18 Watt power amplifier

Rohit Balkishan Dubla

Note: I have updated the amp to use a better biasing scheme for the o/p stage. Check Version-1 and Version-2 for versions using the darlington and compound pair output stages respectively.

The Op-Amps used are TL074 quad devices. This IC is pin compatible with LM348, but has a very high slew rate (13V/µS) as compared to LM348 (0.5V/µS). Use of LM348 results in a somewhat degraded performance.

The supplies to the Op-Amps have been separated from those of the power transistors, so that the Op-Amps are fed from regulated ±15V @ 1A supplies and the power transistors are supplied from the usual high current (>= 5A) ±15 to ±18 Volts supplies which may or may not be regulated. This has been done so that the Op-Amps are unaffected by the varying current drain (and the accompanying voltage drops) that occur in the power-stage power supplies, especially when operating at or near full power. Another reason for having a separate supply for the Op-Amps is for use with active crossovers employing similar Op-Amps. In this case, the volume control R2 must be omitted & instead placed before the crossover network (The non-inverting input of A2 is now directly connected to the o/p of A1, via a 100Ω resistor). All the Op-Amps will now be fed from the regulated supplies.

The o/p transistors are in "compound-pair" configuration instead of the more common "darlington" configuration. This has been done to reduce crossover distortion to negligible levels & make the biasing extremely simple (in spite of the very elementary biasing arrangement employed, the performance of the amplifier is quite good. The 50K pot R6 did not need any adjustment to remove crossover distortion, after I built it). The diodes must be in contact or in close proximity with the driver transistors. To adjust R6, set R6 to maximum (wiper completely towards base of Q1), connect a proper audio source to the amp and set the volume to the lowest audible level. Look for any "crackle" in the sound, especially that which seems to be riding on low frequency sounds. If it is heard, then reduce R6 very slowly till the crackle disappears. Remember, R6 must be set to the maximum value which removes crossover distortion. Any lower value could result in unnecessary heating up of the o/p transistors & current drain.

Due to the very high slew rate of TL074, the capacitor C5 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 (A2). To solve this problem, simply increase the value of C5, 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 (all these symptoms have one common reason - the Op-Amp (A2) gets "kicked" into high frequency oscillation, the cap at it's o/p ensures that this doesn't happen). Choose C5 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 33pF capacitor (C7) across R4 sets the upper -3db frequency to about 30KHz. Reduce this if you find the amp lacking in highs, or altogether omit it (not recommended) - a value not less than 12pF is recommended. The transistors Q2 and Q4 can be changed to BD240/MJ2955 and BD239/2N3055 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 (2N3055/MJ2955 and the like). Impedances less than 3Ω 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.

Due to a ±15V supply to the Op-Amps, the max. o/p swing will be about ±12 V(p-p) or a power of 18W into a 4Ω load. This value of power is a very modest figure and as such this amp won't be very loud. To get a system that gives appreciable power, it's better to use this amp for bi/tri-amping with active crossovers. Assuming an absolute maximum power of 20W into 4Ω, we can obtain 40W (60W) by bi-amping (tri-amping). For a stereo system, this equates to 80W (120W) of total power output, which is quite sufficient for home use. The amp has a very good frequency response, and the highs are extremely clear, hence it is recommended that this amp be used in a bi/tri-amplified system instead of stand-alone. 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 is shown above. The o/p transistors are powered by the upper transformer/rectifiers/filters. The diodes (for the ±18V supply) must have a continuous current rating of at least 5A and the 1N4007 diodes are just about OK. Replace each diode with a bigger one or use 2 paralleled diodes if the diodes become hot during normal operation. The lower transformer has a 15-0-15V o/p since it is used to drive the voltage regulators. If the regulators are not used, then the transformer must have a 12-0-12V o/p which after rectification and filtering will give a DC voltage of ±17V for the Op-Amps. This voltage, though within the absolute maximum rating of the Op-Amp, is not recommended since the Op-Amps will become slightly warm during normal operation. 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.