Panel switch wiring

As the equipment is mains operated, the battery compartment can be removed. The loudspeaker can also be removed as it is no longer required; the mains transformer may be fitted in its position. lf so desired, you · may add a neon indicator and a mains switch as well. Assembly cost and time As to the cost, you are lucky if you are in Delhi and can shop around Lajpatrai Market. The cost would be around Rs 400 or even less if you know the way the Delhi market works. Outside Delhi, the cost would be around Rs 450 much less than you would pay for a mono cassette recorder. lt is certainly worth every rupee that you spend as it would enable you to use the stereo system that you already own. Assembly time would depend on your familiarity with the electronic circuitry, but it would be a weekend well spent. (Time you need for shopping for the mechanism and parts would be extra.)  Two direct·coupled transistors are used in the preamplifier, with base bias for the first provided from the emitter of the second. This system of biasing results in heavy DC ° feedback that ensures stable operation of the circuit.


With the DC operating conditions stabilised, as mentioned, the AC characteristics of the preamplifier are also stabilised. The AC feedback from the collector of the second A transistor to the emitter of the first mainly provides frequency equalisation high frequency de-emphasis to compensate for the recording characteristics. To keep up a high signal- to-noise ratio in the high frequency range, the highs are boosted during recording. (This is called pre-emphasis.) A reverse treatment is therefore essential in playback to reproduce the original high-frequency signal correctly. An R- C combination, 22k with 5nF capacitor, carries out this function adequately.

Output from the preamplifier is fed to a 741 op-amp set for a gain of about ten. The total gain is thus around 1000 so as to provide a 100mV signal from 100uV input normally expected from a cassette head. Use of discrete preamplifier at the input ensures a good signal-to-noise ratio, and the op-amp at the output with a wide dynamic range ensures that there is no chance of signal being driven into saturation. The signal from the preamp is fed to inverting terminal number 2 of the op-amp through 5.6k resistor. Negative feedback from the output terminal number 6 through 68k is also provided to terminal number 2. The non-inverting terminal number 3 is set to half the supply voltage. That sets the DC voltage at terminal number 6. Thus an optimum swing of the output is possible at terminal number6. Positive supply terminal number 7 is taken to the supply positive, and terminal number 4 returned to ground. The output from the op-amp is taken out through a47k resistor to the standard 3.5 mm socket used in cassette recorders. The socket fits in a window provided in a ‘Sony’ cabinet for mono cassette recorder. `