Heard very good things about them! Only thing is, you have to run them through a Pre-amp for it to be any good. You can make your own from piezzo buzzers available from guys like RS Components, if you want to do a test run. Matching the impedance of the buzzers to the pre-amp is sometimes a challenge with which I can be of no help at all :-[...which is why I don't have these on my guit...

patrik steens wrote: anybody who can tell me more about the k&k trinity system for acoustics ? I want to amp my Voyage air guitar, but due to the folding neck a normal piezo pick up is out of course. I heard about the k&k range, and I don't like the sight of soundhole pickups

any advice is welcome

I have decided to standardize and since I hate having batteries inside my guitars, I went for K&K pure mini's in all my guitars, and a Trinity (Pure mini plus mic - comes standard with a dual channel pre-amp and a stereo cable) in my gigging guitar. I'm very happy with them. The battery sits inside the pre-amp's housing. They provide very natural acoustic tone comparing with systems costing 4 times more.
Yes, since the pickups attach under the bridge plate inside the guitar and the mic anywhere near the sound hole, it would be a good choice for your folding neck. You can go for the Pure mini only - it works well with my Ultrasound plus DI/Pre-amp, and with the Baggs Venue and older Para-Acoustic DI and of course the dedicated K&K Pure XLR pre-amp.