LM3886 monolithic power amplifier

The power amplifiers are based on the National LM3886 monolithic power amplifier. This is a powerful device, it can put out 68W RMS into an 8Î© load, with a +/- 35V supply. The circuit configures the LM3886 as a non-inverting amplifier, with a gain of 23. Pin 8 is the muting input, it has been connected to the negative rail using an RC-network comprising R4 and C6. This causes the amp to be muted for a short time after power-up. In practice, this was found to not be enough muting, so an external speaker-protection relay is used, as described later. C7 provides HF rejection, C1, C2, C4 and C5 provide supply decoupling. C1 and C2 should be mounted physically close to the device. R7/C8 form a Zobel network at the output, shielding the amplifier from the nonlinear impedance of the loudspeaker. R5 has a coil of wire wound around it (10 turns). It forms an inductor in parallel with R5, providing protection against speaker lead capacitance effects. Note that the speaker’s ground return should not go back to the PCB, it must go to the junction of the filter capacitors in the power supply. This prevents ground loops. Likewise, the power ground of the amplifier should not be connected to the ground of the signal at any point other than the filter capacitor junction.

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