This reference design demonstrates the use of the MAX98400 Class D audio amplifier in a stereo audio docking station application. The MAX98400 2.1 demo box is a complete powered speaker dock that uses two MAX98400 ICs to drive a 3-channel speaker system consisting of two 2in satellite speakers and one 5in subwoofer. The reference design is intended to be used with a portable audio player as its main music source. The overall solution size is very compact and features active equalization，powersupply monitoring，and dynamic equalization for the subwoofer.

　　The input stage is common to both the left/right and subwoofer sections. A stereo， log-taper potentiometer （digipot） is used as a volume control to adjust the signal level to the preamplifier. To avoid ground loops， the input is sensed differentially and the RCA input connectors are not directly connected to the system ground. A 100Ω resistor is used to reduce the common-mode voltage; U3A and U3-D （see schematics below） perform differential-to-single-ended conversion with a gain of 2. The outputs of the input stage are referred to VREF， the reference voltage of power amplifier U2 which is buffered using U100-A.