Voltase hobby

This 1-watt amplifier leads itself par excellence for usa as driver for a low impedance headphone or as output stage in a hi-fi preamplifier driving an active loudspeaker. The schematic of this amplifier consists of an opamp type LF356 and a push-pull transistor output stage. Low-pass filter R1/C2 at the input limits the slew rateof the input signal. The fixed quiescent current of 30 mA draw by the output transistors, and set by diodes D1…D4 in conjuction with emitter resistors R7 and R8, ensures very low crossover distorsion.

Feedback resitors R3 and R4 fix the gain at about 15dB. The consequent overall distortion with a 3dB bandwidth from 10 Hz to 30 Hz is only 0.1 percent.
The amplifier delivers a maxiumum powder of 1 watt into 8 ohm for an input signal of about 500 mV rms. High-impedance headphones and 4 ohm loudspeakers may also be connected withoutdetriment.
The amplifier is best build on the printed circuit boar shonw bellow. The two transistors (BD139, BD140) shoud be mounted on heat sinks - do not forget the insulating washers and the head conducting paste!

To drive high-imperdance headphones at high volume, you need a +/- 15 volts regulated powder supply