High-gain audio amp with speaker and many applications

It is surprising how often a simple audio power amplifier can be useful when building and servicing electronic equipment. Amplifiers of this type can also be useful for incorporation into more complex items of equipment and can serve as headphone or guitar practice amps. The design described here gives a maximum output power of about 150mW RMS into a high impedance loudspeaker, and requires only about 25mV RMS (into approximately 200k) at the input to produce full output.

The Circuit

The only active device used in the circuit is a TBA820M integrated circuit, as can be seen from the circuit diagram which is shown in Figure 1. The TBA820M is a small, class B, audio power device which requires few discrete components.

RV1 is the volume control which also biases the input of IC1. The input signal is coupled to RV1 by way of DC blocking capacitor C2. The closed loop voltage gain of IC1 is determined by an internal feedback resistor and discrete resistor R1, the specified value for R1 giving a voltage gain of about 112 (41dB). C1 is the main supply decoupling capacitor and C4 decouples the supply to the input stage of IC1.

R2 and C7 form a Zobel network that aids the stability of the circuit. C5 is used to introduce negative feedback at high frequencies which rolls-off the high frequency response of the circuit, also to improve stability. C6 provides DC blocking at the output of IC1.

Under quiescent conditions, the current consumption of the amplifier is only about 4mA, but rises to 20mA or so at high output levels.

Construction

Details of the Veroboard component panel and wiring of the amplifier are shown in Figure 2.

C2 is not mounted on the component panel as it is more convenient to wire it direct between SK1 and RV1. Do not run input and output leads right next to each other as this could result in poor stability. Also, be careful not to overlook the three link wires connecting the panel components.