In the 20 watt stereo amplifier circuit observed only one stage of the system because the circuit stereo, second channel are identical. The figures in parentheses are equivalent to the terminal for second channel. The core of this project is the integrated circuits from National Semiconductor, the LM1876, which provides two power operational amplifier with mute and standby function.

The input signal, that having been conditioned and leveled, will enter into non-inverting amplifier input. At the output of op amp part of the resulting signal is fed back to the inverting amplifier to form a feedback network. Because the circuit is internally balanced when working with source item is not necessary to install the output BootStrap capacitor.

This system requires an operating voltage + / -28 volts and 2 amps current. In this case the transformer should have a primary according to the mains voltage (220V) and a secondary ct of each branch 20v (40v from end to end). The diodes must be 100V / 3A 1N5406 or similar type. Can also be used a bridge rectifier, which makes it easier and reduces the number of tracks / space. The filter capacitors are 4700μF x 50v.

And be sure to provide a heatsink to the IC amplifier.

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Features chips as follows :

The device has built-in overload protection, output current limiting, suppression of the “click-and-pop” when you turn on and off. There is also a programmable gain amplifier and switch-off temperature.

As can be seen, the device is connected to two power supplies – 22 V – for analog and digital 5-12V. If it is assumed that the chip will be powered by 12 volts, the findings for the power supply can be combined. Otherwise, it is enough to feed the analog part to connect any integral stabilizer – TIPPING or LM.

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IC Features :

In the chip implements two identical channels of amplification and specifications are for the stereo mode of the amplifier. Chip can also operate in mono – in bridged mode. In this case, the output power at CG = 0.5% at 120 watts. The other characteristics remain practically unchanged.
Below is a schematic of the LM4780 Mono channel mode :

Parts list :

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On this chip there is a difference in the headphone sense pin pin number 4. This will detect when a headphone jack inserted into the connector. For more information see the datasheet here.

Parts list :

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