Bridge piezo problem

[erstokke]

I have a Michael Kelly FSE that gives a very low output even though there is an active preamp. On inspecting the bridge I found that the piezo element is put in a grove in the bridge. The upper part of the bridge is split in two and the element lies in a grove between.

The piezo element itself is flexible and looks just like shielded copper wire. Touching the element itself gives a loud noise, so I expect the element and preamp to be OK. And yes, I have checked the battery!

What I suspect is the fault, is that the grove in the bridge is so large that the piezo just rattles around inside the bridge. Shouldn´t a piezo element have close contact so that the vibrations from the mandolin causes pressure to the element - which again induces current? I´m thinking about adding some firm material in the grove so the piezo will be somewhat squeezed - or maybe it should have been glued in place?

[mandofiddle]

Have you tried it through a different pre-amp? What preamp are you using now?

[erstokke]

It is called Fishman system 1 - but it is an OEM product that they don´t sell separately. I also tried it through a Fender Acoustic "something", but it had to be cranked up all the way . which normally had filled the Albert hall with sound.

Scratching the piezo gives a lot of noise, that´s why I´m wondering if it´s the large grove that is the problem.

[John Flynn]

Scratching the piezo gives a lot of noise, that´s why I´m wondering if it´s the large grove that is the problem.

That seems like a reasonable conclusion. My understanding is that a pizeo does need to make solid contact with a vibrating surface to be effective. The question is what to do about it. I would contact both MK and Fishman. They must have had this issue before and may either stand behind it or have a fix. If I were you, I would neglect to mention that you have taken it apart! If you get no satisfaction from them, you may need to find a luthier who is experienced with such things.

[siren_20]

You might try putting in some thin shims of paper or cardstock to try to get additional pressure on the piezo element. If that gives the desired results, some thin wood veneer might do the trick as a permanent fix.

[david blair]

I have had one of these I repaired for a friend, and one issue was that the posts were to short, and the top part of the saddle was able to rock, or pronate and not sit perfectly flat on the bottom half. Also we removed the element and added enough epoxy to make sure that the element was making solid contact underneath. Let the epoxy level itself flat and to dry complete. The top half should ideally be modified to push hard against the element everywhere, and flat everywhere else. Probably easiest to glue a small piece of bone the size of the slot to the top piece for best results. If you can do all this with good results, I would again remove it and permantly glue it together. Those elements are made for guitar type saddles and need the full pressure of the strings to work best. The also work pretty well simply stuck on the top against the bridge with some gummy reusable stuff.

[erstokke]

Thanks for great tips.
I cut a slice of 0.6 mm polypropylene sheet and put into the grove. The top part of the saddle now has pressure against the piezo element. The output increased significantly.

On my M-audio audio interface I still have to use the "Mic"-setting, not "Line". But now I have to turn gain all the way down instead of all the way up. And it sounds all right as well.

I still think it should have been able to create a line level output, but I will try it with different amps and PA before I do anything more. I am a bit tempted to mold the whole thing in epoxy, but then there is of course no way back.