The printed circuit board is made according to the diagram shown. With the exception of power supply (already rectified and stabilized voltage is supplied to the board). If you plan to use one common fan, you must:

1) install jumper F---F'

2) do not install parts with a star (*).

For reliable operation of the amplifier, it must be equipped with several additional functions that would delay the connection of the speakers until the end of all transient processes when the supply voltage is applied, as well as turn off the acoustics in the event of an emergency, for example, failure of the arm. In this case, almost the full supply voltage of one of the arms appears at the output of the amplifier and the speakers may simply burn out, not counting the possibility of windings flying off and other undesirable consequences.

If the amplifier is powerful enough, then its cooling system is usually built using fan blowing, which significantly saves the area of ​​radiators and reduces the dimensions of the structure. It is clear that the constant noise of the blades does not bring much pleasure, therefore, to reduce noise, the fan supply voltage can be either reduced or turned off completely, depending on the current heating of the radiators.

If fans are used, it cannot be ruled out that the fan will become clogged or fail, in which case overheating of the amplifier can become critical. To prevent this from happening, the protection system must turn off the load before the radiators (and the transistors on them) reach a dangerous temperature. You can use 70*C as a guide.

It is also advisable to assign a preventive measure to control the AC resistance to the protection system. For example, if there is a short circuit in the acoustics or speaker wire, the load resistance will drop significantly and the amplifier will be overloaded in output current. If the amplifier has current protection, failure will not occur, but so that the user is aware of the problem that has arisen from the very beginning, it makes sense to check the load resistance at the time of power supply, and if it is below the permissible level, then do not allow it to be connected to the amplifier.