When can't you install an under-bridge piezo pickup on an acoustic guitar? What is the alternative?
Under-bridge piezo pickups have become almost standard on acoustic guitars in recent years. They are unobtrusive to the point of invisibility, and provide a clean and moderately accurate sound, well suited for amplification if not quite so suited for recording.
But to be able to install a pickup under the bridge, the bridge has to be of the right shape and configuration.
Some instruments - vintage instruments and modern replicas, for instance - have saddles that are glued in. Not much chance of gaining access. Or there simply might not be space under the saddles within the saddle slots.
Of course any instrument can be modified, but the cost of the modification and alterations to the instrument might be unacceptable.
There are also other guitar-like instruments where the bridge sits directly on the belly of the instrument. There is simply nowhere to put the pickup.
Fortunately piezo technology is adaptable. Pickups can be constructed that attach to the underside of the soundboard. They can be glued on, hopefully in the best position to capture the vibrations of the instrument.
Of course the sound will be different. The under-bridge pickup captures vibrations of the strings very directly. This might seem good, but it's not the strings that are heard when the instrument is played acoustically.
The level of sound that comes from the strings is very low - you only have to listen to an unplugged solid-body electric guitar to realize that.
Most of the sound of the acoustic guitar comes from the soundboard and from air vibrating inside the instrument.
So attaching a pickup to the soundboard is actually a good approach, like the Amulet system pictured above.
In fact it might be worth considering having both under-bridge and glued-on contact pickups (if an under-bridge pickup is of course feasible). That way you would get the best of both worlds, to mix together as you please. A mic in front of the sound hole would complete the picture.