This is an auto-wah/envelope filter effects unit that adds expression to your playing. It changes the cutoff frequency of the filter according to the volume (envelope) of the input signal, allowing you to achieve a dynamic wah effect without using foot controls. https://en.wikipedia.org/wiki/Auto-wah#:~:text=%5BImage%201%5DA%20Mu,filter. It's great for funk, soul, and fusion, and works perfectly with bass. I was using this until recently, and I've checked that it's working. Knowledge is required for use and installation. No claims and no returns. Please see the photos for the condition of the item. This is a used item, so please refrain from bidding if you are extremely sensitive or looking for a new-like condition. I will ship within 2-4 days after the bid. Management number: 22