10W/8Ω power amplifier based on LM1875

Rohit Balkishan Dubla

Figure 1 shows the amplifier schematic (one channel only - two ICs are required for stereo). It is based on the 20W power op-amp LM1875 from National Semiconductor. The circuit is based on the typical application given in the datasheet but with a few changes as follows:

• 1. Voltage gain is decreased from that used by the typical application.

  2. The supplies are reduced from those shown in the typical application to about ±15V because the IC is only expected to deliver about 10W into 8Ω. This also ensures cool operation and increased reliability.

  3. The zobel network at the output uses different values for the resistor and capacitor.

The power supply is ±15V DC. The supply must be properly decoupled using 10µF decoupling capacitors bypassed with 100nF capacitors (shown as 10µF + 100n) close to pins 3 and 5. With this, the amplifier will deliver about 10W into an 8Ω load. Also, note that the schematic shown has a different component labelling than that on the datasheet schematic - bear this in mind while comparing the two.

This amplifier is part of an active 3-way setup. Click here for photos.