K&K Mandolin Twin Internal

The instruction booklet says that installation is a job for a luthier. Actually, it can be done on your own if you have time, patience and some skills, but don't underestimate the time and the courage needed to drill a 12-mm hole in your precious instrument.
After the installation, the visual result is excellent - you just see a high-quality metal endpin. I found also the sound excellent; before installation, I had experimented with a cheaper external sensor and the difference is striking. This K&K provides a high-quality output signal which is pleasant and natural when amplified, and is very good also for recording.
The package includes a very simple metal tool designed for installing the sensors in an f-hole mandolin. For my oval-hole mandolin it was useless, and actually, considering the price, they could have provided a better tool capable of reaching deeper into the instrument. This is my only negative remark.
After some experiments, I installed the sensors not on the soundboard, but on the back of the instruments (the part which touches your belly). I am very happy with the result because the level of the output signal is still very good and the pick noise is greatly reduced, but of course this depends on the instrument.

I used this on my flat top mandolin and it sounds amazing ! I did the fitting myself after watching some youtube videos, not too hard but you have to be comfortable with using tools like drill and file and most important you have to have a strong heart to be able to drill through your Mandolin (toughest part). I used a supper glue as advised. It's not obvious at all and doesn't ruin your mandolin's sound or appearance in any way.
The quality is excellent and the jack is very sturdy. Used it with an L.R Baggs Preamp and the sound is amazing.
Highly recommend for live gigs.
From my experience this is the best solution for live gigs unless you need a pickup that can be removed and installed on different instruments.
Highly recommend.

This pickup is not easy to fit in an F-type mandolin, but it is possible (I got my guitar tech to do it). Once fitted, it is only the endpin jack that gives away its presence.

The pickup produces more than enough signal and a great sound. Remember to run it into a 1Mohm impedance (ideally a pre-amp so that you can apply some EQ) otherwise the sound will be muddy and disappointing. Some tuner/fx pedals have the correct input impedance.

I have tried other piezo pickups and I always come back to K&K. I prefer their passive systems because there is no onboard electronics to affect the sound of my instruments and no extra holes to drill (except for the endpin jack). I can then choose what I use to shape my tone externally. This pickup fits the bill perfectly IMO.