Audio cassette player amplifier. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Audio cassette player amplifier. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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This amplifier can be used in the modernization of car radios, as well as simple stationary or portable tape players. It features low self-noise and electronic tone control.

The player's amplifier is designed to play audio tracks from compact cassettes through external speakers and can be used to upgrade car radios.

The advantages of the described amplifier include a low level of intrinsic noise, a sufficiently large output power and a depth of tone and balance control. It differs from similar devices in ease of setup, small dimensions, high sound quality.

The device (its schematic diagram is shown in Fig. 1) contains a two-channel playback amplifier on a DA1 K157UL1A chip in an unusual switching circuit - it is described in detail in [1]. The reduced output voltage of the SW (about 30 mV) necessitated an additional linear amplifier assembled on the DA3 K548UN1A microcircuit [2]. The signals amplified by it are fed to the integrated tone and balance control DA2 TDA1524 [3] and then through the volume control to the power amplifier, which is the DA4 TDA1557 chip [3].

(click to enlarge)

The playback volume could be adjusted by electronic control of the TDA1524 chip and without a dual variable resistor, but in this case noise is heard in the speakers even at minimum volume. All microcircuits used, except for DA4, are supplemented with supply voltage stabilizers.

Main Specifications

Output power, Рout, W, for RH= 4 Ohm......2x20
for RH= 8 Ohm......2x10
Depth of balance adjustment, dB......38
Tone control, dB, at LF (40Hz)......-20...+18
RF(16kHz)......±16
Frequency response range, Hz, for IEC-I tape......31,5...14000
IEC-II......31,5...18000
Relative noise level, dB......-65
Consumption current, A, at Upit = 12... 18 V. no more......5

The device is assembled on a printed circuit board made of one-sided foil fiberglass with dimensions of 145x20 mm, which allows it to be placed in most cases of industrial car radios. The heat sink for the TDA1557 chip is an aluminum plate with an area of 200 ... 400 cm2, depending on the specific case design.

The player's amplifier uses foreign-made oxide capacitors due to their smaller dimensions. Capacitors C17, C18, C27, C29, C30, C32 - K73-9 or K73-17, all others (non-polar) - KM-5 or similar. In the amplifier, you can use transistors of the KT315 series with any letter index (VT1-VT3), KS210B zener diodes (VD1-VD3). You can also use D814V by connecting them with an anode to a common wire. Diode VD4 from the KD209 or KD208 series with any letter index.

All resistors, except for variables, MLT-0,125 W (or 0,25 W). Variable resistors R7, R14, R17 - any type with a linear adjustment characteristic; R20 - double, with exponential control characteristic (option B).

It is advisable to use AC loudspeakers with a resistance of 4 ... 8 ohms and a power of 20 ... 30 watts.

For the experiment in the assembled structure, the author tried to install various magnetic heads [1], and in most cases good results were obtained.

A drawing of a printed circuit board and a layout of parts are shown in fig. 2. Resistors R1, R2, R4, R5, R24 are installed perpendicular to the printed circuit board, and resistor R6 is placed above the DA2 chip.

(click to enlarge)

It is advisable to use power wires with a cross section of at least 0,5 mm2. It is advisable to bring them out with a twisted pair. The negative wire is soldered to the contact pad 8 of the board, and from it to the case and other components of the device. All wires to the volume control must be shielded. The wires connecting the balance and tone controls with the board can not be shielded.

The wires connecting the magnetic head to the board carefully shield two wires of each channel in their screen, the screens are connected on contact pads 1, 6 only from the side of the board. The screens must be insulated from the outside, the wire sections to the magnetic head leads outside the screen should be made as short as possible.

Establishing a properly assembled device comes down to tuning to the required frequency of the resonant circuits formed by the head sections BG1 and capacitors C2 and C3. In some cases, to correct the range of tone control, you need to select the resistance of resistors R3, R6 and check the operation of voltage regulators for DA1-DA3.

When using the device in a stationary audio cassette player, it is supplemented with a 15 V mains power supply, rated for current up to 5 A with a low ripple level (possibly stabilized).

Literature

Shikhatov A. Playback amplifier on the K157UL1A chip. - Radio, 1994, No. 4. p. 14.
Ataev D. I., Bolotnikov V. A. Analog integrated circuits. - M.: Radio and communication, 1991, p. 68.
Microcircuits for audio and radio equipment. Encyclopedia of Repair, vol. 3. - M.: DODEKA, p. 214, p. 220.

Author: O. Shcherbich, Vladikavkaz, North Ossetia

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