Subwoofer compensation circuit based on acoustic feedback

Abstract

The utility model discloses a kind of heavy bass compensation circuit based on acoustic feedback, it includes microphone MIC, RC passive high-pass filter, buffer, first-order inverting low-pass active filter, power amplifier, loudspeaker SP, Butterworth high-pass filter, +15V is sequentially through R9, R8, MIC connects ground, R8 is connected in parallel with C10, the non-ground end of MIC is sequentially through C9, the resistance body of P1 connects ground, the output end of IC2b is connected with inverting input end short circuit and constitutes buffer, IC2a, R4, R5, R6, C8 constitute first-order inverting low-pass active filter, signal Ui is sequentially through C1, C2, C3 connects the non-phase end of IC1, the output end of IC1 is connected with its inverting end short circuit, the output of IC1 is through R4 and connects the inverting end of IC2a, the output of IC2b is through R7 and connects the inverting end of IC2a, the output of IC2a is driven SP sound emission by power amplifier, acoustic wave signal is received by microphone MIC, constitutes negative feedback loop.

Description

Technical Field

[0001] This utility model relates to a bass compensation circuit based on acoustic feedback. This compensation circuit can achieve good bass reproduction without expensive large speakers. Instead, it adds acoustic feedback after an active filter to enhance the bass, thus achieving good bass reproduction with inexpensive subwoofers. This is a further innovation in the design of super bass circuits. Background Technology

[0002] The sound reproduction range of an audio system can generally be divided into eight ranges: sub-bass, bass, mid-bass, mid-range, mid-high, second-high, high, and extra-high. The bass frequency range of an audio system is between 30 Hz and 100 Hz, and the treble frequency range is between 5000 Hz and 20000 Hz.

[0003] Mid-bass speakers and subwoofers differ significantly in their sound production principles. The former mainly relies on the compression of the diaphragm and the diaphragm to produce sound, while the latter generates low-frequency sound through the resonance of the diaphragm.

[0004] In electronic music, "subwoofer" is a term used to describe a lower frequency band with a longer wavelength. In English, it means "super bass," referring to bass frequencies with heat and energy. As the name suggests, subwoofers enhance the low-frequency range, boosting the longer wavelengths in an audio signal. Because ordinary speakers have limited size and cannot produce longer wavelengths of low-frequency sound, a larger subwoofer is needed to create a powerful low-frequency effect.

[0005] In addition to developing high-performance loudspeaker playback systems, the realization of subwoofer in an audio system also requires the design of high-performance subwoofer circuits. Although the forms of subwoofer circuits may differ, their common feature is to boost and amplify the frequency components of the audio signal from 30 to 100 Hz to drive the subwoofer loudspeaker, thereby obtaining good sound quality. At present, subwoofer circuits can be basically divided into two categories: frequency compensation type and bandpass filter type.

[0006] These two types of subwoofer circuits are mainly located in the preamplifier stage of the power amplifier. The subwoofer circuit reduces the high frequencies in the audio and boosts the low frequencies (sound waves below 100Hz, including infrasound, which are inaudible to humans but still have an effect). Because the human ear is not sensitive to low frequencies, a stronger bass is needed to produce a subwoofer effect.

[0007] Both of these types of subwoofer circuit designs are relatively complex and not cost-effective. A subwoofer compensation circuit based on acoustic feedback can be used instead. This circuit does not use expensive large speakers to achieve good bass reproduction, but adds acoustic feedback after the active filter to enhance the bass. Thus, a good bass reproduction effect can be obtained with an inexpensive subwoofer, which is more cost-effective. This is a further innovation in subwoofer circuit design. Summary of the Invention

[0008] The technical problem to be solved by this utility model is to provide a technology that is simple in structure, low in cost, reliable in use, and can achieve good bass reproduction effect using inexpensive small speakers.

[0009] To achieve the above objectives, this utility model provides a bass compensation circuit based on acoustic feedback, which includes an electret microphone circuit, an RC passive high-pass filter circuit, a buffer circuit, a first-order inverting low-pass active filter circuit, an external power amplifier circuit, a subwoofer circuit, and a VCVS third-order Butterworth high-pass filter circuit. Resistor R9, capacitor C10, capacitor C11, resistor R8, and microphone MIC constitute the electret microphone circuit. The +15V power supply is connected sequentially through resistor R9, resistor R8, and microphone MIC. Connecting to ground, resistor R8 and capacitor C10 are connected in parallel. Capacitor C9 and potentiometer P1 constitute the RC passive high-pass filter circuit. The non-ground terminal of the microphone (MIC) is connected to ground in sequence through the resistor body of capacitor C9 and potentiometer P1. The output terminal of operational amplifier IC2b and its inverting input terminal are shorted to form the buffer circuit. Operational amplifier IC2a, resistor R4, resistor R5, resistor R6, and capacitor C8 constitute the first-order inverting low-pass active filter circuit. Operational amplifier IC1, capacitor C1, capacitor C2, and capacitor C3... Resistors R1, R2, and R3 constitute the VCVS third-order Butterworth high-pass filter circuit. The audio signal Ui is connected sequentially to the non-inverting input of operational amplifier IC1 through capacitors C1, C2, and C3. The output of operational amplifier IC1 is shorted to its inverting input. Simultaneously, the output of operational amplifier IC1 is connected to the junction of capacitors C2 and C3 through resistor R2. The junction of capacitors C1 and C2 is connected to ground through resistor R1. The non-inverting input of operational amplifier IC1 is connected to ground through resistor R3. The output of operational amplifier IC1 is connected to the inverting input of operational amplifier IC2a ​​through resistor R4. The output of operational amplifier IC2b is connected to... Resistor R7 is connected to the inverting input of operational amplifier IC2a. The low-frequency signal at the output of operational amplifier IC2a ​​is amplified by the external power amplifier circuit and drives the subwoofer SP of the subwoofer circuit to produce sound. The sound wave signal emitted by the subwoofer SP is picked up by the electret microphone MIC of the electret microphone circuit. The electret microphone MIC, high-pass filter circuit, buffer circuit, first-order inverting low-pass active filter circuit, external power amplifier circuit, and subwoofer circuit form a negative feedback loop through acoustic feedback. The input signal Ui of this circuit is compared with the feedback signal generated by the subwoofer to compensate for the bass. If the bass is insufficient, the gain of the low-frequency signal is enhanced through feedback; otherwise, the operation is reversed.

[0010] In the first-order inverting low-pass active filter circuit, the output terminal of operational amplifier IC2a ​​is connected to the inverting input terminal of IC2a ​​through resistor R5, and resistor R6 and capacitor C8 are connected in series and then in parallel with resistor R5.

[0011] The RC passive high-pass filter circuit adjusts the parameters of capacitor C9 and the sliding terminal parameter of potentiometer P1 to keep the cutoff frequency of the high-pass filter below 20Hz.

[0012] The compensation frequency of the electret microphone circuit is determined by resistor R8, potentiometer P1, and capacitor C10, and their time constants should be equal to the time constants of resistor R6 and capacitor C8 of the first-order inverting low-pass active filter circuit. Attached Figure Description

[0013] Appendix Figure 1 Appendix Figure 2 Appendix Figure 3 Appendix Figure 4 This document is provided to provide a further understanding of the invention and forms part of this application. Figure 1 This is the schematic diagram of a first-order low-pass filter circuit with inverting input; attached. Figure 2 It shows the amplitude-frequency characteristics of a first-order low-pass filter circuit with inverting input; (Attached) Figure 3 This is a circuit diagram of a second-order Butterworth high-pass filter; attached. Figure 4 This is a schematic diagram of a bass compensation circuit based on acoustic feedback. Detailed Implementation

[0014] The embodiments of this utility model are further described below with reference to the accompanying drawings.

[0015] First-order inverting low-pass filter circuit This design will utilize a first-order inverting low-pass filter circuit and a third-order Butterworth high-pass filter circuit. Therefore, we will first briefly introduce the working principles of both. The first-order inverting low-pass filter circuit is as follows: Figure 1 As shown.

[0016] Figure 1 In the circuit, operational amplifier A, input resistor R1, negative feedback capacitor C, and compensation resistor R3 constitute a standard inverting integrator circuit. The input signal Ui(s) passes through resistor R1 and then through feedback to capacitor C. However, it is assumed that the initial charge of the capacitor is zero at this time, so the capacitor is charged. If the time constant R1C is large enough, the voltage on capacitor C can only rise slowly when an external voltage is applied. That is, the output voltage Uo(s) is approximately proportional to the time integral value of the input voltage Ui(s).

[0017] The integrating circuit has a low-pass characteristic, but when the frequency approaches zero, the voltage amplification factor approaches infinity, and its amplitude-frequency characteristic is as follows: Figure 2 As shown by the dashed line.

[0018] As the preceding analysis shows, the passband amplification factor is determined by the negative feedback network composed of resistors. Therefore, by connecting a negative feedback resistor R2 in parallel with capacitor C in the integrating circuit and setting the signal frequency to zero, the passband amplification factor can be obtained.

[0019] circuit transfer function

[0020] Replace s with jω, let The voltage amplification factor is obtained.

[0021] passband cutoff frequency f p = f 0, amplitude-frequency characteristics as follows Figure 2 As shown by the solid line in the image.

[0022] Butterworth high-pass filter circuit A high-pass filter, also known as a low-cutoff filter or low-impedance filter, is a filter that allows frequencies above a certain cutoff frequency to pass through while significantly attenuating lower frequencies. It removes unnecessary low-frequency components or low-frequency interference from the signal.

[0023] A high-pass filter is a combination of capacitors, inductors, and resistors that allows signal components above a certain frequency to pass through while significantly suppressing signal components below that frequency. Its characteristics can be described in the time domain and frequency domain using impulse response and frequency response, respectively.

[0024] Butterworth filters are a type of electronic filter. A key characteristic of Butterworth filters is their remarkably flat frequency response curve within the passband, exhibiting minimal fluctuations, while gradually decreasing to zero in the stopband. On a Bode plot of the logarithm of amplitude versus angular frequency, starting from a certain boundary angular frequency, the amplitude gradually decreases with increasing angular frequency, tending towards negative infinity. Figure 3 This is a second-order Butterworth high-pass filter circuit.

[0025] Figure 3 This is a second-order Butterworth filter circuit using a general-purpose operational amplifier 741 or 351. The center frequency is...

[0026] Gain is

[0027] This design will employ a third-order Butterworth active high-pass filter, the principle of which is... Figure 3 Similarly, in this design, since most speakers cannot reproduce lower frequencies, a third-order Butterworth active high-pass filter with a cutoff frequency of 37Hz is used to filter out the input signal.

[0028] Bass compensation circuit based on acoustic feedback This type of bass compensation circuit based on acoustic feedback, such as Figure 4 As shown, the circuit includes an electret microphone circuit, a high-pass filter circuit, a buffer circuit, a first-order inverting low-pass active filter circuit, an external power amplifier circuit, a woofer circuit, and a third-order Butterworth high-pass filter circuit (VCVS).

[0029] The electret microphone (MIC), high-pass filter circuit, buffer circuit, first-order inverting low-pass active filter circuit, external power amplifier circuit, and woofer circuit form a negative feedback loop through acoustic feedback. The input signal Ui of this circuit is compared with the feedback signal generated by the woofer to compensate for the bass. If the bass is insufficient, the gain of the low-frequency signal is enhanced through feedback; otherwise, the operation is reversed.

[0030] The speaker feedback signal is buffered and amplified by a voltage follower. The electret microphone MIC1 is placed very close to the woofer SP. It can monitor the movement of the woofer cone. The +15V power supply provides DC bias to the MIC through resistor R8. The monitoring signal is connected to the working ground through the DC blocking capacitor C9 and potentiometer P1. The capacitor C9 and potentiometer P1 form a high-pass filter to filter out high-frequency signals to ground. The sliding end of the potentiometer is connected to the non-inverting input of the operational amplifier IC2b.

[0031] The output of op-amp IC2b is shorted to the inverting input to form a standard voltage follower. A voltage follower (also called a buffer) does not amplify or invert the input signal, but provides isolation between two circuits. It has a high input impedance and a low output impedance, avoiding any load effect within the circuit. When the output is directly connected back to one of the inputs, the total gain of the buffer is +1.

[0032] Third-order Butterworth active high-pass filter and Figure 3 The circuit diagram is similar to that of a second-order Butterworth high-pass active filter. The high-pass capacitors C1, C2, and C3, the positive feedback resistor R2, the current-limiting resistors R1 and R3, and the operational amplifier IC1 constitute a third-order Butterworth high-pass filter.

[0033] The input signal of the third-order Butterworth high-pass active filter circuit is the audio signal U to be amplified. i The output terminal of op-amp IC1 is shorted to the inverting input terminal, which also forms a voltage follower with a voltage gain of "1" and acts as a buffer.

[0034] This design is based on an acoustic feedback-based bass compensation circuit, capable of handling subwoofer frequencies down to 20Hz. i The audio signal is amplified by about 20dB, but since most speakers cannot reproduce such low frequencies, a third-order Butterworth active high-pass filter with a cutoff frequency of 37Hz is formed using IC1 to filter the input signal, so as to prevent the speakers from receiving low-frequency signals that cannot be reproduced. The cutoff frequency of the high-pass filter can be changed by changing the values ​​of capacitors C1, C2, and C3.

[0035] First-order low-pass active filter circuit Resistors R4 and R7, op-amp IC2a, resistors R5 and R6, and capacitor C8 constitute an inverting input first-order low-pass filter circuit, which, together with... Figure 1 Similarly, the working principle is also similar.

[0036] The output signal U of the third-order Butterworth high-pass filter is... o1 The voltage drops through resistor R4 and enters the inverting input of operational amplifier IC2a. The output signal U of the voltage follower IC2b is... O2 After passing through the step-down resistor R7, the voltage also enters the inverting input terminal of the operational amplifier IC2a, U o1 with U o2 This is an operation for summing in reverse phase.

[0037] After the op-amp circuit IC2a ​​outputs voltage, the feedback current flows to capacitor C8. If the initial charge of the capacitor is zero, then capacitor C8 is charged at this time. If the time constant R6C8 is large enough, when an external voltage is applied, the voltage on capacitor C8 can only rise slowly. That is, the output voltage of op-amp IC2a ​​is approximately proportional to the time integral value of the input voltage.

[0038] The function of resistor R5 is to Figure 1 The resistor R2 is the same. The passband gain of this low-pass active filter is determined by the negative feedback network composed of resistors. Therefore, a negative feedback resistor R5 is connected in parallel with capacitor C8 in the integrating circuit. The calculation process is the same as... Figure 1 Similarly, I will not go into details.

[0039] To summarize the function of the first-order low-pass active filter circuit, it can transform the input audio signal Ui to be amplified in this circuit into the output signal U of the third-order Butterworth active high-pass filter. o1 Feedback signal U generated by the speaker o2 By making comparisons, the bass frequencies are compensated.

[0040] Within the operating frequency range of the circuit, the gain of IC2 is 20dB. When the frequency exceeds 300Hz, the gain of IC2 drops to 0dB. It compensates for bass frequencies below the cutoff frequency and acts as a buffer for microphone signals.

[0041] Debugging and Precautions Potentiometer P1 is used to adjust the magnitude of the feedback signal according to the rated output power of the external power amplifier and the microphone efficiency. If the feedback signal is adjusted too high, the frequencies above the cutoff point will also be compensated; if it is adjusted too low, it will have almost no effect, and the 20~300Hz signal will increase with the standard first-order characteristic.

[0042] Experiments show that when the distance between the microphone and the woofer cone is about 10mm, the correct feedback frequency can reach 500Hz. In order to make the compensation absolutely correct, this design sets the high-end compensation frequency at 300Hz. Beyond this frequency, the compensation effect gradually weakens (but it still has a compensating effect on the phase characteristics of the speaker).

[0043] If the crossover frequency of the speaker crossover filter is at 300Hz, it is best to change the values ​​of R6 and C8 so that the low-pass cutoff frequency of this circuit is lower than 300Hz.

[0044] Microphones are best selected through experimentation, especially when using high-power amplifiers. Practice has shown that the microphone used in the prototype can work normally with low-power systems with lower efficiency. If other models of microphones are used, the values ​​of bias resistors R8 and R9 must be appropriately changed so that the voltage drop across the microphone is about half of the power supply voltage.

[0045] In addition, the cutoff frequency of the high-pass filters C9 and P1 should be kept below 20Hz. Therefore, it should be noted that when there is no signal on potentiometer P1, the amplification factor of the negative feedback circuit will increase. The compensation frequency of the microphone is determined by resistor R8, potentiometer P1, and capacitor C10. Their time constants should be equal to the time constants of resistor R6 and capacitor C8 of the first-order active low-pass filter.

[0046] This circuit is suitable for speakers equipped with an active crossover. When using it, the speaker phase should be shifted by 180 degrees. . °, to avoid positive feedback. If this requirement cannot be met, an inverting buffer can be added before socket K2.

[0047] So, what is the phase of a loudspeaker? When an audio signal is transmitted to a loudspeaker, it produces sound by the vibration of a cardboard box or diaphragm. When the cardboard box vibrates, it may extend forward or retract backward. This is the phase of the loudspeaker when it vibrates, and it is a physical quantity that reflects the state of the alternating current at any given moment.

[0048] Microphones also have phase. If multiple microphones are used simultaneously, and one microphone is out of phase, it will cause an imbalance in the overall sound pickup. Microphones that are close together will cancel out the signal. You can do an experiment: place two dynamic microphones with the same phase at 180° to each other to pick up sound. You will clearly feel that the sound pressure level is lower, or even no signal output.

[0049] Therefore, the phase-related issues in this circuit refer to the phase relationship between the speaker and the microphone. Methods to adjust the phase difference include using a phase inversion switch or circuit, adjusting the crossover frequency, and changing the filter type.

[0050] This circuit consumes approximately ±6mA of power, with the microphone consuming only 0.6mA.

[0051] Finally, in the design concept of this device, the reproduction of subwoofers is based on two approaches: one is the use of a subwoofer speaker, and the other is the design of a high-performance subwoofer circuit. Since subwoofer speakers are too expensive, the innovation of this design lies in the novel design of the subwoofer circuit. The electret microphone (MIC), high-pass filter circuit, buffer circuit, first-order inverting low-pass active filter circuit, external power amplifier circuit, and subwoofer circuit are connected to form a negative feedback loop through acoustic feedback. If the bass is insufficient, the low-frequency signal gain is enhanced through feedback; otherwise, the operation is reversed.

[0052] The above embodiments are only used to illustrate and not limit the technical solutions of this utility model. Although the utility model has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the utility model without departing from the spirit and scope of the utility model. Any modifications or partial substitutions should be covered within the scope of the claims of this utility model.

Claims

1. A bass enhancement circuit based on acoustic feedback, characterized in that, The bass compensation circuit includes an electret microphone circuit, an RC passive high-pass filter circuit, a buffer circuit, a first-order inverting low-pass active filter circuit, an external power amplifier circuit, a woofer circuit, and a VCVS third-order Butterworth high-pass filter circuit. Resistor R9, capacitors C10 and C11, resistor R8, and microphone (MIC) constitute the electret microphone circuit. The +15V power supply is connected to ground sequentially through resistors R9, R8, and microphone (MIC). Resistor R8 is connected in parallel with capacitor C10. Capacitor C9 and potentiometer P1 constitute the RC passive high-pass filter circuit. The non-ground terminal of microphone (MIC) is connected to ground sequentially through the resistors of capacitor C9 and potentiometer P1. The output terminal of operational amplifier IC2b and its inverting input terminal are shorted to form the buffer circuit. Operational amplifier IC2a, resistors R4, R5, R6, and capacitor C8 constitute the first-order inverting low-pass active filter circuit. Operational amplifier IC1, capacitors C1, C2, and C3... Resistors R1, R2, and R3 constitute the VCVS third-order Butterworth high-pass filter circuit. The audio signal Ui is connected sequentially to the non-inverting input of operational amplifier IC1 through capacitors C1, C2, and C3. The output of operational amplifier IC1 is shorted to its inverting input. Simultaneously, the output of operational amplifier IC1 is connected to the junction of capacitors C2 and C3 through resistor R2. The junction of capacitors C1 and C2 is connected to ground through resistor R1. The non-inverting input of operational amplifier IC1 is connected to ground through resistor R3. The output of operational amplifier IC1 is connected to the inverting input of operational amplifier IC2a ​​through resistor R4. The output of operational amplifier IC2b is connected to... Resistor R7 is connected to the inverting input of operational amplifier IC2a. The low-frequency signal at the output of operational amplifier IC2a ​​is amplified by the external power amplifier circuit and drives the subwoofer SP of the subwoofer circuit to produce sound. The sound wave signal emitted by the subwoofer SP is picked up by the electret microphone MIC of the electret microphone circuit. The electret microphone MIC, high-pass filter circuit, buffer circuit, first-order inverting low-pass active filter circuit, external power amplifier circuit, and subwoofer circuit form a negative feedback loop through acoustic feedback. The input signal Ui of this circuit is compared with the feedback signal generated by the subwoofer to compensate for the bass. If the bass is insufficient, the gain of the low-frequency signal is enhanced through feedback; otherwise, the operation is reversed.

2. The bass compensation circuit based on acoustic feedback according to claim 1, characterized in that: In the first-order inverting low-pass active filter circuit, the output terminal of operational amplifier IC2a ​​is connected to the inverting input terminal of IC2a ​​through resistor R5, and resistor R6 and capacitor C8 are connected in series and then in parallel with resistor R5.

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