Software oscilliscope precautions

[jaapie]

I want to start using a software oscilliscope, but I don't want to risk frying my soundcard. At this point i'll only be working with pedal voltages and currents, but i haven't been able to find any data on the maximum input voltage/current for my sound card. It's probably worth noting that I haven't used an oscope since I took a circuit analysis class a couple of years ago, and I've never used a software one.

I figure if i take the current across a 1M resistor at the output of the circuit (imitating the 1M resistor to ground at the front of my amp), I should get good results without risking frying my soundcard input with too much current. Am I thinking along the right lines here?

[R.G.]

Nope, you're thinking along the wrong lines. The voltage across a 1M is, for most purposes, the same as what it would be if the 1M wasn't there. That's why 1Ms are often used as input/output pulldowns.

What you want to do is to limit the voltage into the sound card. Soundcards are intended to work up to about nominal line voltage, about +/- a volt, although they probably don't specify that very well. They *may* work OK with up to about +/-2.5V without damage. The better thing to do is to put a buffer in front of the sound card that (a) takes the hit for the team if something really ugly is connected to the input, like an accidental connection to AC power line voltage (b) limits the output voltage of the circuit to what is safe for the soundcard.

An even better thing to do is to additionally NOT use the sound input circuits in your motherboard, and buy a cheap plug-in sound card  for o'scope use. That way, what dies is more likely to be the plug in sound card and not your motherboard.

[jaapie]

Thanks for your reply!

Unless I can find a really good deal, I'll probably have to do without an external sound card for the next week or two (my disposable income is pretty much nil at the moment). I'm pretty sure I've got enough parts around to put together an appropriate buffer; would it be reasonably safe to just use a buffer into the motherboard if everything (except the laptop) is powered by 9v batteries? There's obviously the possibility to put considerably more than +/-1V into the sound card, but it would at least remove the possibility of accidentally putting mains power into my motherboad.

The circuit i want to look at is a jfet distortion I've been breadboarding. If i check the AC voltage with a meter and attenuate it with a pot wired as a  voltage divider before plugging it into the buffer/computer, i could make sure the output is at a safe level before it's hooked up to the expensive stuff.

I'm not trying to disregard good advice, I'd just like to get as far as possible with my current means. I can wait until i can get an external soundcard, but i really want to take a look at this circuit as soon as possible!

[jaapie]

well, i just googled and there's a ton of super cheap USB sound cards. would something like this (http://www.amazon.com/External-Sound-Adapter-Desktop-Notebook/dp/B000BUDHNQ) be appropriate?

[R.G.]

Unless I can find a really good deal, I'll probably have to do without an external sound card for the next week or two (my disposable income is pretty much nil at the moment). I'm pretty sure I've got enough parts around to put together an appropriate buffer; would it be reasonably safe to just use a buffer into the motherboard if everything (except the laptop) is powered by 9v batteries? There's obviously the possibility to put considerably more than +/-1V into the sound card, but it would at least remove the possibility of accidentally putting mains power into my motherboad.

The circuit i want to look at is a jfet distortion I've been breadboarding. If i check the AC voltage with a meter and attenuate it with a pot wired as a  voltage divider before plugging it into the buffer/computer, i could make sure the output is at a safe level before it's hooked up to the expensive stuff.

I'm not trying to disregard good advice, I'd just like to get as far as possible with my current means. I can wait until i can get an external soundcard, but i really want to take a look at this circuit as soon as possible!

I understand having low disposable income from personal experience. Have a look here:
http://xoscope.sourceforge.net/hardware/hardware.html

This seems to be what everyone points to for "soundcard oscillscope buffer" in google. I have not looked through it in any detail, but it gets a lot of play.

[blueduck577]

What I'd do is make a simple op-amp buffer, followed by a set of clipping diodes just like we find in distortion circuits.  Maybe 2 in series on each side, limiting the output voltage to +/- 1.4 V