This project consists of a single channel solid state audio amplifier capable of providing up to 50 Watts to an 8 Ohm load. Provided are the basic schematic and a board layout pattern.
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The two output transistors should be mounted on an external heat sink as they will dissipate considerable heat at high power levels. There are also two diodes marked as being located on the heat sink as well. These are intended to decrease bias voltage on the outputs as the transistors heat to limit thermal runaway and prevent component failure. The output transistors must be mounted using an insulator since the case of those devices are the collector lead and as such must not be grounded.
The amplifier requires 36 volts, complementary supply (positive and negative with reference to ground) to provide the specified output. The supplies should be able to supply 3 Amps, well filtered, with respect to ground. If you are building a stereo version (two amplifiers) the current required will double.
Output impedance of the amplifier is 4 or 8 ohms. Do not attempt to operate into a lower impedence as damage to the output could result. The input impedance is 2 K ohms. The power amplifier can be driven from any commercial single-ended pre-amp or mixer. 2 Volts P-P will drive the power amplifier to its full rated output level.
Set-up is easily done. There is one control which adjusts the output bias level. Normally it is set so the output transistors draw about 100 milliamps under a no-signal condition. Setting it too high will result in excessive heat being dissipated by the outputs; Too low will cause crossover distortion to increase. Bias can be determined by measuring the voltage drop across the emitter resistors of the output transistors.
The schematic of the Main Output Board :
This is a PC Board Layout and component placement drawing. A scale is provided to adjust to the proper size for component dimensions.
Finally a schematic is given for the Power Supply and Speaker Protection Circuit. The Protection circuit does two things. First of all it delays the connecting of any speakers when the amplifier is first turned on to allow voltages to stabilize. This prevents the loud "Pop" sometimes associated with a solid state amplifier when it is powered up.
In addition the protection circuit monitors the speaker outputs for any DC potential. This might occur should the amplifier become unstable due to excessive heat or in event of a component failure. Should this occur the protector circuit will cause the relay to drop out immediately disconnecting the speaker before damge could occur.