I have a synthex in my studio, but I am doing a total recap of my mixer so I can´t listen to the synthex atm. But from memory there are two different "laser harp" sounds as presets on the synthex, first with no attack added and the other a bit softer sounding.
I have done the sound on a Prophet-5 which got pretty close to the real deal. It´s a sync sound with LFO added to the pwm of one of the oscillators ....
Blah Blah Cafe, the Jean-Michel Jarre forum
