Ableton Live Sweeps Technique from Matt Sonic on Vimeo.
Here is one thousands of techniques for producing long build-up sweeps. Hope this provides some ideas. The basic steps are:
1) Start with a short sample or synth tone that you like.
2) Add a reverb as an insert effect.
3) Make sure the reverb has a long time and no predelay. Also cut the highest high and lows so that it sits in the mix.
4) Set the reverb to 100% wet so none of the original sample plays through.
5) Automate the freeze setting to generate an unusually long tail.
6) Record the output of the reverb into another audio file.
7) Reverse the audio file.
8) Trim the end of the reversed audio file so that you only hear the tail of the reverb.
9) Automate the volume of the sweep with a slightly exponential curve.
10) Add a lowpass filter and automate the cutoff using a similar curve.
11) Add a delay to give the sweep more motion.
From here, you have a nice sweep. You can add many more filters, flanges, phasers, etc. to taste.
