Multiband active speaker system. The art of audio

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Multi-way active speaker system

The art of audio

Multi-way loudspeakers provide high-quality sound because each speaker is specifically designed to reproduce a particular frequency band and is optimized accordingly. Most often, in multi-band acoustic systems, the sound spectrum is divided into two or three bands.

To ensure a horizontal resulting frequency response, the frequency bands reproduced by each speaker must overlap smoothly, complementing each other. The mismatch between the sound pressure levels in the bands and the expansion of the zone of joint action of the speakers lead to distortion of the frequency response. Therefore, for the correct choice, it is important to know the dependence of sound pressure on the crossover frequency between the bands (Fig. 1). The upper curve corresponds to pink noise, the lower curve corresponds to modern music:

Multi-way active speaker system
Ris.1

For example, for a three-way system with a power of 100 W with crossover frequencies of 400 Hz and 3 kHz, the power distribution is as follows (with the same speaker sensitivity):

LF channel - 50 W;
MF channel - 35 W;
RF channel - 15.

Both passive and active filters can be used for band separation, but currently active filters are much cheaper than high-quality passive filters that use coreless inductors and non-electrolytic capacitors. In addition, active filters do not have the main disadvantages of passive ones:

power loss;
an increase in output impedance (from the side of the speaker) and the associated deterioration in damping;
complexity of calculation and adjustment due to the frequency dependence of the speaker impedance, the need to use Zobel-Busche compensators.

However, active filters can only be used with separate amplifiers for each frequency band, and it is convenient to use monolithic integrated amplifiers. In some cases, complex filters are not required, and to separate the bands, it is sufficient to use the simplest RC chains with a frequency response slope of 6 dB/octave. Excellent results are achieved due to the fact that such a filter is free from phase and transient distortion. However, the low attenuation of the simplest RC filters requires the use of speakers that can operate without distortion and outside the filter's bandwidth.

Multi-way active speaker system
Ris.2

An elegant and no less effective solution - a filtering amplifier (Power Filter) - was proposed by SGS-THOMSON. The proposed circuit combines a power amplifier and a second (12 dB/octave) or third order (18 dB/octave) filter. The operation of the circuit is based on the fact that there are two identical common-mode voltages at the signal input and the feedback input of the amplifier, which is required for the operation of the active filter. The resistance from the OOS input side is usually about 100 ohms, from the signal side it is very high, which also contributes to the correct operation of the circuit. Schematically, they are similar to Sallen-Key filters. Figure 2 shows a diagram of an RF filtering amplifier with a cutoff frequency of 900 Hz, which implements a 3rd order Bessel filter.

Figure 3 shows a diagram of a three-way active acoustic system built according to the proposed principle. 2nd order Butterworth filters with crossover frequencies of 300 Hz and 3 kHz were used. The midrange section consists of two high-pass filters (R10R11C10C11) and low-pass filters (R12R13C12C13) connected in series. In the low-frequency link, indirect current unloading is used. The buildup signal of the output transistors is taken from the resistors in the power supply circuit of the amplifier. With a supply voltage of 36 V, the output power of the LF channel is 25 W at THD=0,06% and 30 W at THD=0.5%.

Multi-way active speaker system
Ris.3

The gain of the midrange and high-frequency channels is chosen in accordance with the sensitivity and impedance of common dynamic heads (the sensitivity of midrange and high-frequency heads is usually 3...4 dB higher than low-frequency ones). If necessary, you can adjust the sensitivity of the bandpass amplifiers by adjusting the OOS circuits (R6, R15 and R22). To prevent self-excitation, the gain should not be set below 20 dB, i.e. the resistance of these resistors should not be less than 1 kOhm. As the practice of repeating this circuit has shown, the resistance of the current sensor resistors R7 and R8 can be increased to 2,2 ohms. As a result, due to the redistribution of power towards transistors, the heating of the microcircuit is somewhat reduced at high signal levels.

Electrolytic capacitors must have an operating voltage of at least 50V. The power of compensating circuit resistors should be 2 W. Protective diodes VD1-VD6 - any silicon ones with a permissible reverse voltage of at least 50V and a direct current of at least 1A, for example KD243. The output transistors VT1 and VT2 can be replaced with a traditional complementary pair of KT816/817 or KT818/819. You can also use a more modern pair of KT864/865. Transistors must be with the same letter index. Instead of tdA2030A, you can use a functional analogue of domestic production - K174UN19A (the harmonic coefficient will increase to 0.1 ... 0.5%). When using this microcircuit, to increase the reliability, the supply voltage should be reduced to 30 ... 32 V, which will practically not affect the output power. During installation, it must be taken into account that the microcircuit case is connected to pin 3.

To further improve the sound quality, it is worth moving away from high-capacity oxide coupling capacitors at the output of the amplifier and switching to a bipolar power supply. A variant of the circuit for this case is shown in the following figure. Capacitors C1, C4, C14, C21 are better to use non-polar. The remaining recommendations for replacement of parts and installation remain valid.

Multi-way active speaker system
Ris.4

There are two main design options. In the first version, bandpass amplifiers are built into the speakers and a separate preamplifier is used. With an input speaker impedance of only 600 ohms, the characteristics of the connecting cable will not affect the signal. But the preamplifier is required with a sufficiently powerful output that can drive a load with a resistance of 600 ohms. It can be performed on an op-amp with a "parallel" output stage or on a powerful op-amp K157UD1.

Any shielded cable or even a twisted pair cable without a shield is suitable for connecting an active speaker in this embodiment, if the length is up to 2-3 m. You should not only lay signal and power cables side by side and in parallel.

The second option uses passive speakers and a full three-way amplifier. The disadvantage is that each speaker will have to run three pairs of wires. This option can increase the input impedances of bandpass amplifiers up to 10 kΩ, which will allow the use of common preamplifier circuits.

Unstabilized supply voltage for the bipolar version + -18 volts at a load current of at least 2A (per channel). The transformer should give a voltage of 2x16.5 volts (winding with a tap from the middle). Rectifier filter - at least 2x22000 microfarads with a common power supply for all amplifiers and 2x10000 microfarads - with separate ones for each channel.

You can install a separate PSU in each speaker or use a common power supply and separate the DC voltage. This option is also suitable, but the filter capacitances will have to be divided into two parts and one of them installed in the AC to eliminate the influence of the resistance of the power wires.

Publication: www.bluesmobile.com/shikhman

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