short circuiting the input still produces hum. BUT!!! when the amp is connected to a source (pc/ cd player/ protable cd player powred with only batteries) the hum dissapears completely.

Trying fig2 does not remove hum but produces noise and instabilities.

The amps and PSU have been insterchanged for several other models but the problem persists. This issue is also happening with some commercial apparatus i purchased.

Would fig3 solve the problem or do i need to check all my mains wiring at home?

P:S:- i was living in another place before no issues were met am getting theses since i moved to a new place.


Thanks for the support

Hi zeus_threat,

you have a serious internal ground loop. See below for grounding suggestion.
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Old 13th December 2007, 10:21 PM   #3
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Default Re: Re: strange hum problem

Quote:
Originally posted by Nico Ras

you have a serious internal ground loop. See below for grounding suggestion.
What changed between the last place and the new? There's not enough clues to take a stab at what's going on.

mike.
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Old 14th December 2007, 04:06 AM   #4
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Default Thanks for the reply

thanks for the reply guys. Well the PSU is already using star grounding, but like i mentionned its occuring with many amps and PSUs. Well actually mike the same appartus are being used in the new house, no new equipment has been added. I even unplugged all apparatus at home to test the amps but still the hum is present. The mains wiring have been made by a "qualified electrician" . Need to have a check at it. Is there anything you could suggest to investigate.

Thanks
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Old 14th December 2007, 07:10 PM   #5
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Default Re: Re: Re: strange hum problem

Quote:
Originally posted by MikeBettinger


What changed between the last place and the new? There's not enough clues to take a stab at what's going on.

mike.
I will make one suggestion to see if it is the problem, but this impairs the safety aspect of the equipment. Remove the ground connection to the mains plug and see if the hum goes away.

If it does then there is an earth fault somewhere in your building.
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Old 14th December 2007, 10:37 PM   #6
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Default Re: Re: Re: Re: strange hum problem

Quote:
Originally posted by Nico Ras


I will make one suggestion to see if it is the problem, but this impairs the safety aspect of the equipment. Remove the ground connection to the mains plug and see if the hum goes away.

If it does then there is an earth fault somewhere in your building.
I don't see an earth ground in the first drawing and one included in the second, which I guess didn't help. The third adds a noise filter which might help if it was RF noise.

What I think might be happening is the supply voltage might be lower (I have seen this). What kind of amp are we talking about here, DIY? Are there any regulators for the input stages? If the input voltage to the regulators was lower because the line voltage to the house is lower (country location, old house and wiring, tired transformer on the pole outside) the regulator performance can get marginal and hum is a side effect. If this is the case listening late at night should possibly have less hum.

Other then that there's not enough info for me to troubleshoot this further.

Regards, Mike
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Old 17th December 2007, 07:42 AM   #7
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Default thanks for the reply

Thanks for the replies. Well as you see in fig1 i was not using the mains earth at all but still the hum is present.

Well regarding the amps it happends with diy and commercial amps(not all though i got a pioneer and yamaha map which do not have any issue at all, these are double isolated and do not use mains earth at all).

The amps (diy and commercial) that have this issue are class AB some are discrete others are ic based but none use regulators for the input stages.

I will try to test it at night to check if the hum still remains.

I measured the mains and its at 240V. I have also measured transformer secondaries (for the amps getting problems) against their rated output voltage and the rated value is being output

But the thing that buzz me in all that is how the hum suddenly dissapears when the amps are connected to a source(pc, cd player powered only by batteries) but when the amps inputs are unterminated the hum is there.

Any clue on what could be causing this?

Thanks
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Old 17th December 2007, 12:35 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
could you add a shorting plug to one and both input terminals and see if the input loading cures the hum?
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Old 18th December 2007, 02:55 PM   #9
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Default even more strange

Hi well shortcircuiting produeces hum, connection to source there is no hum really can't figure out what's causing this
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