Rackeys Subwoofer Filter Bass Board with Inbuilt Phase Inversion Switch Rackeys RAC-SF1 subwoofer audio filter bass board is single supply subwoofer filter for driving subwoofer amplifiers. It has wide input single-supply voltage range (9v to 18v) and a gain of approximately +14 dB 31Hz. There is an onboard phase invertion switch to invert the phase of signals at the output. The control of output gain is achievable via an onboard gain control.