single-chip 50 W AF amplifier
SINGLE-CHIP 50 W AF AMPLIFIER
The chip on which the amplifier isbased, a Type LM3876, is a memberof the Overture family from National Semiconductor,All members of this family arepin-compatible and mutually interchangeable.They are typified by an internal protection(called SPIKE). In practice, the differencebetween them is the power output.The series was described on the basis of theLM3886 in an earlier issue*.The PCB has been designed so that itcan accommodate the LM3876 (50 W) aswell as the LM3886 (150 W). Because ofthis, pin 5 of the IC on the board is connectedto the positive supply line. This connectionis not needed for the LM3876,since its pin 5 is not (internally) connected(NC).
The IC is located at the side of the boardto facilitate fitting it to a heat sink as shownin the photograph.An important aspect for optimum performanceis the decoupling of the unregulatedsupply lines by C7–C10. All earth connectionsgo to a single terminal on the board.Air-cored inductor L1 consists of 13turns of 1 mm dia. enamelled copper wirewith an inner diameter of 10 mm. Thecompleted inductor is pushed over R7 andits terminals soldered to those of the resistor.All electrolytic capacitors must bemounted upright.
The amplifier can be muted with asingle-pole switch connected to the MUTEinput (pin 8). This function is enabled whenthe switch is open. If muting is not required,solder a wire bridge across the muteterminals on the board.Boucherot network R6-C6 is not normallyrequired in this application, but provisionhas been made for it for use in otherapplications.