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When there are more speakers than channels...
Acoustics to the radio is usually connected according to the principle "one channel - one speaker", however, there are combined ways to connect the load. They use the features of building bridge amplifiers of modern radio tape recorders and allow you to achieve better sound quality. The proposed solutions do not require a serious investment of time and money and have been repeatedly tested in practice. For this, radios with bridge amplifiers of high power (above 20 watts per channel) are suitable. Models of radio tape recorders with a two- or four-channel low-power amplifier will not be considered - here "medicine is powerless." The sound quality of even an inexpensive radio can be significantly improved. In most small-sized coaxial or full-range front speakers, due to poor acoustic design, the low-frequency components of the signal cause overload and distortion already at an average volume level. Obviously, to eliminate this defect, it is necessary to apply HPF. Good results are achieved when using first-order filters with a cutoff frequency of about 90-180 Hz (depending on the characteristics of the head and acoustic design). Another solution is to move to a spaced (component) acoustics, in this case, mid-bass heads can be installed in a suitable acoustic design. In this case, it is convenient to switch to separate amplification (the so-called bi-amping), then it will be possible to adjust the tonal balance not only with the tone controls, but also with the appropriate power distribution of the amplifiers. When using a ready-made set of two-way speakers, it is easiest to use a regular crossover, separating the HPF and LPF inputs for connection to the front and rear channels, respectively. With a crossover frequency of 5 ... 7 kHz, which is typical when using inexpensive HF heads, radios with an unequal power distribution between the front and rear channels (usually 2x7 W front + 2x25 W rear) will perform best in this inclusion. For radio tape recorders with channels of equal power, the channels working for "tweeters" will use no more than a third of their capabilities. In this case, it makes sense to think about reducing the crossover frequency to 1,5 ... 3 kHz, but this requires high-order high-pass filters and special-design HF emitters with a low fundamental resonance frequency. Their cost is considerable, so a three-way front speaker in this case may turn out to be even cheaper, and it has much wider possibilities for setting up the scene. Available midrange heads (or broadband as midrange) with a "soft" diffuser up to 13 cm in diameter can be used only with a high-pass filter, without limiting the frequency band from above, since their frequency response in the operating frequency range is quite uniform and smoothly falls off at high frequencies. Heads of larger diameter usually have a significant frequency response unevenness, in this case, band-pass filters must be used for midrange radiators. Low-frequency heads of three-way systems at crossover frequencies above 500 ... 800 Hz can be turned on without a low-pass filter, using the natural frequency response decay and acoustic design features. This will reduce the losses on the active resistance of the filter coil. However, high-rigidity cone drivers often have several resonances in the midrange region, creating overtones. In this case, the LPF is needed. The diagrams below show the cutoff frequencies provided by the most common standard capacitor values. Dotted lines show optional elements. Depending on the structure of the radio amplifiers, the connection diagram changes. Let's look at them in order of seniority. Radio with two-channel amplifier and output fader. There are still quite a few such "dinosaurs" produced in the mid-90s in operation. A characteristic feature of the built-in amplifiers of such devices is that they are designed for a load of 2 ohms and have a solid power reserve. This is due to the fact that in the middle position of the fader, the front and rear speakers are connected in parallel. However, it is impossible to use component acoustics in conjunction with them. The reason is simple - when a fader is introduced, the output impedance of the amplifier increases, which leads to a change in the cutoff frequency of the crossovers. Therefore, the only possible use of the amplifiers of such devices with modern acoustics is a two-channel configuration, in which the fader is excluded from operation. In this option, you can add a subwoofer by turning it on according to the "mixed mono" scheme. Since it is impossible to adjust the ratio of the levels of the front speaker and the subwoofer, one of the connection options is selected depending on the sensitivity of the used heads (Fig. 1.).
Option А used when the sensitivity of the front speakers is 3-5 dB higher than that of the subwoofer. In this version, they are connected to the amplifiers not via a bridge, but according to the usual scheme, and the power supplied to them does not exceed 7 W / 4 Ohms. To obtain sufficient volume of front acoustics, it is desirable to use high-performance heads with a sensitivity of about 91-93 dB. Pay attention to their phasing - one of the "semiamplifiers" is inverting, the second is non-inverting. The cutoff frequency of the HPF formed by capacitors C1 and C2 can be chosen arbitrarily, however, the presence of capacitors is fundamentally important - they do not pass the constant component from the output of the amplifiers to the speakers. Front heads are conventionally shown as wideband, but they can be coaxial or component. Option Б used when the sensitivity of all heads is close. The presence of capacitors C1,C2 is no longer necessary for normal operation, since a bridge connection is used. In the absence of capacitors, the heads in the passband of the subwoofer filter operate in parallel. This reduces the load impedance to 2 ohms, but the amplifier is designed for this. For the subwoofer, a second order filter (C3L1R1) is used in both cases. Radio with combined amplifier. The amplifier of these radio tape recorders can be used in two versions - as a two-channel bridge with a maximum power of 2x25 W or as a four-channel with a conventional load connection and a "virtual ground". The power in this case is 4x7W. The most famous representative of this family is Sony 1253/1853. To work with a subwoofer, you must also use a two-channel configuration.
The choice of switching circuit, as in the previous case, is made taking into account the sensitivity of the heads. For option А capacitors C1, C2 play the role of HPF. They can be abandoned if the used heads provide undistorted reproduction of low frequencies. The separation of the DC component is carried out by the internal capacitors of the radio. However, for the option Б it is obligatory to have two high-pass filters for stereo channels and a low-pass filter for the sum channel. Their task is to exclude the parallel operation of the heads at low frequencies. Because the drivers have almost equal DC impedance at low frequencies, the absence of filters can lead to an overload of an amplifier designed for a 4 ohm load. For stereo channels, filters of the first order (C1, C2) are used, for the total - of the second (C3L1R1). Another variant of mixed load connection is also possible, but already within the same amplifying channel. It is convenient for connecting a two- or three-way speaker system. For the woofer, a bridge connection is used, and for the midrange / treble or treble - the usual one. The mode switch is set to the position for a four-channel configuration.
The dependence of the power supplied to the heads on the position of the fader is shown in Fig. 3.b. In the process of regulation, the power on the bridge load is reduced by a maximum of 6 dB (4 times), since in the extreme positions of the fader the circuit is reduced to the usual one ("the amplifier arm left without a signal" acts as a "virtual ground"). It should be taken into account that in the zone of joint action of the heads they are connected in parallel, but since these frequencies are already affected by the increase in the load impedance due to the inductance of the voice coil, there is no real overload of the amplifier. Tape recorders with amplifiers of different power. According to this scheme, a number of inexpensive devices are still produced: TCH-77 CD receiver (LG Electronics), Daewoo AKF-4087X, AKF-4237X, AKF-4377X, AKF-8017X, Prology KX-2000R, ARX-9751/52 radios , "Ural" (models 206, 207, 208). Powerful amplifiers of models with channels of different power are made according to the bridge circuit, and less powerful ones - according to the usual one. In addition to the mentioned bi-amping option, you can use a traditional or mixed connection. frontal heads to more powerful rear channels, and rear AC ("sub-sound"), which does not require high power, connect to frontal channels according to the conventional scheme or the Haffler scheme (with a difference signal). The front-rear fader functions are reversed, but it's easy to get used to. All that has been said above regarding the combined connection remains valid, only the ability to adjust the power ratio across the bands in the front channel will disappear. Similarly to the options considered, in this case, according to the combined scheme, you can turn on the front speakers and the subwoofer, while the fader controls the signal level of the tweeters. The operating frequency ranges of the front speakers and subwoofer should not overlap, so as not to overload the amplifier.
Radio with four-channel bridge amplifiers. For four-channel bridge amplifiers, which are equipped with almost all modern radio tape recorders, the above options for switching on speakers can be combined in various ways. For example, using "mixed mono" and the usual connection scheme at the same time (Fig. 5), you can connect a subwoofer and "tweeters" or rear speakers (with bandwidth limitation), and use the remaining channels for midbass or front speakers.
Another option is a three-way front speaker with a combined connection and Huffler sub-sound. The fader allows you to adjust the ratio of MF and LF, the HF level in the process of regulation changes within 6 dB. The load connection order is chosen so that amplifier overload does not occur even when using low-frequency heads in the full frequency band, and midrange - only with a high-pass filter, which makes this circuit especially attractive - no need to wind inductors.
Recommendations Since the inverting and non-inverting outputs of the built-in amplifier are used, pay attention to the polarity of connecting the heads. In cases where, according to the scheme, it is required to connect one of the load terminals to the "ground", this must be done on the radio housing or on the "negative" power wire as close as possible to its housing. Connection to the "mass" of the car is not allowed! For circuits with a bridge option for connecting the load, it is necessary to use non-polar oxide (electrolytic) capacitors as part of the filters. In the extreme case, they can be made up of two polar capacitors with twice the capacity, connecting them back-to-back ("plus" to "plus"), but the sound quality with non-polar capacitors is higher. Capacitors of small capacity are better to use paper or polypropylene. Calculation of the low-pass filter for a subwoofer is performed in the usual way. However, it is desirable to introduce a 1 ohm resistor into the design of the second order filter, as shown in the diagrams. This will reduce the chance of overloading the radio amplifier. The crossover frequency and LPF order for the subwoofer are selected within the range of 80...200 Hz, depending on the location of the low-frequency head. If it is located in the rear of the passenger compartment, the crossover frequency should be chosen as low as possible, and the order as high as possible, to avoid the subwoofer reproducing the "voice" range. However, this requires the manufacture of truly cyclopean inductors. On the one hand, it is undesirable to use ferromagnetic cores in their design, since the distortions caused by the inevitable magnetization of the core significantly degrade the sound quality. On the other hand, the use of a core makes it possible to drastically reduce the active resistance of the coil and the losses in it, which is especially important when using the built-in amplifiers of the radio. So the choice must be made on a case-by-case basis. In a word, there are many options - it would be a fantasy. Publication: www.bluesmobil.com/shikhman
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