Why use the LM358 / LM324 ?

Both are 'general purpose' OpAmps = the LM358 is a dual (in an 8 pin package), and the LM324 a quad (in a 14 pin package). They can be run on either a single rail (3v to 30v) or dual rail (ą1.5V to ą15V) supply. The slew rate is a relatively slow 0.6 V/ľs (LM358 ) or 0.5 V/ľs (LM324) with a 1.1MHz (LM358) or 1.2MHz (LM324) bandwidth - more than 'good enough' for audio applications , where the Line level output 0db is ą1v4v (and is typically limited to ą1v7 peak-to-peak at +4db) For the LM358 I paid £1.42 for 50pcs, so approx 3p each, and for the LM324 £1.52 for 20 (so approx 8p each) both from China via eBay. In use, we have to add the cost of the 'circuit board' eg. a "SSOP to 20pin DIL adapter" (20pin being big enough to mount all the other components as well as the OpAmp)

I paid more than twice the dual price (2*3 = 6p) for the quad (8p) since this minimises the circuit board space needed for an application using 4 Op-Amp's

Note that often items such as these can be found for 1 or 2p each in the CPC 'Bargain Bin', however you have to grab them when you have the chance as they sell out quickly