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70 - 90 watts audio amplifier electronic project

Using electronic diagram below can be build a very simple audio amplifier designed so as to provide an output of 70 W on a load of 8 ohms.
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: 
70 - 90 watts audio amplifier electronic project circuit diagram
70 - 90 watts audio amplifier protection circuit

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