DROK 5W low Profile Amplified Subs Woofer Low-pass Filter Subwoofer Frequency Circuit Board NE5532 Op-amp Chip Potentiometer from left to right are: frequency adjustment, volume control Supply voltage: AC dual 15V (5W) , about the transformer, please check the item ASIN: B00C4A9K8C. but please note that it is a 220 VAC version Frequency range: about 22Hz-210Hz Dimensions: 80 x 40 x 30mm (without potentiometer handle protruding parts) Use Two NE5532 Op-amp to do the signal processing