Posted on Mar 3, 2013 6676
Audio from the telephone is inductively coupled to the base of Ql. which is used as a preamp. The preamp provides a gain of about 75 dB, to boost the input signal from .about 4 mV to about 300 mV pk-pk. If you use a higher gain transistor, increase the value of R9 to produce a Q point, measured from minus to the collector of Ql, of one-half the supply voltage.
The Ql output signal is coupled through C3 to R7, which serves as a volume or drive-level control, to Ul, a dual, 2 w amplifier connected in cascade. Pins 1 through 7 serve as a driver for the final amplifier, pins 8 through 13. Compensation and balance is accomplished by components R1 through R6 and C4, C6, and C7. Pins 3 through 5, and 10 through 12 should be tied to the negative supply rail.