[sdiy] Sound Lab Mini Questions

[Steve Ridley]

> And I'll bet that somewhere out there is a free (or trial) software scope
> that uses your sound card mic input.
> 
> Ideas anyone...?

There are several out there.  "Wavetools" is free and small,
and includes a spectrum analyser and signal generator.
I run it on an old W95 486 laptop.

http://www.maxim.abel.co.uk/utilities.htm

Be aware that soundcard input is nothing like a 
scope input.  It:

a) will block DC
b) will be low impedance (scopes are usually 1Meg)
c) will not like more an +/- a few volts and may die.

You can't do much about the DC except use a DVM
to measure it instead and be aware that a soundcard
scope won't show it.

You can make an input buffer with just a high impedance
opamp (LF351 might be good).  Connect it as a unity gain
buffer and put a DC blocking input cap and a 1Meg pot
on the input to drop and calibrate the level to match your 
soundcard.

A 1K resistor and a pair of back-to-back 4v3 zeners between
the opamp output and the soundcard input would give extra
protection.  You could probably run the whole thing off a
couple of 9v batteries.


Steve




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