"Foster subwoofer plate amplifier. 115W into 4ohms, 80W into 8ohms. 4th order (24dB/octave) low pass adjustable from 50-200Hz. 0-180 degree phase switch, RCA and speaker level inputs, volume control. Unit has a 3rd order (18dB/octave) high pass filter at 29Hz with a Q of 0.707 built into the subwoofer section. Can easily be modified for any Q and cutoff frequency that you want by changing R615, R617, R618, C607, C608 and C609. The amplifier also has a spectral tilt control that is adjusted to give about 10dB of shelving. It can be manually adjusted with a pot on the PCB. See response curves linked below. A drawing of the amplifier and schematic are both linked below. Two of these amplifiers can be bridged together to produce 220W into 8ohms. See the FAQ below for details of bridging and filter modifications."