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Old 2nd September 2012, 07:19 PM   #611
TheGimp is offline TheGimp  United States
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The Mu Metal shield will drop line interference. I think it was about a 15dB reduction in my case.
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Old 2nd September 2012, 07:57 PM   #612
disco is offline disco  Netherlands
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Do soundcards give reliable numbers these days already?
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Old 3rd September 2012, 12:43 PM   #613
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Quote:
Originally Posted by TheGimp View Post
The Mu Metal shield will drop line interference. I think it was about a 15dB reduction in my case.
Where did you put the ultraperm? I have some I can use. Did you put it on the inside surfaces of the plastic box or what? I want my 15dB!
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Old 3rd September 2012, 01:13 PM   #614
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One other thing to address -- when you are measuring in the nano-Volts it's imperative to use shielded twisted pair. It's also helpful to read the grounding tutorial on Rane Electronics website. Grounding and Shielding Audio Devices

When I was performing the regulator noise tests for the upcoming Linear Audio article I experimented with many cables, configurations of connectors and housings. Use the shortest cable possible for your tests. Shielded twisted pair (balanced) with Belden cable and XLR's, wired to the Rane application note, were best for knocking down low frequency power supply garbage. Better than the commercial XLR interconnects I usually use.

Last edited by jackinnj; 3rd September 2012 at 01:18 PM.
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Old 3rd September 2012, 01:53 PM   #615
SY is offline SY  United States
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Quote:
Originally Posted by disco View Post
Do soundcards give reliable numbers these days already?
They have done so for a long time, if you use a calibrated reference to establish absolute levels.
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Old 3rd September 2012, 03:21 PM   #616
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They have done so for a long time, if you use a calibrated reference to establish absolute levels.
What's the noise floor of your setup? I haven't found a soundcard that was better than the stuff I'm trying to measure. Even the external soundcards typically available aren't all that clean. Most are something like -110dB or so at best in a loop back test. I would think good instrumentation should be much lower than that.
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Old 3rd September 2012, 03:42 PM   #617
SY is offline SY  United States
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Depends on bin size and frequency, but the PCI card I have (nothing exotic) gives me -155 to -160dB relative to full scale. Knock off 8dB or so for ref to 1V.

Of course, you can put a low noise amp in front as well. for measurement of very delicate signals.
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Old 3rd September 2012, 04:28 PM   #618
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Quote:
Originally Posted by SY View Post
Depends on bin size and frequency, but the PCI card I have (nothing exotic) gives me -155 to -160dB relative to full scale. Knock off 8dB or so for ref to 1V.

Of course, you can put a low noise amp in front as well. for measurement of very delicate signals.
Wow, that's impressive, what card are you using? I've never seen a card rated for that low of a noise floor.
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Old 3rd September 2012, 04:51 PM   #619
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Hi all,

I think that SY don't speak about noise floor, but rather the noise level of the computed FFT. This floor depend on the FFT averaging and FFT size.It can be much more lower than effective noise floor.

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Old 3rd September 2012, 04:59 PM   #620
TheGimp is offline TheGimp  United States
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I actually got 20dB reduction in Float mode vs 16dB in Gnd Mode. But this is all relative, and not with any real cal standard so take it with a grain of salt.

You don't even have to install the Mu Metal to see the effect, I just cut the sheet in half and made a sandwich with the interface and I saw over 10dB reduction.
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File Type: jpg Mu.JPG (114.5 KB, 207 views)
File Type: jpg MuMetal.JPG (120.3 KB, 217 views)
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