UMZCH for portable radio. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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Using the example of the revision of portable radio tape recorders "Vega", the author showed the possibility of improving the sound quality of such devices when replacing the UMZCH and the power supply unit in them.

Many are familiar with the depleted sound of small-sized portable audio equipment. Of course, there are also models that form the "correct" frequency response with two- or four-band tone controls. But their capabilities are also limited by low-quality output links, primarily UMZCH. A typical example is the domestic Vega RM-252S radio tape recorder with an output power of UMZCH 2x2 W when powered from the mains and 2x1,2 W from batteries.

Increasing the output power of portable equipment in order to improve sound quality is advisable for mains power mode. In this case, it is quite realistic to talk about an increase in the output power by several times.

There is a fairly large selection of integrated UMZCH with unipolar power supply in SIP2 and MULTIWATT cases on the market, both separately and in the form of assembled printed circuit boards. However, not all of them are suitable for battery-powered equipment. Moreover, their power is often indicated without specifying the modes, without specifying some parameters.

For a portable radio, economical UMZCH should be used, for example, TDA1516BQ, TDA1518BQ chips from Philips. They are distinguished by a low saturation voltage of the output stage transistors (about 0,5 V) and a small quiescent current. Some characteristics of these microcircuits are given in Table. 1. It should be noted that for a supply voltage of 12 V, the maximum power at a load of 2 and 4 ohms is reduced by 40%.

Nevertheless, these microcircuits can be recommended for use in portable equipment, for example, in the Vega RM-252S radio tape recorder. Alteration comes down to dismantling the old amplifier and installing a new one, assembled on a separate board according to a typical diagram (Fig. 1).

The capacitance of the filter capacitor in the power supply should be increased to 4700 uF. The conversion is simple, but the output power of 4...5 W per channel at a load of 4 ohms in some devices is still insufficient to fully use the power of the built-in heads or an external speaker.

Increasing the supply voltage of the equipment (in AC mode) allows you to significantly increase the maximum power of the UMZCH. For example, the UMZCH radio tape recorder "Vega RM-235S-1" is operable at a supply voltage of 22 V from the mains and 9 V from batteries. This is very rational, since when working from the network, a large output power and, accordingly, sound quality is achieved, and when working from batteries, it is economical. By the way, the scheme of this UMZCH was also retained in subsequent models of Vega radio tape recorders, but the supply voltage from the network was lowered to 15 V [1].

Among the integrated UMZCHs, there is a TDA2009 microcircuit, the maximum supply voltage of which is 28 V, and the minimum supply voltage is 8 V. This supply voltage range allows you to significantly increase the power when powered from the mains.

To "power up" portable audio equipment, for example, the Vega RM-235C-1 radio, we can recommend the TDA2009 switching circuit shown in fig. 2. In it, the circuits of the OOS dividers R1, R2 and R3, R4 are placed outside the separating capacitors C5, C7 and do not increase the quiescent current (unlike the standard switching circuit). This does not affect such parameters as the gain, the coefficient of nonlinear distortion, since the OOS in both cases acts on alternating current with a constant depth. Eliminating DC current flowing through the dividers improves economy in battery mode.

The proposed circuit does not use capacitors that shunt the UMZCH inputs in the standard circuit, since the band is limited from above, as a rule, in the radio itself - in the preamplifier. The capacitance of the input isolation capacitors C1, C2 is reduced in comparison with the original circuit by more than an order of magnitude, and an HPF with a cutoff frequency of about 60 Hz is formed at the UMZCH input, which is useful for reducing non-linear distortion at low frequencies; while increasing the economy. Self-excitation that occurs at an increased supply voltage was eliminated by introducing chokes L1, L2 into the load circuit.

When powering the UMZCH, the power supply should be modified. It is advisable to replace the existing mains transformer with a stacked W-shaped magnetic circuit with a new one with a ring magnetic circuit made of high-quality steel. Due to this, the power of the transformer is obtained noticeably more with the same dimensions. For our purpose, transformers with magnetic circuits OL40 / 64-32 (transformer power of about 40 W), OL40 / 64-25 (30 W), OL40 / 64-20 (24 W) and, in extreme cases, OL32 / 50-32 (19 W). The secondary winding is calculated for a voltage of 17,5 V.

The maximum current consumed by the new UMZCH increases significantly, so the capacitance of the filter capacitor in the power supply should be increased to 5000 ... 10000 uF. The required capacitance can be made up of two or more capacitors connected in parallel, for example 4700 uF and 2200 uF. It is also necessary to replace the rectifier diodes or the bridge with more powerful ones, for example, KD257.

To power the rest of the radio, a simple 12 V voltage regulator is assembled on a K142EN8B chip. When powered by batteries, this stabilizer must be turned off by a separate button or contact group, closed when the power cord connector is removed from the radio housing.

The scheme of the modified power supply is shown in fig. 3. New or changed at face value elements installed when finalizing the power supply are highlighted with a thick line. Instead of one filter capacitor (2200 uF at 16 V), capacitors C2, C3 are installed.

When working from the mains, the SA1 button must be pressed. In this mode, the UMZCH is powered by a voltage of 24 V, and the preliminary stages, a universal amplifier and a radio receiver are supplied with a stabilized voltage of 12 V. When switching to battery power or external from a 12 V source, press the SB1 button (its contacts turn off the stabilizer).

In the UMZCH and the power supply, MLT-0,125 resistors are used, non-polar capacitors - K73-9, K73-17 or KM-5 (C9, C10), oxide - any imported ones. Coils L1, L2 of 24 turns are wound on a mandrel with a diameter of 8 mm in two layers with wire PEV-2 0,85.

The TDA2009A chip should be installed on a finned or pin heat sink with an area of ​​about 200 cm2. The switch SB1 - the PKN62 (or P2K) button is mounted so that its movable stem extends to the rear or side surface of the radio housing.

Full modernization according to the above methodology provides the characteristics shown in Table. 2.

Of course, the best sound quality is achieved when the radio is powered from the mains and connected to an external speaker. If an equalizer is installed in the radio, then the sound quality is not inferior to more expensive music centers.

Author: A.Pakhomov, Zernograd, Rostov Region

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