NEARLY ANY SYNTH CAN DO THAT SOUND - 1 osc - saw wave , resonance filter 18db (or 24db if u dont have a 18db filter) set to high (tho it depends if the filter cuts the bass out like korgs filter does) compressed and overdriven/distorted to taste

to really emulate a 303 , u have to put a glide on for overlapping notes or use midi control change for portamento on/off for the glide , then adjust the attack (lessen the attack slightly on the filter envelope) , the volume (raise the volume) (nord modular can do this pretty easily from 1 knob)

what seperates the 303 from the rest is the accent and glide - the sound on the mp3 has none so could be anything

here is a version i just knocked up on my supernova2 in seconds using the basic saw wave and onboard overdrive/distortion

http://www.futuretrance.pwp.blueyonder.co.uk/acid.mp3


ok its not exactly the same but with a bit of eq and some compression it could be :)