Disc piezo ontop or underneath?
 

Apart from the obvious cosmetic issues, does placing disc piezos on top of the sound board, rather than underneath, make that much of a difference? I'm looking at a 2-disc setup and I'm thinking of mounting externally so I don't have to drill or mod the guitar - and can remove relatively easily if I want to. (it comes with a mount for the input which attaches to the strap button)

Well, the place most people place SBTs is on the bridge plate, directly under the saddle line. The problem with other locations tends to be that things are too variable as far as the vibrations. The guitar tends to feedback or just act differently at higher volumes with areas that have less predictable vibration. Putting the sensors on the saddle line tends to tame things somewhat, add more presence and some directness from the string vibration.

Placement tends to matter a lot with SBTs, and different locations sound different. So you might find other locations work for you, especially at lower volumes. If a location on the top as opposed to the soundboard sounds good to you, I'd imagine inside or outside wouldn't matter.

But do you really want disks glued or taped to your top, with wires running off of them visibly? How will you secure them so they don't pull off with the weight of a cable?

I vote for inside.

I tried outside. Donner makes discs that are easy to mount to the outside, and the wires made a lot of noise whenever I moved. Inside, the wires don’t move around. BTW, I recorded both Donner and K&K inside, and K&K was the clear winner.

Doug, thanks for the reply.

I will be using it a fairly low volumes - I have a acoustic duo, we play small cafes etc. I have a flattop with the same discs (three, under the bridge plate) and haven't had any problems so far.

Its actually for my archtop. The dealer suggested for archtops just before each f-hole, (in line with the bridge) rather than directly under the bridge. I guess this way I can experiment before commiting.

It comes with a clip to attached to the endpeg which will hold the input jack. I've seen similar setups on mandlins and even violins.
The wires - well not ideal but I can run them under the trapese. If it works out well I can commit to getting someone to doing it "properly" - I did the flat top myself but wrangling wires through f-holes is above my paygrade.

HogsNRoses - noted. I'll keep an open mind! The pickups in mind are JJB prestige 220. I've got good results with the 330's on my flat-top.

An archtop may be quite different. I've never experimented with SBTs in an archtop. Give it a try, you never know.

A piezo disc will sound the same on the top as on the bottom, assuming the bottom is not a bridge plate.

Archtops are a challenge, since getting inside can be very difficult.

The best place to mount them is inside on the spots where the bridge feet bear down.

Best outside place would be as close to the bridge feet as you can get, either behind or to the side. If you have a choice, pick a spot where there is no brace underneath.

With a K&K like device, you'll get a decent sound but will likely have feedback issues, even at moderate levels.