From K&K Sound:
The Pure Pickup is our award-winning, signature pickup for acoustic guitars. Installed on the guitar's bridge plate, the Pure Pickup is the the best choice for achieving a rich, natural guitar sound -- even without a preamp!
Our customers tell us that it's the best passive pickup on the market, which means it's the perfect plug-and-play solution for the musician who wants a warm, full-range signal without worrying about cluttering up their instrument, adjusting onboard electronics, or having to remember to change the battery.
Pure Classic: 4-head transducer for nylon string acoustic guitar
Our Volume Control is a non-destructive, plug-and-play addition to any single-source passive internal transducer system, like a Pure Pickup, FanTaStick, Twin Spot, or Big Twin.
You can order it as an integrated element with any single-source passive pickup, or as an Add-On for an existing pickup. Simply unscrew the existing endpin jack, remove it from the endpin hole, and plug it into the 1/4" plug of the Volume Control Add-On. The 1/4" plug has a self-adhesive fastener, which can be secured inside the guitar. The lightweight thumbwheel control mounts with a peel-and-stick installation to the inner edge of the soundhole. Mount the new endpin jack into the endpin hole, and you're done!
This passive volume control works without a battery. It does not boost the volume, but it allows you to turn the pickup signal down.