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Old 29th March 2009, 10:39 PM   #1
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Question Question - THD Measrement/Spectrum Analyzis

Hi !

I have a few questions regarding measurement of THD and their spectrum.

1) What PC software and sound card would you recommend? I am not in the mood to buy stand-alone spectrum analyzer for many thousands of $$$ just for hobby purposes.

2) I have constructed dummy load (to reduce output voltage from amplifier) using 10 Ohm 25W Russian-made wire potentiometer (rheostat) + speaker cables (D1.5mm 10sm no shield) + shielded audio cable (5 sm) with standard connectors.

The measured inductance of this pot is 20 uH (70+ rounds of thick wolfram alloy wire over ceramic tube).

I think parasitic inductance of this pot and short non-shielded cables should not induct considerable level of hum, since resistance of this circuit is very low (10 Ohm max).

Please correct me if I am wrong.

Thanks in advance for any suggestion(s).
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Old 30th March 2009, 12:18 AM   #2
Jeb-D. is offline Jeb-D.  United States
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First, I'd get some 8ohm dummy loads that aren't inductive.

I recommend the E-MU 0202 soundcard if your only going to be measuring analog equipment. It's pretty inexpensive and does a pretty good job.

For software, I like Multi-Instrument 3.1. It seems to offer more variety of testing than any freeware. I'm currently using the trial version. When my time expires, I'll probably buy it, though I'm not sure which level I will get. I think the "device test plan" add-on is a must.

Here are a couple examples of measurements that I haven't seen supported with other software. This is a single ended amplifier, so don't be too surprised on the non-linearity.

Output voltage vs. THD. (This checks clipping characteristics)
Click the image to open in full size.

Input vs. output voltage (checks for amplitude distortion)
Click the image to open in full size.
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Old 30th March 2009, 12:22 AM   #3
Jeb-D. is offline Jeb-D.  United States
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If you are on a really tight budget, just use your stock soundcard and some freeware such as visual analyser or daisyweb. You will find it very limiting, but it's better than nothing.
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Old 30th March 2009, 02:10 AM   #4
kevinkr is offline kevinkr  United States
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Trial version of Arta works well, can't save setups or print files, but will give you a sense of what you can do.

I currently use Arta and audiotester.de software which are not freeware. Audio Rightmark has its adherents and is quite good - and is free.

Hardware I use is all M-Audio, depending on system either the transit (inexpensive usb codec) or M-Audio Audiophile 2496 PCI - both available reasonably priced on eBay.
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