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Old 27th April 2004, 09:54 PM   #1
maylar is offline maylar  United States
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Default sound card input levels

A fella on another forum wants to use his laptop and an oscilloscope program to check clipping distortion on his car audio amp. I'm telling him he needs a voltage divider to reduce the levels, but I'm not sure what levels to shoot for.

Anybody have an idea what's the appropriate input voltage range for line level and mic level inputs on a typical laptop computer?

Input impedances?

Thanks a lot.
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Old 28th April 2004, 01:39 AM   #2
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About below 1 volt and 70 ohms, however this is a very dangerous thing to be doing as you usually need to buy a 'proper' speaker to line level box.

Make sure you know what you're doing this is very dangerous, especially if his amplifier is anything above 30 watts.

Even with <30 watt amp it's dangerous for both your amplifier and your line level device.

Theres also the very evident risk that you could use your laptop for a enter-watts-here fuse via a thing most people call "ground loops" sending high amps through your soundcard, pci chipset, motherboard then your psu....
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Old 28th April 2004, 07:18 AM   #3
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Ground loops are probably not an issue as at least one of the pieces of equipment is battery operated.

Whilst distortion can be measured to an extent, the readings will be quite a way off the mark due to the limited bandwidth of the soundcard. e.g. for a 1kHz tone, only 4 harmonics will be measured.

If you can't get your head around designing a potential divider with the correct impedance for the soundcard input (i.e. no more than 1/10th the soundcard input impedance) as well as it reducing the voltage to a safe level, then buy a dedicated attenuation box.
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Old 28th April 2004, 10:50 AM   #4
lucpes is offline lucpes  Europe
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http://www.hut.fi/Misc/Electronics/c...r_to_line.html
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