FM VERİCİ

The chip on which the amplifier is based, a Type LM3876, is a member of the Overture family from National Semiconductor, All members of this family are pin-compatible and mutually interchangeable. They are typified by an internal protection (called SPIKE). In practice, the diftection ference between them is the power output. The series was described on the basis of the LM3886 in an earlier issue*.



The PCB has been designed so what it can accommodate the LM3876 (50W) as well as the LM3886 (150W). Because of this, pin5 of the IC on the board is connected to the positive supply line. This connection is not needed for the LM3876, since its pin5 is not (internally) connected (NC).



The IC is located at the side of the board to facilitate fitting it to a heat sink as shown in the photograph.



An important aspect for optimum performance is the decoupling of the unregulated supply lines by C 7-10. All earth connections go to a single terminal on the board.



Air-cored inductor L1 consists of 13 turns of 1mm dia. enamelled copper wire with an inner diameter of 10mm. The completed inductor is pushed over R7 and its terminals soldered to those of the resistor.



All electrolytic capacitors must be mounted upright. The amplifier can be muted with a single-pole switch connected to the MUTE input (pin8). This function is enabled when the switch is open. If muting is not required, solder a wire bridge across the mute terminals on the board.



Boucherot network R6-C6 is not normally required in this application, but provision has been made for it for use in other applications.



According to the manufacturers, both chips are optimalized for a load of 8 Ohm. The output power is lower when a 4 Ohm load is used or when the supply voltage is reduced. When a 4 Ohm load is used, the SPIKE protection becomes active when the supply voltage is about 27V, resulting a in a reduction of the power output to 10W. This means that it is not advisable to use loudspeaker with an impedance < 8 Ohm.



For best result you can expand power amplifier using BPA-200 Amplifier

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Part list



Resistor:

R1, R3 = 1 k

R2, R4, R5 = 18k

R6 = see text

R7 = 10R, 5 Watt

R8, R9 = 22k



Capacitors:

C1 = 2.2 uF

C2 = 220 uF, 160 V

C3 = 22 uF, 40 V

C4 = 47 pF

C5 = 100 uF, 40 V

C6 = see text

C7, C8 = 100 nF

C9, C10 = 1000 uF, 40 V



Inductors:

L1 = 0.7 uH - see text



Integrated circuits:

IC1 = LM3876T



Miscellaneous:

Heat sink for IC1 < 1.5 K W-1

Single-pole switch - see text





















Schematic and PCB Layout LM3876

This is design is very similar to the National Semiconductor BPA-200 (Bridge/Parallel Amplifier) which uses 4x LM3886 per channel and an input buffer:





Nothing Special in design but usefully for more watts!

My one will use 2x BrianGT LM4780 kits and a balanced line driver to bridge them, like this:





 

The LM4780 is a stereo audio amplifier capable of typically delivering 60W per channel of continuous average output power into an 8 load with less than 0.5% THD+N from 20Hz - 20kHz.

The LM4780 is fully protected utilizing National's Self Peak Instantaneous Temperature (°Ke) (SPiKe^TM) protection circuitry. SPiKe provides a dynamically optimized Safe Operating Area (SOA). SPiKe protection completely safeguards the LM4780's outputs against over-voltage, under-voltage, overloads, shorts to the supplies or GND, thermal runaway and instantaneous temperature peaks. The advanced protection features of the LM4780 places it in a class above discrete and hybrid amplifiers.



Each amplifier of the LM4780 has an independent smooth transition fade-in/out mute.

The LM4780 can easily be configured for bridge or parallel operation for 120W mono solutions.





The total effect is (2x LM3886's paralleled) x2 Bridged and should give approx 225 watts into 8 ohm and 335 watts into 4 ohm when used with a sufficient power supply.



The line driver part is based on the DRV134 and a PCB from digi01, you can see the details here.



In the end mine was dual mono in a recycled amp case using 2x 300VA (18VACx2) toroidal transformers, 54,400uF of capacitance per channel with a snubber. The DRV134s add 6dB of gain so I set the LM4780s to about 16x gain to compensate (they are set to 33 normally). I get no DC offset, no hum, no crackle or thump on turn on or turn off, silent as.



Pics.....