Class B Amplifier

This is a low power general purpose amplifier using standard easily obtainable parts. The circuit is similar in design to my 'Basket Case' amplifier but without the preamp and tone controls. The schematic diagram is shown below. It can be seen that the push pull output transistors have absolutely no bias at all, and this is the essence of class B operation. When there is no audio applied to the input, there is no current flow in the output and the transistors are completely turned off. Therefore the quiescent current is just that of the opamp itself, which in the case of a TL071 is about 1.7mA, making the circuit ideal for use with a battery or solar panel

The disadvantage of class B is crossover distortion. Most normal push pull amplifiers are biased in class AB mode, meaning the output transistors are already conducting a little, so the 0.6 volts required to turn them on has already been overcome. In this design, it is the audio signal itself that has to turn on the transistors, which takes time, and produces gaps in the output waveform. But if an opamp with a high slew rate is used (meaning fast), this distortion can be greatly reduced by incorporating the output transistors within its feedback loop, effectively tracking and cancelling out errors in the signal. It's not a perfect solution, but the better the opamp the more effective the cancellation, and although the TL071 is not the best opamp around, it does a pretty good job. The amplifier is configured as inverting with the volume control (also in the feedback loop) setting the gain between 0 and 25. A veroboard layout is shown below

The opamp is fitted in a socket that has pins 1, 5 and 8 cut off, as they are not required by the TL071. Pin 7 is 'blobbed' with solder to the track above it (where pin 8 would have been) to feed power to the left side of the board without having to use a wire link. Also not having pin 5 connected allows the ground rail to run straight through. This all goes to make the veroboard layout simpler

The finished amplifier