This a Turbo Bass Circuit that used single IC LM324 based op-amp amplifying low signal input-output. By using IC LM324 have two op-amps as two part. The first part as U1 in circuit diagram is used to buffer the audio input signal and the signal will be filtered by the second part as U2. Which the second filter can give more bass and eliminate the High-Frequency signal. So this circuit very matched for Bass Power Amplifier or Subwoofer Amplifier. See the Turbo Bass circuit using IC LM324 below:
At the schematic circuit above is include the symmetrical Power Supply circuit, you just need AC Voltage Input 12V - 15V AC. It's so simple and easy to make, not only the schematic diagram, below you can see the PCB Layout design for making Turbo Bass Circuit.
3 Channel Equalizer LM324, Tone Control Circuit LM324. Good Luck DIY'ers