Latest 50Watt LM3876 Power Amplifier Circuit diagram and explanation

My cause of construction of this project was to develop a compact design for a stereo amplifier can be integrated by a wrong (but the sound quality conscious!) Living student at a university or a college dormitory.
audio amplifier 50W circuit

audio amplifier 50W circuit
The amplifier feeds a pair of speakers with two LM3876 amplifier integrated circuits (50 watts per channel), or a pair of headphones Meier Crossfeed through a filter and a dual OPA2134 Opamp. There are four selectable line inputs, and output buffers with line level for the registry. The design with readily available components of good quality, and is divided into four BPC, a power amplifier for each channel, for the nutrition board, and for the pre-amp / headphone driver.
Above the diagram of part of the preamp board .
The output selector is sent to pins J1 and J3. Looking at the left channel, C1 and R2 form a low pass filter with a-3dB point of 40 kHz, which rejects any RF interference picked up on the interconnections. R2 also includes the impedance of the device, in this case, 47k ohms. R1 ensures Opamp U1 is presented with an impedance equal to its two inputs, contribute to improving the performance of the distortions described in the datasheet OPA2134.
The value of R1 (9K1) is universally accessible, close to the value of the parallel combination of R3 and R4 (22k and 15k, respectively). R3 and R4 set the gain at that time, just below 2.5 in this case. This gives ample space for a wide range of signal sources, which could be as much as 3VRMS. In this case, the peak output voltage of 10.6V would be fine with the project to supply ± 15V.

This initial gain brings the signal to a level that the output of the volume can lead the power amp circuits directly without any additional benefit, and allows the helmet of the driver circuit to operate with a low gain, gives lower noise level. C7 forms 100kHz a lowpass filter with R3, to fall on the gain of unity at very high frequency, and to help promote stability in the Opamp. It is not strictly necessary for the proposed OPA2134 allows the unit, but down substitute cheaper but more likely oscillation device, such as the NE5532, if budgets are tight. C19 couple the AC output of this phase for volume control, and with a 50k potentiometer, the-3dB point of the response of the headphone amps at 1.4Hz (power amp has more HIGH PASS FILTER). The capacitor is very important because all the other stages are DC coupled and DC C19 prevents any of the source components, amplification and presentation of headphones or speakers.

Resistance R9 binds the production of inputs to a recording device like a VCR or mini-disc. This helps to prevent the source being loaded in the diet of both the gain stage input and the recording device and protects the source, the output should be shorted to earth for a reason whatsoever. The output from J5 and J6 are introduced into the volume control pot, which should be good quality. Finally, C3 to C6 and provide decoupling of the power supply rails, C5 and C6 high frequency decoupling, C3 and C4 lower decoupling.