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Old 29th January 2014, 01:34 AM   #1
skidave is offline skidave  United States
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Default ARTA Measurement of Power Amp

I'm looking for some feedback on some ARTA testing that I have been doing. I finally feel very comfortable with the software and E-MU 0204 combo.

Below is a screen shot of a solid state power amp I have as a test amp for my bench. This image is the amp at 95 watts into 8 ohms.

So I can never really get rid of all the 'grass' on the high end. That does not bother me that much...but I'm curious about the other spikes between the harmonics.

I'm looking for feedback on what I have captured here.

Also, many thanks to RNMarsh, DennisMiller55, Davada and 1audio. Your knowledge, posts and some of your direct emails have been greatly appreciated!

Dave

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Old 29th January 2014, 02:27 AM   #2
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I would think the first thing is to look at the FFT of a loopback signal of the 0204 in your system. Once you determine that is clean you go ahead. What does the 1 watt FFT look like ?
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Old 29th January 2014, 05:10 AM   #3
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Not to bash ARTA, I use it all the time, but if you want to get distortion numbers I would suggest REW (Room EQ Wizard) using the RTA module as a spectrum analyzer.

For instance I took these measurements of a chip amp when I was trying to reduce ripple and mains feedthrough via the power supply:

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Old 29th January 2014, 07:04 AM   #4
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The 'STEPS' program part of ARTA gives very accurate FR and distortion measurements. I would suggest you run this with at least a 96kHz sample rate on your sound card to get some baseline distortion v. frequency measurements at your desired output level first.

There's a lot 60Hz power-line harmonics visible in the original post!
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Old 29th January 2014, 11:40 AM   #5
skidave is offline skidave  United States
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Quote:
I would think the first thing is to look at the FFT of a loopback signal of the 0204 in your system. Once you determine that is clean you go ahead. What does the 1 watt FFT look like ?
My loop back is clean with low distortion numbers and just a bit of grass above 10K. I don't have a capture of 1 Watt, but I will look next time and compare.

Quote:
The 'STEPS' program part of ARTA gives very accurate FR and distortion measurements. I would suggest you run this with at least a 96kHz sample rate on your sound card to get some baseline distortion v. frequency measurements at your desired output level first.

There's a lot 60Hz power-line harmonics visible in the original post!
I have used STEPS, just last night I was using the spectrum analyzer portion of ARTA. The 60 Hz noise is from the HP generator I was using. The noise is not there when I use the internal sine generator.

Quote:
Not to bash ARTA, I use it all the time, but if you want to get distortion numbers I would suggest REW (Room EQ Wizard) using the RTA module as a spectrum analyzer.
I believe I have REW's latest version, I just have not loaded it yet.

What are you using for a voltage divider / interface box for your soundcard?

Thanks everyone!
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Old 29th January 2014, 11:53 AM   #6
esgigt is online now esgigt  Netherlands
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I Guess your HP Generator needs some TLC...
Review the smoothing caps. And the grounding
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Old 29th January 2014, 12:57 PM   #7
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Quote:
Originally Posted by esgigt View Post
I Guess your HP Generator needs some TLC...
Review the smoothing caps. And the grounding
Yes. I see line frequency and rectifier-produced harmonics, supply stuff.

Jan
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Old 29th January 2014, 01:35 PM   #8
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Some of harmonics may be due to quality of load resistor and resistors in voltage divider.
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Old 29th January 2014, 02:00 PM   #9
skidave is offline skidave  United States
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Quote:
Some of harmonics may be due to quality of load resistor and resistors in voltage divider.
This is what I was thinking about. I'm using a non-inductive load resistor [8 ohms @ 100 Watts] attached to a very large heat sink sitting on the cold floor of my concrete basement.

For the voltage divider, I'm using 1% metal film resistors to create a divide by 10 or divide by 20 ratio.
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Old 29th January 2014, 04:33 PM   #10
1audio is offline 1audio  United States
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The harmonics generayed by modern resistors will not be measurable with that hardware. I see lots of ground/usb noise in the plot. Its a common and difficult problem to deal with.

1)tie a seperate ground from the DUT to the computer
2) use differential inputs to the 0204. This means making a passive differential divider to tie to the amp output.
3) explore the ground switch o the bottom of the 0204 (I think it has one).

You still may have problems with the output of the 0204 sharing a ground with the DUT.

These are common problems with USB audio interfaces.
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