how to make Op-Amp Audio Amplifier

Op-Amp Audio Amplifier

The above circuit is a versatile audio amplifier employing a low cost LM358 op-amp. The differential inputs give the amplifier excellent immunity to common-mode signals which are a common cause of amplifier instability. The dotted ground connection represents the wiring in a typical project illustrating how the ground sensing input can be connected to the ground at the source of the audio instead of at the amplifier where high currents are present. If the source is a power supply referenced signal then one of the amplifier inputs is connected to the positive supply. For example, an NPN common-emitter preamplifier may be added for very high gain and by connecting the differential inputs across the collector resistor instead of from collector to ground, destabilizing feedback via the power supply is greatly reduced.

My utility amplifier was built into an aluminum Bud box and eventually ended up bolted to the bottom of a shelf as shown. The well-behaved and ready-to-go amplifier is really handy.

As is often the case, the circuit values are not critical. Other op-amps will usually work but a bit of experience may be necessary if problems develop. The two 4.7 ohm resistors in the emitters may be replaced with a single 10 ohm resistor in either position - I just like the symmetry!