MIMF

Thought I would give some feedback (excuse the pun). I completed the small nylon string archtop earlier this year and just received the K+K "pure archtop" pickup last week. Having fitted it now, I can say that the sound is incredibly natural. I did want a pickup that did not "enhance" the sound of the guitar (which is very sweet and pure).
There is a problem with feedback though, on my bigger valve amp it can only be used at very low volume (generally fine), I also have a small el-cheapo Ibanez acoustic amp which works quite well. K+K say I probably need to change phase - but that is beyond my understanding of electronics (zero:-).

I don't know how relevant this is exactly, but a very good friend of mine, and probably the best known bass player in the region (mountain west United States) had similar issues with K&K pickups. He blogged about it not too long ago:

http://basstony.wordpress.com/2012/06/2 ... ll-circle/

Perhaps there's some insight there for you.

There may be nothing you can do about it, short of EQ settings on whatever amp you use. This is just one of the trade-offs that you get with these types of pickups. It's characteristic of all piezo pickups.

Directly into the PA or small amp into the PA works best for me.

Phase is relative to distance away from speaker, and frequency as well. I'm surprised K&K would recommend that as a solution. You will probably find that small changes in the positioning of the guitar relative to the amplifier will have a dramatic effect on the feedback.
Most purpose built acoustic amplifiers or preamplifiers will include a notch filter, as there is usually some resonant frequency where the feedback is worst, and dialing in this notch will allow you to bring the rest of the frequency range to a reasonable level.
Like Mark says, this is just one of the inherent problems with amplifying acoustic instruments.

Murray, That is a really cool guitar. Would you mind giving us a peek inside - for instance - Dimensions, top thickness, bracing Etc.

I've been romancing the Idea of a Nylon string AT for a while, I have a real weakness for the sound of Nylon.

Aside from phase, feedback is a function of how much gain there is in the loop. If the room can hear the guitar acoustically, then the guitar can 'hear' the room, and you'll get feedback when there's enough sound in the room to keep the top of the guitar going. Usually feedback on acoustics happens at one of the two pitches in the 'bass reflex' couple; the so-called 'main air' pitch, generally around G on the low E, and the 'main top', an octave or so higher. These can be a fair amount higher in pitch on archtops. If you know those two frequencies you might be able to notch them out. They're usually reasonably stable, so once you figure them out you'll be at least close.

Iirc, the K&Ks use several piezo elements on the top. This can make it problematic to phase the unit, since the different elements can be sensing a given top resonance in locations that show different phases. If that's the case, it might (just) be possible to find locations for them that have exactly equal amplitude at opposite phase, so they'll cancel out. The 'main top' and 'main air' resonances have pretty much the same phase everywhere, with the top possibly reversing phase right out near the edge. You can get some idea of this using Chladni patterns, and there are a couple of other ways I could think of to look at this stuff.