LP-2020A+ power supply noise - diyAudio
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Old 27th February 2014, 10:03 PM   #1
Dunk02 is offline Dunk02  Australia
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Default LP-2020A+ power supply noise

Hi Everyone,

I have a new LP-2020A+ and I have trawled through as many firms as I can but my brain is going crazy - so forgive me if someone has already posted on this topic. I have also posted same question to Audikarma and Vinylengine forums seeking help.

The provided PS is a 12V 5A unregulated AC/DC unit (see picture). Not sure what its intended use is but this is what was provided with the amp.

With this PS I get a significant hum through the 8ohm bookshelf speakers and flickering volume lights. This noise is unexpected and present with and without the mp3 player connected to the input jack. The hum intensity is NOT increased volume. The frequency of the audible hum at the speakers appears to be ~130Hz as measured with my iPhone fourier analysis app. I have a hobby oscilloscope and I think I have measured a 2.1V ripple at 7.8Hz at the PS DC plug - no load. There also appears to be a periodic wave at about 1.25Hz as well. I know little about these matters but a 17% ripple in the DC appears to be large.

I also have a variable output AC/DC-DC/DC laptop PS and it produces no hum from a 12V DC supply nor from a 240V/50Hz mains supply. I only hear a little white noise at high volume levels. Happy with this.

I have attached the waveform for analysis and an mp3 of the noise.

I tried different power points in the house with no change. Someone at Audiokarma suggested that PS might not be the problem but instead the input power capacitor - not sure about this as my the other PS is noiseless.

I also note that an AM radio placed near this PS picks up a lot of noise emanating from the PS and it is of the same frequency that is audible through the speakers. The radio signal coming from the other PS is much less. The PS supplied with the amp does jot appear to be adequately shielded perhaps??

The conclusion I am drawing is that the power supply is either not fit for purpose (RFI is too high, ripple is too high) or it is damaged.

I need to identify the problem so i can persuade the seller to refund my costs.

As you can see, the PS I received is not a wall wart and it could be an LCD or laptop PS - who knows! If I just blindly buy another unit it may also be unsuitable. The problem I have is that I can only by from eBay so if someone can identify an eBay item of known performance I would appreciate it.

Perhaps the provided PS can be modded/fixed cheaply - but I'll leave that for later.
Attached Images
File Type: jpg IMG_2389.jpg (53.8 KB, 77 views)
File Type: jpg PS output.jpg (48.1 KB, 73 views)
Attached Files
File Type: zip Humming Amp.mp3.zip (123.8 KB, 1 views)
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Old 27th February 2014, 11:07 PM   #2
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Can you clarify that you have powered the amp in question from other power supplies that generate 12VDC?

Measuring that PS unloaded may well generate quirky AC signals on the regulated 12VDC nominal, due to the low-load regulation method employed - but that is likely a red-herring.

Are you separating the speaker leads from power supply and leads as much as possible?

On balance, I'd say you have a poorly regulated or faulty power supply - the amp appears to be fine - so you can only really have a go at getting a replacement power unit - they may send you another gratis.
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Old 28th February 2014, 01:19 AM   #3
Dunk02 is offline Dunk02  Australia
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Hi Troy,
Yes the second PS is a 5A AC/DC unit providing variable volutes - I used 12/DC. The cable payout was identical in both situations. I moved the PS around with no effect. The speaker cables were not near the PS.
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Old 28th February 2014, 07:02 AM   #4
Dunk02 is offline Dunk02  Australia
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Well the eBay seller kindly refunded my money so I promptly broke open the PS.

I wonder if anyone would like to to discuss the layout of this PS, indicate its deficiency and tell me why it hums, and perhaps indicate anyway that it can be improved/salvaged.

Thanks
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Old 28th February 2014, 09:15 PM   #5
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Unless you purchase another such PS, and confirm it does the same, or is different, then it would be difficult for you to determine if it is faulty or just a poor design.

Without a circuit diagram and advise on what may be an improvement on a poorly designed PS, and then some test tools to measure before and after performance, it is difficult to go down that road for what is a $1 power supply.

Of course you could blaze away with some possible 'fixes', but without good information and assessment, any such fixes are just scatter gun approaches with a blind fold on.
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Old 1st March 2014, 07:55 PM   #6
Dunk02 is offline Dunk02  Australia
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Exactly, so can someone please exactly identify an eBay seller and PS that works?
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Old 2nd March 2014, 06:37 AM   #7
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12 VDC 5A Power Supply Bulk LOT X 32 | eBay

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Old 2nd March 2014, 07:34 PM   #8
Dunk02 is offline Dunk02  Australia
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Thanks but link does not work.
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