45 W audio amplifier

A classic 70's-80's design. 45W in 8 ohms speaker with a 70V 2A power supply. 685mV peak at input gives 27V peak (that is 27/1.414. = 19 Vrms) to the output making 45 W RMS (19Vx19V/8ohms). After building the amp and before applying the supply voltage preposition the 2 pots at mid course. With 0V at input (shorted input), adjust the first pot to measure half supply DC voltage between the two last transistors (i.e. 35V DC when supply is 70V). Keeping input shorted, adjust second pot for a resting (that is with shorted input) current of 50mA in the last upper transistor emiiter (use measurement of the voltage across the 0.33 ohm emitter resistor and multiply by the 3 to get the resting current ( i.e. 13mV/0.33 = 50mA). Redo triming the two pots until getting stable values. Inputing a sine signal 1kHz 685mV peak should give 27V peak to the output on a 8 ohms load, that is 45 W RMS. 1st transistor is BC560C. Final transistors are TIP35C mounted on a large radiator with isolation pads and nylon screws. The other NPN transistors are BD139 and the second PNP is a BD140. The BD139 is ruling the resting current of final stage and should be mounted on the same radiator with an isolation pad as well (it acts as a temperature transducer and resting/bias current regulator for the final transistors). Power supply could range from 50 to 70V DC. Source : Elektor n°397-398 p44 (07-08/2011). Adapted.

published 2 years ago