The BOSS PSA240 power supply is DC regulated and smoothed for BOSS guitar effects pedals and other 9v equipment. The BOSS PSA-240 delivers 9v DC at up to 200mA.
The BOSS PSA-240 AC/DC regulated power supply converter converts UK mains voltage to DC 9v for most BOSS guitar and bass guitar effects pedals, stomp boxes and for some other Roland and BOSS products. The PSA240 power supply (PSU) output is regulated and smoothed. The voltage and current are constant with almost no hum.
A DC power supply with low hum is essential for guitar effects pedals, especially distortion pedals. Humbucker pick-ups reduce hum, hence the name, while single coil pick-ups are more prone to hum. To reduce hum, when using distortion pedals, switch to the humbucker pick-up if your guitar has one, or switch to two single coil pick-ups combined if your guitar has single coils. An example of a humbucker guitar is the Gibson Les Paul and a single coil guitar is the Fender Stratocaster.