E.W.S. Subtle Volume Control or SVC is a device based on Scott Henderson's idea, which provides better control of your overall volume than a traditional volume pedal. With subtle increases or decreases in volume, your audience may never realize that you've changed your volume.
The SVC goes in your amp effects loop. This allows you to control your volume without changing the gain you have dialed in at the front of the Preamp. The SVC can also be used between your last pedal and the front of the amp, but keeping your gain consistent while changing volume will not be as effective.
Because the SVC is not an effect, it does not require any power source such as batteries.
The SVC is designed to be placed on your pedal board and foot operated. Since the signal coming from the amp effects loop is low impedance, the cables running from the amp to the SVC will not degrade the sound.