K&K Mandolin Twin Internal

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.

The signal is clear and accurate. Probably too accurate. It picks up a lot of unwanted noise. The pick hitting the strings is distractingly loud. I've tried EQ'ing it away (LR baggs preamp / EQ on amp / Boss notch EQ pedal) it can be minimized, but at the expense of the sound quality of everything else, which kind of defeats the purpose.
In fairness to K&K, it seems that retrofitting a pickup to a mandolin does not seem to ever give satisfactory results (to a gigging musician).

If it wasn't for the unwanted noise, I'd be quite happy with this pickup.
That being said, I won't be using this pickup for gigs... (or anything else)

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.