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Old 27th September 2005, 08:53 AM   #1
leander is offline leander  Malta
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Default Windows Oscilloscope signal voltage input

I dont have money to buy a new oscilloscope

Can I use this oscilloscope for tesing an amplifier output power ?

Would I damage the soundcard with a 40v rms output from the amp when connected to the mic in ?
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Old 27th September 2005, 09:08 AM   #2
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Would I damage the soundcard with a 40v rms output from the amp when connected to the mic in ?
Who knows? Try it and let us know

Even if it doesn't get damaged the input will clip at few hundred mV and you won't be able to measure nothing above that. Is it not obvious that you need an input attenuator?
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Old 27th September 2005, 09:40 AM   #3
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I too have looked into trying out a "Windows Osc".
The Info I have found warns that going over 2.7Volts RMS through the Mic input will damage the sound card in your computer. Good Luck!
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Old 27th September 2005, 09:55 AM   #4
MikeB is offline MikeB  Germany
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40volts into the mic-in ?

You really shouldn't use mic-in, these are typically very poor, use at
least the line-in, and use a voltagedivider like 47k+1k to reduce the
level down to ~1v rms. Be careful to not exchange cables with
grounding, you might fry your computer and/or amp !

I haven't tried this particular software, but using a pc as scope is
quite good to observe waveforms and do some fundemental thd

But it can't replace a real scope to verify stability.

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Old 27th September 2005, 11:48 AM   #5
SY is offline SY  United States
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40V will smoke a sound card.

I built an external box to act as a buffer/attenuator. It's just a unity-gain buffer (I used a BUF03) with a switchable 10:1 attenuator and an input impedance resembling a scope (1Mohm in parallel with 20pF). In conjunction with a couple of scope probes (a 1x and 10x), I can switch between no attenuation, 10:1 and 100:1.

The sound card route will not really do most of the things you'd want a scope to do because of the very limited bandwidth; it's good for things like spectral analysis, but terrible for troubleshooting or looking at square waves or impulses. A decent used scope can be had for perhaps $200 and should be considered an essential tool. Doing electronics without a good scope is like working on a car without using wrenches.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 27th September 2005, 12:50 PM   #6
anatech is offline anatech  Canada
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Sooo right. If you don't use a wrench, I guess you are using a hammer.

I measured the input impedance of my SB Live! 24 bit card on line in and got around 8K8. I will retest to make sure as I used a Shallcross decade resistance box and looped line out to line in and used RMAA to show -6dB. I don't know how much it's own output would be loaded down because this computer is no where near my bench.

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