Showing posts with label Cassette. Show all posts
Showing posts with label Cassette. Show all posts
Monday, January 26, 2015
Low Cost Playback Amplifier For Cassette Deck
For some time now, there have been a number of tape cassette decks available at low prices from mail order businesses and electronics retailers. Such decks do not contain any electronics, of course. It is not easy to build a recording amplifier and the fairly complex magnetic biasing circuits, but a playback amplifier is not too difficult as the present one shows.
The stereo circuits in the diagram, in conjunction with a suitable deck, form a good-quality cassette player. The distortion and frequency range (up to 23 kHz) are up to good standards. Moreover, the circuit can be built on a small board for incorporation with the deck in a suitable enclosure. Both terminals of coupling capacitor C1 are at ground potential when the amplifier is switched on.
The stereo circuits in the diagram, in conjunction with a suitable deck, form a good-quality cassette player. The distortion and frequency range (up to 23 kHz) are up to good standards. Moreover, the circuit can be built on a small board for incorporation with the deck in a suitable enclosure. Both terminals of coupling capacitor C1 are at ground potential when the amplifier is switched on.
Cassette Deck Playback Amplifier Circuit Diagram:
Because of the symmetrical ±12 V supply lines, the capacitor will not be charged. If a single supply is used, the initial surge when the capacitor is being charged causes a loud click in the loudspeaker and, worse, magnetizes the tape. The playback head provides an audio signal at a level of 200–500 mV. The two amplifiers raise this to line level, not linearly, but in accordance with the RIAA equalization characteristic for tape recorders. Broadly speaking, this characteristic divides the frequency range into three bands:
- Up to 50 Hz, corresponding to a time constant of 3.18 ms, the signal is highly and linearly amplified.
- Between 50 Hz and 1.326 kHz, corresponding to a time constant of 120 µs, for normal tape, or 2.274 kHz, corresponding to a time constant of 70 µs, for chromium dioxide tape, the signal is amplified at a steadily decreasing rate.
- Above 1.326 kHz or 2.274 kHz, as the case may be, the signal is slightly and linearly amplified. This characteristic is determined entirely by A1 (A1’). To make the amplifier suitable for use with chromium dioxide tape, add a double-pole switch (for stereo) to connect a 2.2 kΩ resistor in parallel with R3 (R3’). The output of A1 (A1’) is applied to a passive high-pass rumble filter, C3-R5 (C3’-R5’) with a very low cut-off frequency of 7 Hz. The components of this filter have exactly the same value as the input filter, C1-R1 (C1’-R1’). The second stage, A2 (A2’) amplifies the signal ´100, that is, to line level (1V r.m.s.).
Wednesday, October 1, 2014
Stereo Cassette Player Circuit Diagram
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. `
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. `
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