Is there an effect in Kontakt that make a single instrument to sound like an ensemble?

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Sort of, but probably not as realistically as you would wish.

You can use the chorus effect, but it will sound like a chorus effect (not a true ensemble).

You can use various techniques to double the samples (perhaps doubling with adjacent samples, detuning at random, etc.). But it will always be an effect. If you're thinking of transforming a solo violin into a string orchestra, it just won't sound real.

 

Exactly what David said. But you can try simulating it depending on what type of instrument you use. If you use a trumpet which is not multi tonic then duplicate the midi and transport the copies with an octave/quart/quint, play chords etc... For violin it's more tricky i think, you need to replace the instruments for the transported ranges with it's correlated instruments as bass, chello, viola, violin. Chello can cover quite big range. Depending on your needs...

 

Try this script. Use it in 1 - 5 slots of the script editor, and play with the parameters for getting the best results...