Monoprice 38172 Audio Control Device -

This Impedance Matching Volume Control allows you to connect multiple speaker pairs to a single power amplifier or receiver. It increases (or decreases) the impedance of the connected speaker pair to ensure that the overall impedance level presented to the amplifier is high enough to prevent damage to the amplifier. It can support amplifiers with up to 100 watts of output power and fits into standard single-gang low-voltage junction boxes or mounting brackets.