M5218AP solid bass filter circuit, LM741 opamp amplfier built on symmetrical power supply + 56 – 56 volts symmetrical bass filter section 15 volt zener regulated by whether they have been power supply. Stereo 50kω … 200hz frequency setting... Electronics Projects, 100W Subwoofer Amplifier Circuit "audio amplifier circuits, transistor amplifier, " Date 2014/06/22