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Noise Measurements


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Also, it seems Aussie-land has a lot of line noise problems. If so, why?
Interference from all that chatter, perhaps? - Pommy bashing, moaning about the current state of the Baggy Greens, general mouthing off, ...

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One has to know what they are doing to measure AC power line noise without damaging the test equipment or having the test equipment become part of the noise problem.

 

But the real measurement should be how does the noise affect the audio output of the hi-fi system. For that a battery powered RMS DMM and/or a battery powered O-scope at the amp's speaker terminals is the best test.

Well designed hi-fi equipment should be able to deal with a dirty AC power line.

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Here's a couple of shots showing noise. Worthwhile to remove.

 

Fig 8 FFT Plot of output from JC1 amplifier with Stock power cord.jpg

 

This is the FFT at the speaker terminals of a JC1 amplifier with a stock power cord. It's full of harmonics and shite.

 

Signal at Speaker Terminal 0db 250Hz FFT.png

 

This is a screen shot of my speaker's terminal voltage FFT. Just the fundamental exists.

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Here's a couple of shots showing noise. Worthwhile to remove.

 

[ATTACH=CONFIG]30993[/ATTACH]

 

This is the FFT at the speaker terminals of a JC1 amplifier with a stock power cord. It's full of harmonics and shite.

 

That's an 8-bit scope. You won't get a cleaner reading than that.

 

[ATTACH=CONFIG]30994[/ATTACH]

 

This is a screen shot of my speaker's terminal voltage FFT. Just the fundamental exists.

 

What measurement device is that? What is the measurement supposed to prove?

 

These images show the presence of a 60 Hz tone. If this is power line hum making it to the amp output, it only proves that your amp has a problem, nothing about the quality of the mains power.

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Ok. The TEK scope picture is from the Polk Audio site posted by Raif Smith to determine which AC power cord works best at attenuating 60Hz and harmonics. Measurements were made at the speaker terminals. If you google : Studies On Residential Power Line Noise Part 1-Statement SC Power Cord you can also find more references.

 

The article refers also to Parts1, 2, & 3 however these links are now gone from the Pol audio site. If anyone wants the full set, I have the posts, all 8 parts, PM, and I might upload to Dropbox.

 

The other screen shot is from a Fluke 195C scope which shows a 250Hz signal fed into an amp measured at the speaker terminals. Unlike Raif Smith's TEK scope photos, there's no harmonics, just the 250Hz fed by Jriver.

 

The 60Hz crud could be conducted leakage currents at mains frequencies by a device that loves to make them. At this power I can only think of a power amp/pre amp perhaps. Diode/cap load a possibility that can create harmonics like this.

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I think when we discuss various types of transformers to isolate/balance AC power we are on solid ground. Beyond a well constructed AC cord I have a hard time figuring out why I don't want to do all the filtering in my power supplies themselves.

 

 

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