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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Three-channel multimedia UMZCH. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers Despite the many models of multimedia speakers of industrial production, the interest of radio amateurs in the independent manufacture of such structures does not decrease. Of particular interest are systems with a common low-frequency channel. The low-frequency loudspeaker - a subwoofer - is placed in a separate housing, which can significantly reduce the size of the speakers of the left and right channels. For such designs, the amplifier described here with a total rated power of about 20 watts is intended. The features of the multimedia complex include the relatively small size of the video monitor and the corresponding dimensions of the acoustic system, which is usually located in close proximity to the listener. In this regard, the maximum power of amplifiers for such speakers usually does not exceed 10 ... 20 watts. The proximity of a multimedia speaker system often limits its allowable size, so it is common to place a low-frequency head in one common housing - a subwoofer, and stereo speakers act here as "satellites". To form the signal of the bass channel (subwoofer), an adder and an active filter are usually used. As an example, in fig. 1 shows a diagram of this node.
On the DA1.1 op amp, an inverting adder is made, combined with a first-order filter, on the DA1.2 op amp, an active second-order Butterworth filter. The cutoff frequency of the resulting third order filter is approximately 180 Hz. Divider R1R2 sets the op-amp mode for direct current. The frequency band of the MF-HF loudspeakers (satellites) is limited by first-order filters at the input of the stereo UMZCH. However, it is not necessary to use active filters to isolate the subwoofer frequency band. Two versions of the UMZCH for a computer [1] were published on the pages of the magazine, in which an original method of generating a signal for a subwoofer was used, which does not require a separate filter. Unfortunately, in the first version of the design, two different versions of the TDA1519 chip were used, which are not always available for sale. In the second version - hopelessly outdated TDA2005, which do not meet modern requirements in terms of distortion and noise; this chip requires a lot of external elements. Using modern UMZCH microcircuits designed for automotive radio equipment, you can somewhat simplify the circuit and significantly improve the performance of the UMZCH. It is convenient to make a power amplifier based on the common TDA1554Q chip (Philips). It includes two inverting and two non-inverting amplifiers with a gain of 20 dB, their input impedance is 60 kOhm. There are two options for enabling them. The first is standard, like a four-channel UMZCH with a maximum output power of 4x6 W (4x11 W) at a load of 4 (2) ohms. The second option is as a two-channel bridged UMZCH with a maximum output power of 2x22 W at a load of 4 ohms. In the proposed design, two inverting channels are used in a conventional connection, and two non-inverting channels, thanks to an original solution, are used in a bridge connection.
The power supply voltage of the amplifier can be within +10 ... 16 V. The current consumed by the device in the absence of a signal is not more than 0,1 A. In the standby mode "Standby" (remote shutdown mode) - 0,1 mA. The rated power is specified at a supply voltage of 15 V and harmonic distortion of about 0,5%. The maximum power, as is customary, is determined at 10% distortion. The amplifier circuit is shown in fig. 2. The device is simplified as much as possible, and the denominations of most elements are unified.
Volume and tone are controlled by dual variable resistors VR1 and VR2, respectively. To avoid overloading the amplifier, the depth of tone control depends on the position of the volume control slider. At maximum volume, the rise in treble does not exceed 2...3 dB (and then - due to the blockage of LF and MF), but increases to 5...6 dB at low volume [2]. Adjustment of the treble tone to "blockage" is not provided, since, as practice shows, it is not necessary. In addition, most PC sound cards have software controlled tone and balance controls. If necessary, the tone control range in the amplifier can be increased to 12 ... 14 dB by setting a variable resistor VR2 with a resistance of 10 kOhm. It is also possible to install a stereo balance control (VR4), although its necessity is even more doubtful. For installation reasons, inverting amplification channels were used for the satellites, therefore, to preserve the original phase of the signal, the dynamic heads BA1, BA2 are connected in reverse polarity. The total signal for the subwoofer is formed on a separating capacitor C13 common for two channels, as in [1]. The cutoff frequency of this filter is 170...180 Hz. The capacitance of the capacitor C13 is indicated for dynamic heads with an impedance of 4 ohms. For heads with an impedance of 8 ohms, its capacitance must be reduced to 220 uF. In relation to satellite signals, the subwoofer signal is formed as an additional function, therefore, under certain conditions (more on this later), a "hump" at the crossover frequency of up to 3 dB may appear on the resulting frequency response. To eliminate this shortcoming, a tunable proportionally integrating filter VR3R1R2C3 was introduced into the subwoofer channel, the cutoff frequency of which varies in the range of 50 ... 150 Hz. When the frequency is tuned, the signal level also changes at the same time, which makes it possible to abandon the usual level control in the subwoofer channel. On fig. 3 shows the theoretical frequency response of filters by electrical voltage; the LF channel family has been shifted down by 6 dB for convenience.
To ensure the operation of two identical amplifiers in a bridge connection, it is necessary to apply antiphase signals to their inputs. This design uses a split load cascade. An inverted signal is taken from the collector of the transistor VT1, and a non-inverted signal from a part of the emitter load. The stage gain on both outputs is about -16 dB, so the input voltage of the LF channel in the maximum wide band mode is approximately 4 dB higher than in the satellite channels. This compensates for the difference in sensitivity between the full-range and low-frequency drivers and provides headroom for the level control in the subwoofer channel. In addition, this measure automatically eliminates the overload of the cascade on the VT1 transistor at the input: due to the difference in gain, the signal clipping at the output of the bridge amplifier will begin earlier than at conventional outputs (from where the signal for VT1 is taken). Due to the deep OOS through the resistors R4, R5, the linearity of the cascade is satisfactory even with large signals. The DC cascade mode is provided by connecting the VR3R1 circuit to the capacitor C13. This capacitor has a constant voltage, approximately equal to half the supply voltage. Another node, so far unusual for such amplifiers, is the phase switch of the SA2 subwoofer signal. However, in home theater systems and car subwoofers, such a cascade is sure to exist. The need for its use is caused by the following: with a spatially separated acoustic system, the resulting frequency response at the listening point will be determined by the phase ratio of the incoming signals. The phase shift, in turn, is determined by the distance to the dynamic heads. On fig. Figure 4 shows the theoretical frequency response in the near field of radiation for the cases of in-phase and anti-phase inclusion of heads with their compact installation. Real frequency response in terms of sound pressure, depending on the distances and characteristics of the heads, can take even more bizarre forms. Obviously, the introduction of a phase switch allows more flexible control of the resulting frequency response.
The SA1 power switch controls the state of the microcircuit, and through it the supply voltage is supplied to the phase splitter stage. In the off state, the outputs are switched to a high-impedance state, and the current consumption does not exceed 100 μA. The purpose of the rest of the details is obvious. The filter capacitance is divided into two parts, since it is difficult to install a large capacitor directly near the power pins of the microcircuit. About the details and design. Oxide capacitors K50-35 or similar imported ones, capacitors C1, C2, SU - ceramic of any type, the rest - K73-17. All fixed resistors are MLT0.125. The variable volume control resistor should be with an exponential dependence of the resistance on the angle of rotation (type B), the rest - with a linear one (type A). The KT315V transistor can be replaced by any npn transistor with a base current transfer coefficient of at least 50. The choice of other parts is not critical. The rectifier is made on KD213A pulse diodes, which will allow, if necessary, to significantly increase the filter capacitance without the risk of multiplicative interference. Any network transformer can be used with an overall power of at least 80 W (better more), a permissible secondary winding current of at least 5 A and an output voltage of 9 ... 11 V. Due to the relative simplicity of the amplifier, it is quite possible to assemble it on a breadboard (in this version, he worked with the author). For a magazine publication, a printed circuit board was designed (Fig. 5), which houses most of the parts. The board is designed for the installation of P2K switches in two directions and the details mentioned above. Capacitor C3 with a capacity of 0,15 uF, if necessary, can be made up of capacitors with a capacity of 0,1 uF and 0,047 uF, for which additional pads are provided on the board.
Variable resistors, connectors and a network transformer are placed outside the board. The jumpers in the signal circuit are made with a thin mounting wire; for mounting power circuits and acoustic systems, it is necessary to use a wire with a cross section of at least 0,75 mm2. The heat sink can be made from a 30x50 mm duralumin corner or used ready-made from a car radio (this is exactly what was used in the author's version). A properly assembled amplifier does not require adjustment. When switching on, it is enough to make sure that the voltages indicated on the diagram are present (permissible deviation ± 10%). In case of interference from the computer power supply, two ceramic capacitors with a capacity of 220 ... 470 pF should be turned on at the input of the microcircuit (between points 6, 7 of the board and the common wire). They can be placed on the side of the printed conductors. To reproduce low frequencies, you need to use a specialized low-frequency dynamic head in acoustic design. The easiest way is to use speakers from domestic household equipment, removing unnecessary details. The author used AC S-30B ("Radio Engineering") for testing. Acoustic design of satellites can be simple, including open. Literature
Author: A. Shikhatov, Moscow; Publication: radioradar.net
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