Electronic crossover amplifier, Electronic Crossover
Keywords: production of electronic crossover amplifiers
Since digital technology, the sound quality of the audio and input system has been greatly improved, and the preamplifier becomes almost just the simple thing of the source selection switch and the volume potentiometer. However, in contrast, the output system is similar to the simulation era, and the reason is mainly due to the principle of the speaker. Since the audio range is wide to nine to ten times, it is difficult to vibrate completely in accordance with the electrical signal vibration in such a wide frequency range in such a wide frequency range. Then require a linear acoustic radioscope, which is almost impossible. A solution is to divide the audio range into several sections, and then use only a number of speakers to show a speech, which is a multi-speaker system, which is common for two-unit and three unit systems. But the divided frequency band requires a crossover network. Insert generally L, C filter between a power amplifier and a speaker. Since the speaker is not a pure resistance component, it is difficult to give the crossover design, which is not easy to get good performance; and the high quality divider requires the use of high quality inductors and capacitors, the price is not. In addition, since the efficiency of various speakers is different (the treble speaker is less than 6 decibels than the low-wave speaker), in order to balance the sound pressure of the entire frequency band, the attenuator needs to be inserted into the frequency divider to reduce the level of high efficiency speakers. It is a combination of several minimum efficiency speakers throughout the speaker system.
In order to change this, a multi-channel amplifier method is generated. Split the band with an active filter after the preamplifier, each frequency band has its own power amplifier and speaker, and the level of each frequency band is adjusted by potentiometers prior to each power amplifier. The advantage of this way is obvious, it cancels the foregoing LC network, and can effectively utilize the efficiency of each speaker; while also reduced the frequency requirements of the power amplifier, the output power can also be small; this structure is shown figure 1. Its critical circuit is an active filter.
The filter is low-pass, a high pass, a bandpass filter, and a strip-resistant filter. The low-pass filter allows the component from zero-frequency to its cutoff frequency, and blocks higher than the cutoff frequency; the high pass filter prevents the component below its cutoff frequency, and allows the component to pass; band pass filter It is allowed to pass the frequency component between its low cutoff frequency and the high cutoff frequency, and prevent all of the frequency components outside of this frequency range.
An active filter using an operational amplifier can cancel the inductor element. And you can get the voltage or current gain. According to the filter cutoff characteristics, it can be divided into Bezier, Libibi Snow and Badworth type. The characteristic curve is shown in Figure 2, mainly in the vicinity of the cutoff frequency, and the Bessel declines slowly. Steep, and the Badworth type is between the two. The cutoff characteristic usually uses a 1x frequency attenuation amount to indicate that the second-order filter is 12 decibels, and the third-order filter is 18 decibels.
Figure 3 is a standard Badworth second-order active filter. Figure 3A is a low pass filter, which calculates the formula as follows:
C = 1 / 2πf r C2 / C1 = 4Q ^ 2
C ^ 2 = C1 × C2 Q = 0.71
Figure 3b is a high pass filter, which calculates the formula as follows:
Rc = 1 / 2πf C R2 / R1 = 1 / 4Q ^ 2
R ^ 2 = R1 × R2 Q = O. 71
Design: low-pass filter with cutoff frequency f = 500 Hz. Select R = 18kΩ. but
C = 1/2 × 3.14 × 500 × 18 × 10 ^ (- 3) = 0.017684μF
C2 / C1 = 4 × (0.71) ^ 2 = 2.0164
C2 = 2.0164C1
(0.017684) ^ 2 = 20164C1 ^ 2
C1 = 0.01245μF = 12450pf. Actually select 12000PF and 470PF parallel.
C2 = 2.0164 × 12450PF = 251 10pF, actually select 22000PF and 2700PF parallel.
Design Example: High-pass filter with cutoff frequency f ≈5khz. Select R = 18KΩ. but
R2 = R1 / 2.0164 = 18kΩ / 2.0164 = 8.927kΩ
R = SQRT (R1 × R2) = 18 × 8.927 = 12.676kΩ
C = 1/2 × 3.14 x5000 × 12.676 × 10 ^ (- 3) = 0.002511μF = 2511PF
R1 actually select 18KΩ, R2 actually selects 9.1kΩ, C actually selects 2200 pf and 270pf parallel.
Figure 4 is a schematic diagram of an audio 12 decibel three-channel electronic divider. Select multi-channel prodrugate than diffuse sound quality after the power amplifier is divided. The frequency range of the three-channel frequency division is low-frequency ~ 500 Hz; medium frequency 500Hz ~ 5kHz; high frequency 5kHz ~. The frequency characteristics they synthesized are shown in Figure 5.
Its low frequency filter and high frequency filter are the front design example: intermediate frequency filters. Combined with a primary high pass filter and a level low pass filter, the calculation of R and C is the same as the design example. Here, the low-pass filter can be set after the high pass filter, and the residual noise can be reduced, and the buffer is provided before the filter facilitates the matching of the sound source, and the 1kΩ and 150pf of the input signal are used to limit the bandwidth of the input signal: each filter The output terminal is adjusted by the 10-turn line of the LKΩ.
The output signals of the three-way filter are connected to the same three power amplifiers, and their circuits are shown in Figure 6. First, the input stage is FET, which is a current buffer. The end-level power unit uses a high-frequency characteristic MOSFET, the bias circuit diode and the resistance, and the semi-variable resistor VR2 is used to set the quiescent current, and the quadrant measurement can be used. Measure the voltage of the source resistor (0.47Ω) when there is no signal, and then calculated using formula I = U / R. The final negative feedback from the source of the MOSFET to the amplifier end. Since the power supply as a driving voltage of the operational amplifier is not too high, limiting the maximum output of the amplifier. If the power supply voltage is ± 15V, the maximum output voltage of the driving level is ± 12V = 24V, the speaker impedance RL = 8Ω. The last level maximum output power P = Vcc × (Vcc / 8RL) = 24 × 24/64 = 9W. This power seems small, but in fact this is just a band of output power, plus another two bands of output power, is fully applicable.
Figure 6. Amplifier output terminal Rx, Cx and LY, RY is provided to stabilize the circuit operation. Since the speaker is not a pure resistance component, when the frequency is increased. The inductance component will become large, which is equivalent to the high frequency load, and the high frequency gain is improved, which may cause circuit oscillations; to add RX equivalent to high frequency load to avoid oscillation. When connecting amplifiers and speakers with a longer cable. Due to the presence of the cable capacitance, it will increase the high frequency load, so that the power amplifier is unstable; add LY, RY to avoid this. LY and RY are 10 homogenesis by a diameter of 1 mm enameled copper wire in a 10 Ω5 W carbon film resistor.
In order to protect the speaker, the fuse of the person 2A is required at the output of each amplifier. In the high frequency channel, but also between the amplifier and speaker 2.5μF capacitor in series with the polypropylene, to protect the tweeter.
As long as the resistance of each of the channel filters, digital capacitance accurately, generally do not need to debug. Adjusting the power amplifier: adjusting VR1 when no signal is input so that the output voltage is 0V, then adjust VR2 voltage across the source resistor 0.47Ω level is 0.1V (approximately 200mA) to.
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