This simple amplifier is capable of producing up to 45W in to 8 ohms at the specified supply rails of +/-35VDC (I wouldn't exceed this voltage without changing the driver and voltage amplification stage (VAS) transistors to something with better Vce specs than the BD139/BD140 (MJE340/MJE350 come to mind). The goal was to build an amplifier that used commonly available transistors and no strange hard-to-get components.

The amplifier comprises constant current sources (CCS) Q3, which controls the current to the long-tail-pair (LTP) input stage and Q4 controlling the current through the voltage amplification stage (VAS) and biasing network. The amplifier is self-biased using two 1N4148 signal diodes (D3/D4), but this is open to modification using a Vbe multiplier, as during testing I found the quiescent current to be around 170mA (which is quite high!). The output stage consists of a Class-B Sziklai compound pair comprising of Q6/Q8 and Q7/Q9 to drive the speaker. A Class-AB design using emitter-followers in Darlington configuration could have also been used, however the Sziklai pair (with common collector output) is just as good.

The overall gain of the amplifier is 23, or 27dB, as set by R6 and R7 (A = R7/R6 + 1), but you could modify this by increasing the value of R7 if you want a lower input sensitivity.

As for the output transistors, they are marked as just Q8 PNP and Q9 NPN on the PCB silkscreen. This leaves it up to you what power transistors to use, and the circuit is accepting of most choices. Examples you could use are TIP2955/TIP3055, TIP36C/TIP35C, MJL21193/MJL21194 (albeit expensive at $16 AU each!). I don't recommend any Toshiba devices in the 2SA or 2SC range, unless you can get them from a reputable supplier as these are the most commonly faked transistors on the market. As for the Vce (voltage collector-emitter) anything with a rating of greater than 80V is fine, with a minimum Ic (current collector) of 10A. 200V Vce is overkill.

My tests were done using supply rails of +/-30VDC and TIP2955/TIP3055 as O/P devices.

Specifications:

Input Sensitivity: 741mV RMS

Power Output - 34W RMS @ 8 ohms, 45W RMS @ 4 ohms

Frequency Response - 20Hz - 22kHz

Distortion (THD) - Unmeasured

Stability - Unconditional (as far as I can tell, it doesn't seem to experience thermal runway or going in to spurious oscillations)

Link to my YouTube video on assembling this project: