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The QA401 test box described by Bob in the first half of the video appears to be a very reasonably priced and useful tool for measuring amplifiers. I can see one of these landing on my test bench in the near future:
Still watching the 2nd half on speaker testing. Thanks for sharing.
Finished the video (almost 3 hours - that was long!!)
Spkr section (Demian M): Positives: I like how his $1 DIY Giradin mic (mounted on end of a 12" brass tube) was very close to the calibrated measurement from an expensive B&K 4135. He mentioned that this mic used a panasonic capsule. Probably the same cheap one that I used when I built Joe D's Mitey Mic several years ago. Also liked his speed of sound rule of thumb: 1ms per foot. So if you see a .5ms glitch on the impulse, look for an obstruction roughly 6 inches from the driver.
Negatives: He talked mainly about measuring finished speaker systems, not designing & building speakers from scratch. Wish someone would post a video showing all the steps involved in building a nice speaker from scratch, including measurements & crossover design. Was also a little disappointed that they ran out of time toward the end. The promised "hands-on" loudspeaker testing demo was shelved. Perhaps this will show up in a future part 2 video.
Amp section (Bob C): Pos: Liked the testing demo at the end of the video. The FR amplifier test worked out quite well. Neg: He had some trouble with the tone burst test. I wish he would have spent a little more time discussing the REW system and how it compares to the QA401. The big issue seems to be getting the right sound card set up on your computer. The QA401 solves that problem.
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What are we measuring with the QA401?
Bill - If you're interested in using REW for amplifier distortion measurements check out this diyaudio thread: https://www.diyaudio.com/forums/software-tools/338511-howto-distortion-measurements-rew.html .Nearly everyone in that thread uses an external USB soundcard for distortion measurements; you'll find recommendations in the thread. You'll probably want one that supports both balanced and unbalanced (i.e. single ended) inputs. Also see post #283 for an input attenuator. Hope this helps,EdPS - Let me know if you take the plunge and get a QA401. It looks like a very capable turn-key device.
Thanks, Ed. I've been scanning that thread for a while now, trying to figure it all out. The REW + usb sound card route would definitely be alot cheaper. At the 1:17 time marker in the video, they discuss this a little bit. The QA401's noise floor might be just a little bit lower than some of the less expensive sound cards, but not sure on this. Somewhere else in the video, Bob talks about how noisy fluorescent and LED light fixtures can really mess up the noise floor. Said he swaps out to incandescent light bulbs when running tests.
There is also a ginormous "QA400/QA401" thread over at diyaudio with more info (extremely long). Doing a quick scan, a good starting point seems to be from post #2400 onwards, where some new posters just purchased new QA401's and have a few questions.