Power amplifier using transistors

 

Using some power transistors and some other common electronic components, can be designed a high power audio amplifier capable to provide a maximum output power of 90W.
This power amplifier based on transistors is capable of provide an output of 70W on a load of 8 ohms or 90W on a 4 ohms load.
If the component values in parentheses are used can be connected speakers with 4 ohm impedance, in which case the amplifier maximum output power will be around 90 watts.
The input signal is brought to the transistor T1 and the reaction is taken on the basis of T2. Current through deferential stage is kept constant at 1 mA current source through the action of T3. The input signal for T4/T8 transistor is taken from the T1's collector in combination with current source T5 forms a control stage class A for power transistors. Current through control stage is quite small (about 7 mA) as T6 and T9 are Darlington power transistors.
Protection circuit from Fig. 2 must also be changed when using a 4 ohm load. R24 and R28 values are then 3k9, R26 and R28 are 220 ohms, and D5, D6 and R30 are all eliminated.
Rectified voltage for 70 W / 8 ohms version is ± 40 V to be in load, no load, this corresponds to about ± 47 V. At 4 ohms, these values are ± 34, respectively, ± 40 V.
The transformer used must provide for alternative 1A 70 W / 8 ohm (mono) and 2.2 A for version 90 W / 4 ohms.

Circuit Diagram: