10x probe question

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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
 
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. :whazzat:
regards
 
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.
 
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).
 
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