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Funny Story in Retrospect
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=7&t=6300
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Author:  gofar99 [ 22 Jul 2018, 17:02 ]
Post subject:  Funny Story in Retrospect

Hi Everyone, As most of you probably know I design and build all sorts of audio gear. I expect it to always work the first time and meet my standards of performance. (I do miss occasionally, but not often) Anyhow I have need of another pair of mono blocks to match the performance of an existing pair for a bi-amp arrangement. Easy enough. Just build them. So after the first one was done I ran it through a bunch of tests on the bench. This is a routine with me to verify that stuff works like I expect it should. It was a class A U/L push-pull EL34 Oddblock. S/N Ok at -90dbv, response at 1 watt 10HZ to 20KHZ +/- 0.5db (-2 at 30KHZ) fine, power output at under 2% thd+noise was 13 watts, again fine. So what was the matter.....I ran a mid range square wave into it and yuck. All frequencies above about 500HZ had severe rounding on the leading edge. Checked out the build, the only thing different was I used a LM317K (big T03 version) instead of the usual LM317HV. Swapped it out....same wave shape rounding. Figured maybe the scope was not putting out clean squares so I dug out a known calibrated signal generator. Same thing. About now the light bulb comes on :idea: Maybe it is the scope, yup. Said lots of bad words #$%^&. Got the identical one from the stereo (yes I use a dual trace PC based scope on the main system). Hooked it up. Same rounding. Bulb comes on again :idea: It has to be software related. This is the first time I needed the scope since the last Microsoft update. Uninstalled the scope software and re-installed it. Squares are now square. Thanks a lot MS for fixing something that wasn't broken. Now the amp shows good squares like it should. Actually pretty decent all the way to 20K. Not a surprise as these amps can respond very well above the audio band and actually need to have a frequency selective NFB network in place to stop the response at about 40K and above. Without it they can go to 70K or so. Since there is nothing of interest in audio out there and lots of opportunities for problems I build in the limit. I can smile now, but to suspect your test gear is not doing its job is not something we usually consider when a project doesn't seem to behave.

Good listening
Bruce

Author:  Geek [ 23 Jul 2018, 02:14 ]
Post subject:  Re: Funny Story in Retrospect

'Tis why I use an analog scope ;)

Author:  M. Gregg [ 23 Jul 2018, 12:02 ]
Post subject:  Re: Funny Story in Retrospect

Geek wrote:
'Tis why I use an analog scope ;)


+1

Regards
M. Gregg

Author:  gofar99 [ 23 Jul 2018, 16:09 ]
Post subject:  Re: Funny Story in Retrospect

Hi, I have one, but the DSO provides much more information and has more features than any stand alone in a reasonable price range.

Good listening
Bruce

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