10x probe question

[NEO Dan]

Would it be a good idea to adapt 10X scope probes for use with a Mic preamp? I've been measuring FR curves with a M-Audio Mobile Pre USB audio preamp. Normally I measure at low voltages typically Consumer and Pro Audio line levels. Not long ago I was measuring an amplifier, I think it went into oscillation and it somehow killed the outputs of the USB piece. There is loads of distortion on the audio outputs now. I am going to buy another similar USB audio preamp, likely a E-MU 0404 USB. I just wanted to know if the 10X probed will allow me to safely measure 10X higher voltages than the USB piece alone can tolerate. Is my logic flawed?


Thanks
Dan

[Juergen Knoop]

it will protect your soundcard from most abuse, but you may not be able to measure something useful with it either.
The probe consist basically of a 9 megohm resistor and will build a voltage divider with the soundcard input with very large attentuation.
Not very helpful for measuring line level signals.
regards

[GLORY_MORRIS]

I mean, you can purchase an oscilloscope used for less than $200 these days. I got one in excellent shape (literally, still wearing some of it's plastic wrapping) for $180 about a year ago.

[NEO Dan]

Thanks guys, I'll give it a try.

[Conrad Hoffman]

A 10X probe will only be a 10X probe if plugged into a 1Mohm input, which your sound card probably isn't. More likely it's lower, and you'll end up with much higher than 10X, i.e., not much signal. You'd be better of building a small interface box with a level control and some voltage limiting between that and the sound card. A limiting resistor and some back-to-back zeners for example.

[unclejed613]

a 10X scope probe has a 10Meg resistor in series with the tip, which makes the 10x probe into more of a 1000x probe if your soundcard inputs are 10k inputs. a better way to make a 10x probe would be to put a 100k resistor in series with the tip. if you're not sure if your line in is actually a 10k impedance or something much higher, put a 10k resistor across the input to assure voltage division. it won't be terribly accurate as far as making actual voltage measurements, but it will protect your sound card. a further protection method would be to add a pair of opposed zeners (back to back but in series). 4.7V zeners will clamp the input at +/-5.4V (in both directions you have a forward biased 0.7v junction in series with a 4.7V zener).