Conclusions

The conventional configuration of the audio amplifier performed as designed, under breadboard conditions. The amplifier drove an 8 ohm speaker with an output of approximately 700 mV when a 500 ohm, 50 mV signal was injected. The add-on transformer was tested and found to ineffective in passing the audio to the load. The transformer has consequently been deleted from the design.

Test Results
1.    Output resistance tested OK.

Pin Resistance (Ohms)

1.    INFINITE
2.    4.05 M
3.    6
4.    220
5.    INFINITE

2.    The project was powered up and found to be not working. Investigation found poor connections of the IC to the board. After repositioning the pins, the circuit worked.

3.    After approximately 1 minute of operation IC was found to be very warm and the heat sink was installed. The IC continued to heat up and eventually oscillated out of control. The circuit was thoroughly checked and capacitors C3 and C4 were found to be out of specifications. The faulty components were replaced and the circuit was retested. The temperature of the IC was then reasonable.

4.    Voltage was measured at the speaker with a Fluke DVOM and was around 700 millivolts. The output voltage level appeared to remain constant with varied ampifier source voltage input. The best audio was obtained with an input of 50 mV or less. Higher input signals overdrove the amplifier unless an attenuator was installed in series.