Two interestingly coupled transistors form the preamplifier stage of the circuit. The uniqueness of the configuration lies with the fact that the the emitter of the second transistor produces a feedback and biases the first transistor.
This DC feedback effect enhances the amplification of the stage significantly. It also ensures overall DC as well AC stability of the circuit under all types of input variations.
Another AC feed back loop added across the emitter of the first transistor and the collector of the second transistor ensures complete frequency equalization. Precisely it provides frequency compensation to the recording characteristics, also called frequency de-emphasis.
During recordings, to maintain a high signal to noise ratio at high frequency ranges, the particular high frequencies are always boosted, called pre-emphasis. Therefore, it implies that during playback the recorded information must be reproduced with equally boosted levels, the inclusion of the RC combination of a 33 K resistor and a 5 nF capacitor adequately ensures this required criterion during playbacks.
The above dimensioned output is next fed to an intermediate op amp stage. The inverting input of the IC 741 accepts the amplified signals from the pr amplifier stage i.e. from the collector of the second transistor via a series coupling capacitor and resistor.
A negative feedback from the output pin #6 of the IC is applied to its inverting input pin #2 through a 56 K resistor which takes care of the gain setting of the stage, while the non-inverting pin of the IC is clamped to half the supply voltage using a couple of 33 K resistors. This ensures very good response of the IC output to the received even weakest of input frequencies.
The feedback resistor of 56 K sets the stage to produce a gain of about ten. This makes the total amplification of the two stages combined, equal to 1000, that means these two stages are able to lift the minute 1 – 10 µV play back head signals up to 100 mV, just enough for the acceptance of the next power amplifier stage.
Pin #7 is terminated to the positive supply where as the negative or the ground potential is applied to pin #4 of the IC.
Power Amplifier Stage:
A single chip 10 Watt amplifier stage incorporating the IC TDA 2003 is employed here for the final amplification. To get a comprehensive explanation regarding the IC, you may refer the article here.
The signal, which is amplified to about 100 mV by the preamplifier stage, is applied to the input of the above power amplifier stage via a variable resistor. The potentiometer is basically used as the volume control by controlling the input sensitivity of the amplifier. The more the slider arm is moved toward the ground, the lower the sensitivity gets, making the output lesser in volume.
The loudspeaker at the output produces a high quality, undistorted output that’s at par with the most sophisticated available tape player units.
Circuit Idea Drawn after Opening and Studying an Old Cassette Tape Player Unit.