I'm attempting to make a spectrum equaliser
I bet you are not, there is no such thing. Spectrum display - yes, graphics equaliser - yes spectrum equaliser - no
Following the instructions in the thread on sparkfun, I've tried samping fast for 50ms, and then using the difference between the max and min, but this value seems pretty random.
That is for detecting the envelope, you want the real signal.
and connected 3.3v to AREF
Doh!
Says:-
If you're using an external reference on the AREF pin, you must set the analog reference to EXTERNAL before calling analogRead(). Otherwise, you will short together the active reference voltage (internally generated) and the AREF pin, possibly damaging the microcontroller on your Arduino board.
With only a X100 gain it looks like it is not enough. I would couple it in like in this page:-
http://interface.khm.de/index.php/labor/experimente/arduino-realtime-audio-processing/