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DIY Audio Analyzer with AK5397/AK5394A and AK4490
DIY Audio Analyzer with AK5397/AK5394A and AK4490
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Old 24th April 2017, 03:44 PM   #1001
cwtim01 is offline cwtim01  Hong Kong
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Quote:
Originally Posted by 1audio View Post
Here are a few measurements from the RTX6001:
First is loopback at 1V
Thanks for these measurements. The loopback measurement shows a THD of -111dB, but the specification states that Analyzer residual THD @1kHz would be "typical -130 dB, < -120 dB @ 0 dBV". How would you interpret this difference.

I already have sound cards (Lynx Hilo and Terratec Phase X24) that can do better than -110dB THD loopback @0 dBV in ARTA, and so the -120dB THD spec is an important element for me.
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Old 24th April 2017, 04:24 PM   #1002
soongsc is offline soongsc  Taiwan
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I am wondering whether it is possible to actually use software to calibrate hardware distortion on a per unit basis.


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Old 24th April 2017, 04:31 PM   #1003
lemon is offline lemon  Greece
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@cwtim01
It is loopback measurement with BW 20-70K with THD -111dB.
For BW 20-20K <-115dB from specifications.
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Old 24th April 2017, 04:47 PM   #1004
anatech is offline anatech  Canada
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DIY Audio Analyzer with AK5397/AK5394A and AK4490
Hi Jens,
Quote:
Calibration is fairly simple and requires only a good voltmeter and a screwdriver.
And also a good commercial audio sine wave generator. That's fantastic! I have some HP audio generators that are more than good enough to calibrate your unit, and the HP 34401A I have will allow extremely close calibration. I can keep those things calibrated and transfer the accuracy to the RTX. So self-calibration and performance confirmation is wonderful! Many thanks for allowing this!
Quote:
Regarding recommendation about how often it should be calibrated, this is still open.
It is will all equipment. These days this is tending towards a 2 year cycle and probably depends on input attenuator and your reference voltage drift. So depending on your voltage reference, that might determine the calibration cycle. These normally drift the most when new and after a year or so the drift settles down to some lower rate per year. So, worst case calibration will be in the first couple years.
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Old 24th April 2017, 07:09 PM   #1005
JensH is offline JensH  Denmark
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DIY Audio Analyzer with AK5397/AK5394A and AK4490
Quote:
Originally Posted by cwtim01 View Post
Thanks for these measurements. The loopback measurement shows a THD of -111dB, but the specification states that Analyzer residual THD @1kHz would be "typical -130 dB, < -120 dB @ 0 dBV". How would you interpret this difference.

I already have sound cards (Lynx Hilo and Terratec Phase X24) that can do better than -110dB THD loopback @0 dBV in ARTA, and so the -120dB THD spec is an important element for me.
I am not quite sure what levels Demian used for the loopback measurement. It doesn't look like 1V to me. The level is measured at -2.1dBFS. If the level was 1V I would expect the measured level to be -10dBFS at the 10dBV input setting (or 0dBFS at the 0dBV setting).

I have attached a measurement, where I have an output of 1V (verified with my HP 34401A). The input is set to the 10dBV range, so the level measured is -10dBFS.
In this case I measure a loopback distortion of -124.7dB.
It is generally a good idea to use levels well below 0dBFS when measuring distortion (THD), because the distortion of the ADC (AK5394A) does increase when approaching full scale.

You refer to the Analyzer residual THD @1kHz, which would be "typical -130 dB, < -120 dB @ 0 dBV"
This is not the loopback figure, but the analyzer (input) only.
I have attached another measurement, where I used a low distortion oscillator with balanced output to drive the analyzer input. In this case I do see a distortion of around -130dB at a level of 1V.

Jens
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Old 24th April 2017, 07:23 PM   #1006
cwtim01 is offline cwtim01  Hong Kong
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Thanks JensH, these Audio Tester graphs are much more re-assuring.
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Old 24th April 2017, 07:53 PM   #1007
agdr is offline agdr  United States
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Quote:
Originally Posted by JensH View Post
You refer to the Analyzer residual THD @1kHz, which would be "typical -130 dB, < -120 dB @ 0 dBV"
This is not the loopback figure, but the analyzer (input) only.
I have attached another measurement, where I used a low distortion oscillator with balanced output to drive the analyzer input. In this case I do see a distortion of around -130dB at a level of 1V.
Good point! It is easy (for me anyway) to forget that in loopback the total distortion is the distortion of the analyzer plus the distortion of the generator. Hence Victor oscillators.
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Old 25th April 2017, 07:22 AM   #1008
DualTriode is offline DualTriode  United States
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Default 60hz power supply hum and resulting series of distortion frequencies

Hello All,

It appears that the first plot in 1audio’s post #990 is much more telling. It better represents how this instrument will be applied. The only thing missing is the DUT placed between the function generator and the input.

We are not going to apply this instrument with its input only or with some high quality standalone oscillator. I am concerned about the presence of the 60hz power supply hum and resulting series of distortion frequencies in the loopback configuration. The 2Khz and 3Khz spikes are very much like those seen in the AP or Keysight U8903B analyzers.

JensH, please show us an audio Tester loopback with a -10dBV signal applied.

DT
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Old 25th April 2017, 09:11 AM   #1009
JensH is offline JensH  Denmark
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Here is a loopback measurement with an output of -10dBV.

Output attenuator set to 0dBV range
Input attenuator set to 0dBV range
Output to DAC = -10dBFS
48 kHz sample rate

The settings in the loopback plot in post #1005 were:
Output attenuator set to 20dBV range
Input attenuator set to 10dBV range
Output to DAC = -20dBFS
192 kHz sample rate (does not make much difference)

Changing from -20dBFS in the previous plot to -10dBFS increases the distortion of the DAC.

The 50/60 Hz hum can be a challlenge, especially when using unbalanced connections. Using the balanced inputs and proper cables will normally keep the noise at an acceptable level.

I agree that using a very low distortion standalone oscillator should not be necessary in most cases. In this case it was used to separate the issues, so that only the distortion of the analyzer was measured. With a loopback connection, and no other reference, you don't know if the distortion you see is caused by the generator or the analyzer.
Using a 1kHz notch filter I recently measured the distortion of the "Low Distortion Oscillator" to be well below the limits of the analyzer input/ADC.
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Old 25th April 2017, 09:41 AM   #1010
zfe is offline zfe
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Regarding the power line hum. It is probably to late for such additions, but how about providing the possibility of an alternative supply power (e.g. battery) by the user.
Easiest would perhaps be a switch cutting the secondary AC and switch the input of the regulated supply to some external connectors, where the user can supply the necessary.
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