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Old 6th October 2008, 06:34 PM   #21
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Hmm, even an el cheapo Sounblaster Live has very lo noise and hum, provided you use its native 48 kHz sample frequency.

Attached a picture of a M-Audio Transit USB card. Red is powered directly from the PC, yellow is powered from an external powered hub, need I say more? Apart from this, Zero Cool is interested in levels > 1%.

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Old 6th October 2008, 07:19 PM   #22
Hardi is offline Hardi  Germany
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There´s still one "problem". THD+N components are only recognized up to 20 KHz.
That is not so important for listening, but it CAN be important for the measuring results.
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Old 6th October 2008, 07:51 PM   #23
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True, but you can measure harmonics separately as well as combined with a wideband true RMS meter. I assume it is known how THD and THD + noise is defined.

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Old 6th October 2008, 08:20 PM   #24
Hardi is offline Hardi  Germany
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I think that´s exactly the problem when you want to align a 339 with a soundcard as source.
The unknown value of components in the 339 measuring range.
LP filtering will help, but it´s still hard to get clear results.
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Old 6th October 2008, 09:41 PM   #25
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Quote:
Originally posted by Hardi
Some soundcards are too noisy while having a clean THD spectrum up zo 20 KHZ. There can be aliasing noise or low frequency hum....Also with some "good" cards.

The 339 measures everything (hum + noise) up to 110 KHz!

It´s also a good idea to use the HP339s 30 KHz filter in this case.
The THD section can measure THD of fundamentals up to 110 kHz, but harmonics up to 330 kHz are included in THD measurements unless rolled of by a low pass filter, according to the specs.

In post #22 you say "THD+N components are only recognized up to 20 KHz." What do you mean by that? The specs would seem to suggest that the instrument "recognizes" components up to 330 kHz (unless rolled off).
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Old 6th October 2008, 09:42 PM   #26
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Quote:
Originally posted by Pjotr
Hmm, even an el cheapo Sounblaster Live has very lo noise and hum, provided you use its native 48 kHz sample frequency.

Attached a picture of a M-Audio Transit USB card. Red is powered directly from the PC, yellow is powered from an external powered hub, need I say more? Apart from this, Zero Cool is interested in levels > 1%.

Can you show the spectrum up to 330 kHz?
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Old 7th October 2008, 08:14 AM   #27
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Quote:
Originally posted by The Electrician


Can you show the spectrum up to 330 kHz?
What’s your own opinion? No, of coarse not with a soundcard at 48 kHz sr. But as stated earlier you can see what the order of the residual D+N is of a 1 kHz sine with the HP 339A, even if it is not calibrated.

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Old 7th October 2008, 09:35 AM   #28
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Quote:
Originally posted by Pjotr


What’s your own opinion? No, of coarse not with a soundcard at 48 kHz sr. But as stated earlier you can see what the order of the residual D+N is of a 1 kHz sine with the HP 339A, even if it is not calibrated.

What's my opinion about what? Whether your spectrum analyzer can show the spectrum up to 330 kHz? I haven't the slightest idea what the capability of your spectrum analyzer might be; that's why I asked.

I can only see what the order of the residual D+N is below 20 kHz. How do we know that there aren't significant spurs above 20 kHz?

I think the issue Hardi is raising is, since the 339A sees spectral components up to 330 kHz, it would be good to know if the output of the sound card has any energy above 20 kHz.
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Old 7th October 2008, 09:39 AM   #29
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IMO all suggestions so far are much too complicated.

An old-fashioned THD+N meter is nothing but an AC voltmeter with a switchable and tunable notch filter to filter out the fundamental.

You first set the total voltage as 100% then you notch out the fundamental and measure the remaining voltage as percentage of the first measurement.

So why don't you just feed the distortion meter a 1 kHz signal from a function generator at 1 kHz and set the reading to 100 % as the first step. Then you switch in the notch and tune to lowest reading. Then you tune the function generator to 2 kHz and feed the distortion meter a reduced level of this 2 kHz. I'd suggest that you do this by using a purpose-built switchable attenuator in order to make this fast and repeatable.

With this method it shouldn't be too difficult to check the accuracy of a THD meter even when the THD of the signal generator isn't very low.

Regards

Charles
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Old 7th October 2008, 03:20 PM   #30
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Did you read post #9 and #12 in this thread? The method you describe has already been suggested, different only in minor details.
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