A very simple high efficiency audio amplifier circuit can be build using this LM12 circuit diagram . This LM12 audio amplifier circuit will provide a high output power on a 8 ohms or 4 ohms load impedance . the maximum output power delivered by this amplifier is around 60 watts on a 4 ohms load and 100 watts on a 4 ohms load on each channel .

This high power audio amplifier is composed on two principals stages :
A preamplifier stage based on the LM381N dual preamplifier integrated circuit and a power amplifier stage based on the LM12 high power operational amplifier .
To obtain the full power from this audio amplifier you will need to use a dual 35 volts power supply circuit  .
If you want to use this audio amplifier circuit as an car audio amplifier you will need a power DC-DC converter  . The input sensitivity of the audio preamplifier must be set to about 300 mv by R1, R2 variable resistors .
To obtain more audio power  from this audio amplifier you can use it in the bridge mode configuration , but in that way you will obtain a mono power amplifier circuit  .
If you want to use it in bridge configuration or stereo configuration just  design the pcb board in the stereo mode configuration and modify the jumpers  in this way :

1. Install jumpers JU2 and JU 4
2. Remove Ju3
3. Remove R13 with jumper
4. Replace R14 with a 113 k resistor  ( 1 % )
5. Replace R15  with a 4.5 k resistor ( 1 % )
6. Remove r10
7. Cut foil between pads from R14 and ground
8. Connect the input signal to the left input
9. Connect the output between Right and Left output