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Old 21st May 2011, 05:56 PM   #1
NYCOne is offline NYCOne  United States
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Default Using PC Oscilloscope freeware as a distortion analyzer

I have Visual Analyzer 2011 – a freeware PC based oscilloscope program. The program has essentially no documentation; it assumes you’ve used an oscilloscope before.

I’d like to do the traditional measurements for an amp and preamp (freq response, distortion, etc). Can someone point me in the right direction for a “how to” to newbies on doing these measurements?
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Old 24th May 2011, 05:29 AM   #2
singa is offline singa  Singapore
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Build pete millet's Soundcard Interface
so you won't fry your soundcard.See if he has some boards left to order.

See old posts of visual analyser there are some links to english documentation.

Last edited by singa; 24th May 2011 at 05:32 AM.
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Old 24th May 2011, 05:50 AM   #3
richiem is offline richiem  United States
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The 'scope software by itself cannot do distortion analysis except to display gross aberrations of waveforms -- assuming you know what the waveform is supposed to look like. The 'scope draws a graph of amplitude (usually voltage) on the Y-axis (vertical) versus time on the X-axis (horizontal) -- a fine analytical tool for many things that I think is absolutely indispensable. So once you establish a reference level and frequency, then you can see level differences at different signal frequencies, which is what frequency response testing is. Fine differences are hard to see, but 1dB of level change is about a 10% change in level, so you can do fairly accurate measurements with the 'scope, especially if you are careful to always check the output signal level of the DUT (device under test) against the actual input signal level driving it.

But the ARTA suite of software (free download) can be used almost without limits in demo mode and uses its signal source through the PC's sound output to do frequency response testing, showing a typical graph plot of level vs. frequency, and can do distortion analysis with its built-in signal source and spectrum analyzer. I think this will be of more use to you than the basic oscilloscope program, although that is a useful tool.

I would suggest that you Google "testing audio amplifiers" and "using electronic test equipment", "using oscilloscopes" etc, then follow links -- it's all out there, but sometimes finding it is hard, especially at the entry level. Initially, of course, the hard part is learning the lingo.
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Old 3rd March 2012, 06:29 PM   #4
NYCOne is offline NYCOne  United States
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Quote:
Originally Posted by richiem View Post
The 'scope software by itself cannot do distortion analysis except to display gross aberrations of waveforms -- assuming you know what the waveform is supposed to look like. The 'scope draws a graph of amplitude (usually voltage) on the Y-axis (vertical) versus time on the X-axis (horizontal) -- a fine analytical tool for many things that I think is absolutely indispensable. So once you establish a reference level and frequency, then you can see level differences at different signal frequencies, which is what frequency response testing is. Fine differences are hard to see, but 1dB of level change is about a 10% change in level, so you can do fairly accurate measurements with the 'scope, especially if you are careful to always check the output signal level of the DUT (device under test) against the actual input signal level driving it.

But the ARTA suite of software (free download) can be used almost without limits in demo mode and uses its signal source through the PC's sound output to do frequency response testing, showing a typical graph plot of level vs. frequency, and can do distortion analysis with its built-in signal source and spectrum analyzer. I think this will be of more use to you than the basic oscilloscope program, although that is a useful tool.

I would suggest that you Google "testing audio amplifiers" and "using electronic test equipment", "using oscilloscopes" etc, then follow links -- it's all out there, but sometimes finding it is hard, especially at the entry level. Initially, of course, the hard part is learning the lingo.
I see that ARTA can be used to test speakers. I cannot tell if ARTA can be used to test amps for THD, etc. I built the test box indicated on the ARTA website, but I have not had the chance to try it yet.
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Old 3rd March 2012, 08:07 PM   #5
tvrgeek is offline tvrgeek  United States
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I use TRUERTA, but I bought the high res version. I have more expensive and fancier tools, but I keep going back.

Take the comments on building an interface seriously. All it takes is one spike and good by sound card.
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Old 3rd March 2012, 10:04 PM   #6
richiem is offline richiem  United States
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@NYCOne -- Yes, ARTA can give spectra of amplifier distortion products and calculate both THD and THD+Noise. Be aware that the residual distortion is limited by the THD of the sound-card/input device you use with it, and that the measurement bandwidth is limited by the sampling frequency Nyquist limit of somewhat less than 1/2 the sampling freq., which means that seeing harmonic up to the fifth will limit the input frequency to about 1/12th the sampling rate of the analog-to-digital converter system.

For the 16-bit, 48kHz converters in many PCs, the limits will be around .005% and an upper frequency limit of about 4kHz for realistic measurements. My PC's 24-bit, 96kHz internal Intel HD Audio system will do around 0.002% with frequencies up to about 8kHz. My EMU 0204 24-bit, 192kHz USB ADC will do around 0.001% and an upper frequency of about 16kHz.

By using a Twin-T active filter to notch out the fundamental by 60dB, and then driving the EMU, I can get THD readings below 0.0005% (5 parts-per-million), which is pretty good. Individual harmonic products can be seen to well below -130dBu.

See my web-page on the active Twin-T at Active Twin-T filter.
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Old 4th March 2012, 12:18 AM   #7
tvrgeek is offline tvrgeek  United States
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I use the e-mu 1616 as it is also a mic preamp for acoustic measurements. 96K @24 bits gives you pretty good to 40Khz at .001 or less. Quite usable.
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