This is a simple audio amplifier using a TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar in complexity to a simple one-transistor radio but the performance is superior .source

This 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.source

This circuit use 4-transistor utility amplifier suitable for a variety of projects including receivers, intercoms, microphones, telephone pick-up coils, and general audio monitoring. The amplifier has a power isolation circuit and bandwidth limiting to reduce oscillations and “motorboating”. source

This simple amplifier shows the LM386 in a high-gain configuration (A = 200). For a maximum gain of only 20, leave out the 10 uF connected from pin 1 to pin 8. Maximum gains between 20 and 200 may be realized by adding a selected resistor in series with the same 10 uF capacitor. The 10k potentiometer will give the amplifier a variable gain from zero up to the maximum.source

This is a very simple headphone amplifier designed by CMoy.Each channel uses a single Burr-Brown OPA134 opamp in a non-inverting configuration. It has adequate current capability to drive most headphones without an output stage.The amplifier is ideal as a booster for power-conserving stereo sources such as portable CD players and for interfacing with passive EQ networks such as tone controls or a headphone acoustic simulator.source