18 Watt power amplifier - darlington pair version

Rohit Balkishan Dubla

The Op-Amps used are TL074 quad devices.

The o/p transistors are in "darlington-pair" configuration (a single darlington power transistor can be used in place of Q1/Q2 & Q3/Q4). The diodes must be in contact or in close proximity with the o/p transistors. VR1 must be adjusted to be within ±10% of it's mid-point to ensure acceptable o/p offset level. It can also be replaced with a fixed 2.2K or 2.7K resistor. The o/p devices will be biased at about 120mA and will dissipate about 3 to 3.5W at no signal.

Due to the very high slew rate of TL074, the capacitor C4 must have a minimum value of 22nF. If after a few moments of powering up, the amp goes into oscillation or produces a humming sound (with speakers connected) or you find that the power transistor supplies drop (this may happen even without connecting the speakers), accompanied by mild heating up of the o/p transistors then the culprit surely is the Op-Amp (X1). To solve this problem, simply increase the value of C4, and test it again. Make sure that oscillation/hum does not set in even when you connect/disconnect an audio source at the input with the amp turned on or when the volume is turned up to maximum. Choose C4 to be the minimum value which stops this behaviour. Use a small (cheap) general purpose speaker for testing, since the oscillation could damage your expensive tweeters.

The transistors Q2 and Q4 can be changed to BD239/BD245 and BD240/BD246 respectively. BD239/240 to be used only if the load impedance is greater than or equal to 6 Ohms. For lower impedances, either retain the shown transistors or use higher current transistors. Impedances less than 3 Ohms are not recommended. The heat sinks need not be very elaborate (except with BD239/240), since the transistors have power ratings well in excess of those required. In spite of this, a large heat sink is always better than a smaller one.

This amp can be used for bi/tri-amping with active crossovers. For a tri-amped system, a 22µF non-polar capacitor must be used in series with the o/p of the amp feeding the tweeter.

The power supply for the entire system can be a simple dual supply using a 12-0-12V/5A (120VA) transformer, which after rectification and filtering will give a DC voltage of about ±17V. Mains wiring must be done with utmost care and precaution to avoid any risk to the constructor/users of the amplifier. This PSU can be used for a bi/tri-amped system without any modifications.