Check the listing for details. Protek B-820 Audio Oscillator. Condition: Used. Listed at 80.00 USD. The Protek B-820 Audio Oscillator is a multi-effects processor designed for professional audio applications. This model offers a wide range of audio manipulation capabilities (200hz - 200khz), making it a versatile tool for testing, sound engineers, and producers. With its user-friendly interface and high-quality build, the Protek B-820 is a reliable choice for enhancing and shaping audio signals in various settings. This unit is has been fully tested and is in good working condition.