Input signal to the Darlington comes from the feed current 741. Since R6 is connected to earth, positive or negative current signal is also going through R4 or R5. The voltage drop across this resistance serves as the input signal to the transistor pairs. A common dc negative feedback from the collector junction with T2 and T4 stabilize the DC circuit and the junction point is maintained at zero volts. So, there is no coupling capacitor needed for the speaker.

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Amplification
One component is used here to amplify the input signal for each of left and right: an op amp architecture-known type LM741 (U1 for the left channel and U2 for the right channel). Gain, and therefore the output power will depend on the position of the slider of the potentiometer (RV1 for the left channel and RV2 for the right channel) variable resistor wired between the output of the op amp and its inverting input. RB call the “right” of the potentiometer (RV1 or RV2), between slider and output op amp, and RA call “left” part, from slider and inverting input. The gain is then defined as the ratio RB / RA :

-If the potentiometer is in the middle position, the resistance of both sides of the slider is the same: RA = RB. The system gain is then:

G = RB / RA = 1 (no amplification or attenuation)

– If the potentiometer is set to 1 / 10 (slider near the input), the resistance RA is lower than RB. The system gain is then:

G = RB / RA = 10 (amplification)

– If the potentiometer is set to 9 / 10 (slider close to the output), the resistance RA is greater than RB. The system gain is then:

G = RB / RA = 0.1 (attenuation)

The resistors R1 and R2 are used to determine the potential of coupling capacitors C1 and C2 to a fixed value of 0V.

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