audio spectrum analyzer

The Elektor Electronics Magazine has made one. Schematics are available in the book 101 test instruments. I don't know if it is for sale anymore. Look in a library ;)
This is a completely analogue circuit. Lot of work to make because of the lot of filters.

Maybe if you know someone that bought the Welleman spectrum analyzer kit, and talk nicely to him, you can get the schematics :)
It doesn't have to be complex

Zetex make a chip (this was discussed in Electronics World last year) called the ZFX36L01 -- it has a mixer in it and you can configure the chip for bandpass or notch. All you need to do is make a sweep generator for the local oscillator. Will it make a lab instrument -- no way -- but it can greatly simplify construction.

Or you can get a HP 3581 analyzer for less than $100 -- and this is a great bargain. Last one traded on Ebay for $72. See the manual I put on the web at