gofar99 Messages: 1909 Registered: May 2010 Location: Southern Arizona
Illuminati (5th Degree)
Hi Everyone, Since this is about audio... I put it here, but Tower might be just as good. I have lots of test equipment and use it all the time to perfect my designs. A project I am working on right now (valve headphone amplifier) was finally assembled last week. I found that I could not use my HP distortion analyzer on it as the output was too low to get accurate distortion measurements. The roughly 100mw across 75 ohms did not generate enough voltage for it to work as designed (0.040 volts vs the typical 0.1 or higher up to 300 volts it was designed for) So I figured I would use my DSO and get the readings off the scope directly. Fine. Response great 20HZ to past 40K. S/N residual was at -95dbv (as low as I can get with my horrible AC and ambient noise) When I started to crank up the output I began to see some dreaded harmonics grow on the screen. At 100mw they were buried in the noise at about -95dbv. I got rather disturbed about them and tried lots of things to get them lower. (translate = much solder, bad words, hair pulling and more bad words) Then the light bulb came on. Actually take the readings from the scope and see what they were and not fret instantly. Surprise, the harmonics were at -40db from the signal up to about 250mw. -40dbv is equal to 1%. I can live with that. It goes to show that test gear is only as good as the interpretation of the results.
Wayne Parham Messages: 18695 Registered: January 2001
Illuminati (33rd Degree)
Dude, I can't tell you how many times I've done something like that. I work in a pretty fast-paced high-tech environment and have to meet deadlines, often rushed, in turnover to QA. Seems like at least one of my projects every time has some fluke like that I fret over and then find I'm looking in the wrong table or not waiting for something to settle. No way to know 'til you know though, you know?
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At 100mw they were buried in the noise at about -95dbv. I got rather disturbed about them and tried lots of things to get them lower. (translate = much solder, bad words, hair pulling and more bad words) Then the light bulb came on. Actually take the readings from the scope and see what they were and not fret instantly. Surprise, the harmonics were at -40db from the signal up to about 250mw. -40dbv is equal to 1%. I can live with that. It goes to show that test gear is only as good as the interpretation of the results. 
Dude, I can't tell you how many times I've done something like that. I work in a pretty fast-paced high-tech environment and have to meet deadlines, often rushed, in turnover to QA. Seems like at least one of my projects every time has some fluke like that I fret over and then find I'm looking in the wrong table or not waiting for something to settle. No way to know 'til you know though, you know? 
