I've used a very crude method to do this that you probably don't want to try.  I had guitar tracks recorded already.  There was one note in one track I wanted to sustain longer.  I manually copied the track, moved the copy towards the end of the original track, and drew fade in/out envelopes to mask the transition.  It took a lot of time, but I eventually got a very clean sounding note that sustained beautifully.

it's much more fun to try to tame the beast in real time

much more visceral


much more inspiring

Hollow and semi hollow bodies work much better at feedback as well.

Danny Danzi

But for what you are looking for, if I'm reading correctly, you pretty much want a guitar note to scream and sound like you pressed a key on the keyboard, correct? Like an infinitely sustained note, right?

Thanks for the lengthy response, Danny (and thanks, everyone else, for all the other responses).
 
Actually what I had in mind was just the opposite.  I was thinking there might be some kind of feedback controller plug that could let a synth or sampler go into a sustained-note mode like electric guitar players seem to routinely do.  I don't have a specific application in mind, but with the introduction of TH2 in X2, the discussion got me wondering again if guitar simulation control suites like TH2 supplied that kind of capability.   Sounds like they do not, but the question seemed worth asking.

Sustain pedals will help. However I myself just wobble the note. I don't really like notes just sitting there flat for long periods so a vibrato adds some spice and feel and the slight grinding against the fret keeps the string ringing out. Even with lower gain/distortion/volume/using sims I can draw a note out the way indefinitely. Probably not what you are looking for though but it's one of my fave techniques. Gives the note life and character instead of just laying there like a dead fish. The right kind of reverb settings can help keep a note alive longer too.

why do it fake...

when it's so much fun to do it for real?

Stupid filter. That word is s--p--i--c--e... as in a delicious curry pho. ;-)

I also wouldn't discount sims for sustain. It's not the same as real feedback (usually) but with the new models out there if there is a signal to be had you can do a lot. But the thing is once the string stops vibrating the only thing that will drag out is the feedback which is different than the original tone. You could just pluck your note and copy/paste/blend it to stretch it out as long as you'd like and that way you get actual note, note residual hardware/sim gack. You'd probably want to clone the track so you have two or three instances of the note and then offset the clips over top of each other so it sound consistent throughout the duration and you don't hear a splice point.

Perhaps not exactly what your looking for but Electro Marmonix makes the Freeze pedal that can sustain note or chords indefinitely.

http://www.youtube.com/watch?v=iOabP5WZLMY

Frank Zappa - Duke of Prunes (orchestral version) - solo at 1:26 - now this is the kind of beautifully controlled musical feedback that I could never replicate. 

http://www.youtube.com/watch?v=Qk_Tr5ZxmNY