Now this is an interesting amplifier and can be built for almost any AF power level you want from or more. I will give you suitable examples for the 30 – 100watts version.

RV must be set to minimum resistance before application of power. To set up the amplifier, advance RV SLOWLY whilst monitoring the DC current through R7. Set RV for about 40mA. If you have a digital voltmeter then you can adjust for 8mV accross R7 and R8. If this standing current becomes too high (more than 80mA without a signal) when the transistors become warm then add a 1K0 thermistor accross RV, increase the value of RV to 470R and readjust when cold. When warm the current should remain stable. The thermistor should be mounted in direct contact with the heatsing upon which TR5 and TR7 are mounted on. If after adding the thermistor the standing current falls below 20mA then you can place a fixed resistor of 470R in series with the thermistor to reduce it’s negative temperature effect (When I say warm I mean after a good blast of music has been applied to “cook” the amplifer a bit). All current measurements should be made WITHOUT the speaker connected.

All transistors except TR1 MUST be mounted on some form of heatsink which must be adequate for the job. You can replace TR4/TR5 with a single NPN power darlington and replace TR6/TR7 with a single PNP power darlingdon transistor. The power supply must be capable of handling more than 10 amperes as well as being quite well filtered. The speaker impedance should be about 8 ohms.

The low frequency response of this 30 – 100watts is not restricted by an output capacitor. There remains the possibility of a standing DC through the speaker. R3, D1 and C3 is included to prevent this from happening. You could replace C2 with a dead short but I prefered to leave it in circuit so that the negative feedback rises to 100% at DC.

This amplifier should not be attempted by those lacking experience since high voltages and powers are available. When DC-coupled high power amplifers decide to “go” then the results can be very spectacular. You therefore cannot afford to make any mistakes during the building of this project.

You can of course reduce the supply voltage and use cheaper transistors  to make a smaller amplifier. If you reduce the speaker impedance to 4 ohms then there is the possibility to get yet more power out of it, but you will need to increase the current rating of the output and driver transistors as well as the standing current through TR2 and TR3. Do NOT use the 2N3055 or similar devices since their Hfe reduces to as low as 5 when pulling more than 10 amperes.

30 watts to 100 watts

R1 47K R2 220R
R3, R4 47K R5 68R
R6 5K6 R7, R8 0R1
R9 2R2 C1 1u0
C2, C3 2200u 3vWKG C4 100n
PNP TR1 2N2904 PNP TR2, TR6 2N4036
NPN TR3, TR4 2N3725 PNP TR5, TR7 NPN 2N6099
Diodes 1N914 RV 330R
+VE +45 volts -VE -45 volts