Small earth hum, what could be a possible cause?

flyingtele · 6th October 2011, 01:44 PM

Okay, I know this is a pretty open ended question. I just got a dB Dynamics Orion subwoofer plate amp.
It works fine, but it has a very slight earth buzz while operating, even if there is no input present.
I have tracked the source to somewhere in the pre-amp circuit, and the pre amp circuit has its own power supply separate to the main power amp.
I tried replacing the capacitors in the power supply, and also the small caps on both the input and output of the preamp board, and it didn't help anything.

I decided to 'listen' to the output of the power supply that feeds the preamp, so I took an old speaker and put a resistor in series with it and connected it between ground and each of the rails, and got a substantial hum so I'm wondering the the power supply is at fault.
It is a basic circuit, with two 330 uF caps across either rail and ground post the rectifier, then both the + and - rails each go through a 12v voltage regulator, and then each output has another cap across it and ground.

Could a damaged voltage regulator in the power supply cause an earth hum on the output? Everything is well earthed, and I'm losing motivation on it. If its going to be more trouble than its worth I'll just leave it, as other than that problem it works fine and the hum is not very loud anyway.

Any help or suggestions would be welcome. I'm relatively new to electrical diagnostic and repair etc, my forte is speaker repairs. This amp is not a very expensive unit, so it wouldn't surprise me if its just down to poor component layout etc, but I thought it would be worth asking some folks who know more on the subject

Th3 uN1Qu3 · 6th October 2011, 04:42 PM

Your speaker test isn't relevant unless you also put a big capacitor in series with it... else you'll be feeding unregulated DC to it and it'll hum anyway.

Either poor component layout or too small main caps. Replace the first caps, the ones directly after the rectifier, with 1000u ones and see if it improves.

tauro0221 · 6th October 2011, 06:18 PM

Hi,
Does the power supply ground is connected to earth ground? Check the dc voltage with a voltmeter in AC. You should read almost zero if you power supply is working alright.

6th October 2011, 01:44 PM	#1
flyingtele diyAudio Member Join Date: Jun 2009	Small earth hum, what could be a possible cause? Okay, I know this is a pretty open ended question. I just got a dB Dynamics Orion subwoofer plate amp. It works fine, but it has a very slight earth buzz while operating, even if there is no input present. I have tracked the source to somewhere in the pre-amp circuit, and the pre amp circuit has its own power supply separate to the main power amp. I tried replacing the capacitors in the power supply, and also the small caps on both the input and output of the preamp board, and it didn't help anything. I decided to 'listen' to the output of the power supply that feeds the preamp, so I took an old speaker and put a resistor in series with it and connected it between ground and each of the rails, and got a substantial hum so I'm wondering the the power supply is at fault. It is a basic circuit, with two 330 uF caps across either rail and ground post the rectifier, then both the + and - rails each go through a 12v voltage regulator, and then each output has another cap across it and ground. Could a damaged voltage regulator in the power supply cause an earth hum on the output? Everything is well earthed, and I'm losing motivation on it. If its going to be more trouble than its worth I'll just leave it, as other than that problem it works fine and the hum is not very loud anyway. Any help or suggestions would be welcome. I'm relatively new to electrical diagnostic and repair etc, my forte is speaker repairs. This amp is not a very expensive unit, so it wouldn't surprise me if its just down to poor component layout etc, but I thought it would be worth asking some folks who know more on the subject

6th October 2011, 04:42 PM	#2
Th3 uN1Qu3 diyAudio Member Join Date: Feb 2010 Location: Bucharest	Your speaker test isn't relevant unless you also put a big capacitor in series with it... else you'll be feeding unregulated DC to it and it'll hum anyway. Either poor component layout or too small main caps. Replace the first caps, the ones directly after the rectifier, with 1000u ones and see if it improves. __________________ "Audio grade" components simply means that they failed at a more critical job.

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6th October 2011, 06:18 PM	#3
tauro0221 diyAudio Member Join Date: Nov 2008 Location: Victoria,TX	Hi, Does the power supply ground is connected to earth ground? Check the dc voltage with a voltmeter in AC. You should read almost zero if you power supply is working alright.

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