low to high impedance converter

[saxtim]

I'm using a simple diy audio probe - a jack with sleeve connected to ground and the tip connected to a capacitor and then a tinned wire with which to probe the circuit.

I've been using this to record signals on my computer within circuit for debugging purposes. (as I have no oscilliscope) Bo Hansen has told me in my DIY Fet mic thread that running it straight into my soundcard's line input like this loads the circuit down. He suggested a simple FET, source follower design to convert impendance before going into the line input of my soundcard for better accuracy.

Can someone point me in the direction of a schematic with parts/values?

thanks

tim

 

[Kev]

check Mark Burnley's
Signal Tracing
http://www.diyfactory.com/data/mbsignaltracing.htm
I know, you're already past that .. this is for others though
also from mark is
http://www.diyfactory.com/data/mbresistors.htm


not sure if this will help but
http://www.diyfactory.com/projects/countryhick/countryhick.htm
and
http://www.uwm.edu/People/msw/ActivePlug/BootStrap.html

might give some ideas

Rod Elliot may have something
ESP Audio
http://sound.westhost.com/index.html

 

[NewYorkDave]

Here's a single-JFET solution which should do the trick. Adjust R1 as necessary to cause the junction of R1 and R2 to sit at half the supply voltage.

This won't drive very low impedances, but it should do fine driving the ~10K or greater input impedance of a PC soundcard.

You may want to add a pulldown resistor to the input side of C1, since this is going to be used in a probe. If you do, it must be a very high value (tens of Megohms) or it'll defeat the purpose of the circuit. Use metal film resistors throughout.

Be sure to check the circuit for noise and distortion before putting it into service.

 

[PRR]

> DIY Fet mic thread that running it straight into my soundcard's line input like this loads the circuit down

Use a Direct Box.

 

[saxtim]

thanks!

tim