Paul Shippey

The piezo under-saddle transducer is the most popular method of amplifying acoustic instruments.
This page outlines the component parts and modifications required when fitting the Headway HE1/M2.FEQ system on a Shippey mandolin, but the principles
are broadly the same for all under-saddle pickups, whether fitted to a mandolin, mandola, cittern or bouzouki.

Piezo cable pickup;
This sits in the instruments' saddle slot. The piezo cable converts vibrations into an electrical signal (which is then amplified).

Preamp and Enpin jack socket;
The preamp with onboard fixed EQ modifies the signal from the piezo. It is designed to produce a more natural (in this case) mandolin tone; reduce the chances of feedback; and send a stronger, impedance matched signal to your PA / anplifier. (the preamp components are inside the metal sleeve labelled 'Headway').
The endpin jack accepts a 1/4" jack plug. Inserting the jack plug switches the battery on. The socket also incorporates a strap button.

Battery;
A 9 volt battery powers the preamp. The battery is held in a pouch which is velcro mounted inside the instrument and is accessible through the soundhole.

A 3mm diameter hole is drilled through the mandolin's top (underneath where the bridge will sit), and the piezo cable is passed through. It then passes through a hole in the bottom of the bridge and runs along the bottom of the saddle slot, with the saddle itself sitting on top of the pickup.
In order for the pickup to function properly, it is critical that the bottom of the saddle slot is absolutely flat and that the saddle slides freely (but is not too loose) in the slot.

The strings passing over the saddle exert a constant pressure on the piezo cable. When the strings vibrate, there is a change in pressure which the piezo cable converts into an eletrical signal which passes through the preamp/EQ to the PA / Amplifier and onto the loudspeakers to be converted into sound.

The cable type of pickup is able to sense vibrations from all directions, so in addition to sensing pressure from string vibration, it can also pick-up vibrations from the soundboard via the bridge. This helps achieve a more 'natural' amplified sound, where some of the particular instrument's character is preserved.

The wiring and preamp are hidden inside the instrument. The only visible clues are the battery pack (visible through the soundhole) and the endpin jack.

The beauty of this method of amplification is its simplicity, reliability, quality of sound and high resistance to feedback.

You can simply plug in and go, or if you want to modify the sound, pedals can be used.