K&K Pure Mini Pickup (for steel string guitar) with volume control add-on.
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.
It functions best when the input impedance of the device you plug your guitar into is 1 Mega Ohm (preamp/amp or mixing board-channel). If this input impedance deviates by a large amount (i.e. 100 Kilo Ohms or 10 Mega Ohms), you may experience diminished range (from "full volume" to "off" within a 1/3rd rotation) or, in extreme cases, a slight hum when the potentiometer is at middle position. This is a normal phenomenon when passive potentiometers are used with piezo based pickups. At "full-on" or "full-off" it will always be noise-free. A lot of customers use the Volume Control like an extended on/off switch.