Microphone front-end circuit

Abstract

The utility model relates to a microphone front-end circuit, which comprises a signal input circuit, a first switch unit, a second switch unit and a signal output circuit, the signal input circuit receives audio input and power supply input of the microphone and is used for converting the audio input into a single-ended signal; the first end of the first switch unit is connected with power supply input, and the second end of the first switch unit is connected with the signal output circuit; the control end of the first switch unit is connected with the signal input circuit and is used for conducting when a single-ended signal is positive; the first end of the second switch unit is connected with the signal output circuit, and the second end of the first switch unit is grounded; the control end of the first switch unit is connected with the signal input circuit and is switched on when a single-ended signal is negative; the signal output circuit is connected with the second end of the first switch unit and the first end of the second switch unit, and is used for converting the received signal into a balance signal. According to the utility model, noise can be prevented from being generated in the sound processing process of the microphone, and the sound quality is improved.

Description

Technical Field

[0001] The utility model relates to the technical field of audio, and more specifically to a microphone preamplifier circuit. Background Art

[0002] Microphone is a common and important tool in voice pickup. In order to achieve better use effect of microphone, voice amplification circuit is usually set in the microphone. In the amplification circuit, the most basic small signal amplifier is based on triode, and various negative feedbacks are usually added to obtain good performance. At present, most microphones have problems such as insufficient sound restoration, low or high pitch, and even broken sound in some cases. Utility Model Content

[0003] The technical problem to be solved by the utility model is to provide a microphone preamplifier circuit in view of the above-mentioned partial technical defects of the prior art.

[0004] The technical solution adopted by the utility model to solve the technical problem is: constructing a microphone preamplifier circuit, including: a signal input circuit, a first switch unit, a second switch unit and a signal output circuit;

[0005] The signal input circuit receives an audio input and a power supply input of a microphone, and is used to convert the audio input into a single-ended signal;

[0006] The first end of the first switch unit is connected to the power supply input, and the second end of the first switch unit is connected to the signal output circuit; the control end of the first switch unit is connected to the signal input circuit, and is used to be turned on when the single-ended signal is positive;

[0007] The first end of the second switch unit is connected to the signal output circuit, and the second end of the first switch unit is grounded; the control end of the first switch unit is connected to the signal input circuit, and is used to be turned on when the single-ended signal is negative;

[0008] The signal output circuit is connected to the second end of the first switch unit and the first end of the second switch unit, and is used for converting the received signal into a balanced signal and outputting the balanced signal to the audio processing circuit.

[0009] Preferably, in an embodiment of the microphone preamplifier circuit of the present utility model, the signal input circuit includes a capacitor EC3, a capacitor EC4, a resistor R8, a resistor R9 and a resistor R10;

[0010] The control end of the first switch unit is connected to the first end of the capacitor EC3 and the first end of the resistor R8, and the control end of the second switch unit is connected to the first end of the capacitor EC4 and the second end of the resistor R9;

[0011] The second end of the capacitor EC3 and the second end of the capacitor EC4 are used to input the single-ended signal;

[0012] The second end of the resistor R8 is connected to the first end of the resistor R9 and the first end of the resistor R10 , and the second end of the resistor R10 is connected to the input end of the signal output circuit.

[0013] Preferably, in an embodiment of the microphone preamplifier circuit of the present invention,

[0014] The first switch unit includes a transistor Q2, a resistor R11 and a capacitor C10; the base of the transistor Q2 is connected to the first end of the capacitor EC3, the first end of the capacitor C10 and the first end of the resistor R11, the collector of the transistor Q2 and the second end of the resistor R11 are connected to the power supply input, and the emitter of the transistor Q2 and the second end of the capacitor C10 are connected to the input end of the signal output circuit; and / or

[0015] The second switch unit includes a transistor Q3, a resistor R12 and a capacitor C9; the base of the transistor Q3 is connected to the first end of the capacitor EC4, the first end of the capacitor C9 and the first end of the resistor R12, the emitter of the transistor Q3 and the second end of the capacitor C9 are connected to the input end of the signal output circuit, and the collector of the transistor Q3 and the second end of the resistor R12 are grounded.

[0016] Preferably, in an embodiment of the microphone preamplifier circuit of the present utility model, the signal input circuit further comprises a first common mode filter circuit, a balanced-to-unbalanced circuit and a frequency selection network;

[0017] The first input terminal and the second input terminal of the first common mode filter circuit are respectively connected to the audio input, the first output terminal of the first common mode filter circuit is connected to the power supply input and the input terminal of the frequency selection network, and the second output terminal of the first common mode filter circuit is connected to the control terminal of the balanced-to-unbalanced circuit;

[0018] The output end of the frequency selection network is connected to the first end of the capacitor EC3 and the first end of the capacitor EC4.

[0019] Preferably, in an embodiment of the microphone preamplifier circuit of the present utility model, the balanced-to-unbalanced circuit comprises a transistor Q1, a resistor R3, a resistor R2, a capacitor C5, and a capacitor EC2;

[0020] The base of the transistor Q1 is connected to the first end of the resistor R2, the first end of the resistor R3, the first end of the capacitor C5 and the first end of the capacitor EC2, the emitter of the transistor Q1, the second end of the capacitor C5 and the second end of the resistor R3 are connected to the input end of the frequency selection network; the emitter of the transistor Q1 and the second end of the resistor R2 are grounded, and the second end of the capacitor EC2 is connected to the second output end of the first common-mode filter circuit.

[0021] Preferably, in an embodiment of the microphone preamplifier circuit of the present utility model, the frequency selection network includes a low-pass filter and a high-pass filter;

[0022] The input end of the low-pass filter is connected to the input end of the high-pass filter and forms the input end of the frequency selection network;

[0023] The output end of the low-pass filter is connected to the output end of the high-pass filter and forms the output end of the frequency selection network.

[0024] Preferably, in an embodiment of the microphone preamplifier circuit of the present invention,

[0025] The low-pass filter comprises a resistor R5, a resistor R6 and a capacitor C19; the first end of the resistor R5 is the input end of the low-pass filter, the second end of the resistor R5 is connected to the first end of the capacitor C19 and the first end of the resistor R6, the second end of the capacitor C19 is grounded, and the second end of the resistor R6 is the output end of the low-pass filter; and / or

[0026] The high-pass filter includes a resistor R4, a resistor R7, a capacitor C6 and a capacitor C7; the first end of the resistor R4 is the input end of the high-pass filter, the second end of the resistor R4 is connected to the first end of the capacitor C6, the second end of the capacitor C6 is connected to the first end of the resistor R7 and the first end of the capacitor C7, the second end of the resistor R7 is grounded, and the second end of the capacitor C7 is the output end of the high-pass filter.

[0027] Preferably, in an embodiment of the microphone preamplifier circuit of the present utility model, the first common-mode filter circuit comprises a common-mode inductor L1, a capacitor C1, a capacitor C2, a capacitor C3, a capacitor C4, a capacitor EC1 and a resistor R1;

[0028] The first end of the common-mode inductor L1 is connected to the first end of the capacitor C1 and serves as the first input end of the first common-mode filter circuit;

[0029] The third end of the common-mode inductor L1 is connected to the first end of the capacitor C2 and serves as the second input end of the first common-mode filter circuit;

[0030] The second end of the common-mode inductor L1 is connected to the first end of the capacitor C3 and the first end of the resistor R1, and serves as the second output end of the first common-mode filter circuit;

[0031] The fourth end of the common-mode inductor L1 is connected to the first end of the capacitor C4 and the second end of the capacitor EC1, and the second end of the capacitor EC1 is connected to the second end of the resistor R1 and serves as the first output end of the first common-mode filter circuit;

[0032] The second end of the capacitor C1 , the second end of the capacitor C2 , the second end of the capacitor C3 , and the second end of the capacitor C4 are grounded.

[0033] Preferably, in an embodiment of the microphone preamplifier circuit of the present utility model, the signal output circuit comprises: a resistor R13, a capacitor EC6, an audio transformer T1 and a second common mode filter circuit;

[0034] The first end of the audio transformer T1 is connected to the first end of the capacitor EC6, the second end of the capacitor EC6 is connected to the first end of the resistor R13, and the second end of the resistor R13 is connected to the output end of the signal input circuit;

[0035] The second end of the audio transformer T1 is grounded, the third end of the audio transformer T1 is connected to the first input end of the second common-mode filter circuit, the fourth end of the audio transformer T1 is the power supply input, and the fifth end of the audio transformer T1 is connected to the second input end of the second common-mode filter circuit;

[0036] The first output end of the second common-mode filter circuit and the second output end of the second common-mode filter circuit are output ends of the signal output circuit.

[0037] Preferably, in an embodiment of the microphone preamplifier circuit of the present invention, the second common-mode filter circuit comprises a common-mode inductor L2, a capacitor C15, a capacitor C16, a capacitor C17 and a capacitor C18;

[0038] The first end of the common-mode inductor L2 is connected to the first end of the capacitor C16 and serves as the first input end of the second common-mode filter circuit;

[0039] The fourth end of the common-mode inductor L2 is connected to the first end of the capacitor C15 and serves as the second input end of the second common-mode filter circuit;

[0040] The second end of the common-mode inductor L2 is connected to the first end of the capacitor C17 and serves as the first output end of the second common-mode filter circuit;

[0041] The third end of the common-mode inductor L2 is connected to the first end of the capacitor C18 and serves as the second output end of the second common-mode filter circuit;

[0042] The second end of the capacitor C15, the second end of the capacitor C16, the second end of the capacitor C17 and the second end of the capacitor C18 are grounded.

[0043] The implementation of a microphone preamplifier circuit of the utility model has the following beneficial effects: it can avoid the generation of noise during the microphone sound processing process and improve the sound quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0045] Figure 1 It is a logic block diagram of an embodiment of a microphone preamplifier circuit of the utility model;

[0046] Figure 2 The utility model is a circuit principle diagram of an embodiment of a microphone preamplifier circuit. DETAILED DESCRIPTION

[0047] In order to have a clearer understanding of the technical features, purposes and effects of the present utility model, the specific implementation methods of the present utility model are now described in detail with reference to the accompanying drawings.

[0048] like Figure 1 As shown, a first embodiment of a microphone pre-circuit of the utility model is shown. Figure 1 In the first embodiment of a microphone preamplifier circuit of the utility model, the signal input circuit 110, a first switch unit 121, a second switch unit 122 and a signal output circuit 130 are shown; the signal input circuit 110 receives the audio input and power input of the microphone, and is used to convert the audio input into a single-ended signal; the first end of the first switch unit 121 is connected to the power input, and the second end of the first switch unit 121 is connected to the signal output circuit 130; the control end of the first switch unit 121 is connected to the signal input circuit 110, and is used to be turned on when the single-ended signal is a high level; the first end of the second switch unit 122 is connected to the signal output circuit 130, and the second end of the first switch unit 121 is grounded; the control end of the first switch unit 121 is connected to the signal input circuit 110, and is used to be turned on when the single-ended signal is a low level; the signal output circuit 130 is connected to the second end of the first switch unit 121 and the first end of the second switch unit 122, and is used to convert the received signal into a balanced signal and output it to the audio processing circuit.

[0049] Among them, the signal input circuit 110 is used to connect a microphone to obtain the audio input of the microphone. Among them, the audio input of the microphone is a balanced audio signal, and the signal input circuit 110 converts the received audio input into a single-ended signal. At the same time, the signal input circuit 110 is connected to the power supply input to provide direct current for the audio input. The first switch unit 121 and the second switch unit 122 form a push-pull circuit. When the single-ended signal output by the signal input circuit 110 is a positive half-cycle signal, the first switch unit 121 is turned on to output the audio signal to the audio output circuit. At the same time, when the first switch unit 121 is turned on, the signal output circuit 130 is connected to the power supply input to increase the positive signal anti-interference ability. When the audio signal output by the audio input circuit is a negative half-cycle signal, the second switch unit 122 is turned on to output the audio signal to the audio output circuit. At the same time, when the second switch unit 122 is turned on, the input of the signal output circuit 130 is grounded to increase the negative signal anti-interference ability. The signal output circuit 130 converts the received signal obtained from the first switch unit 121 and the second switch unit 122 into a balanced signal and inputs it to the corresponding audio processing circuit for audio processing. Only one of the first switch unit 121 and the second switch unit 122 is turned on at a time, which can reduce conduction loss.

[0050] like Figure 2 As shown, in one embodiment, the signal input circuit 110 includes a capacitor EC3, a capacitor EC4, a resistor R8, a resistor R9 and a resistor R10; the control end of the first switch unit 121 is connected to the first end of the capacitor EC3 and the first end of the resistor R8, and the control end of the second switch unit 122 is connected to the first end of the capacitor EC4 and the second end of the resistor R9; the second end of the capacitor EC3 and the second end of the capacitor EC4 are used to input a single-ended signal; the second end of the resistor R8 is connected to the first end of the resistor R9 and the first end of the resistor R10, and the second end of the resistor R10 is connected to the input end of the signal output circuit 130. That is, in the signal input circuit 110, the audio signal can be input to the control end of the first switch unit 121 through the capacitor EC3, and to the control end of the second switch unit 122 through the capacitor EC4. At the same time, the audio signal can also be input to the resistor R10 through the capacitor EC3 and the resistor R8, and input to the input end of the audio output circuit through the resistor R10. The audio signal can also be input to the resistor R10 through the capacitor EC4 and the resistor R9, and input to the input end of the audio output circuit through the resistor R10.

[0051] Optionally, the first switch unit 121 includes a transistor Q2, a resistor R11, and a capacitor C10; the base of the transistor Q2 is connected to the first end of the capacitor EC3, the first end of the capacitor C10, and the first end of the resistor R11, the collector of the transistor Q2 and the second end of the resistor R11 are connected to the power supply input, and the emitter of the transistor Q2 and the second end of the capacitor C10 are connected to the input end of the signal output circuit 130. Specifically, the transistor Q2 and its peripheral circuits constitute a switch unit, and when the input to the base of the transistor Q2 is a positive signal, the transistor Q2 is turned on.

[0052] Optionally, the second switch unit 122 includes a transistor Q3, a resistor R12, and a capacitor C9; the base of the transistor Q3 is connected to the first end of the capacitor EC4, the first end of the capacitor C9, and the first end of the resistor R12, the emitter of the transistor Q3 and the second end of the capacitor C9 are connected to the input end of the signal output circuit 130, and the collector of the transistor Q3 and the second end of the resistor R12 are grounded. Specifically, the transistor Q3 and its peripheral circuits constitute a switch unit, and when the input of the base of the transistor Q3 is a negative signal, the transistor Q3 is turned on.

[0053] Optionally, the signal input circuit 110 also includes a first common-mode filter circuit, a balanced-to-unbalanced circuit and a frequency selection network; the first input terminal and the second input terminal of the first common-mode filter circuit are respectively connected to the audio input, the first output terminal of the first common-mode filter circuit is connected to the power supply input and the input terminal of the frequency selection network, and the second output terminal of the first common-mode filter circuit is connected to the control terminal of the balanced-to-unbalanced circuit; the output terminal of the frequency selection network is connected to the first terminal of the capacitor EC3 and the first terminal of the capacitor EC4. Specifically, the audio input of the microphone is filtered by the first common-mode filter circuit and then input to the balanced-to-unbalanced circuit, and converted to a single-ended signal through the balanced-to-unbalanced circuit input, and the single-ended signal is finally obtained after passing through the frequency selection network. The required frequency signal.

[0054] Optionally, the balanced-to-unbalanced circuit includes a transistor Q1, a resistor R3, a resistor R2, a capacitor C5, and a capacitor EC2; the base of the transistor Q1 is connected to the first end of the resistor R2, the first end of the resistor R3, the first end of the capacitor C5, and the first end of the capacitor EC2, and the emitter of the transistor Q1, the second end of the capacitor C5, and the second end of the resistor R3 are connected to the input end of the frequency selection network; the emitter of the transistor Q1 and the second end of the resistor R2 are grounded, and the second end of the capacitor EC2 is connected to the second output end of the first common-mode filter circuit. Specifically, in the balanced-to-unbalanced circuit, the transistor Q1 and its peripheral circuits form a follower circuit, which copies the voltage of the input signal to the output end, and provides low output impedance and high input impedance, so that the input signal is not affected by load changes, thereby realizing signal amplification and transmission. It also isolates the previous stage circuit and reduces the influence of the subsequent stage circuit on the previous stage circuit.

[0055] Optionally, the frequency selection network includes a low-pass filter and a high-pass filter; the input end of the low-pass filter is connected to the input end of the high-pass filter to form the input end of the frequency selection network; the output end of the low-pass filter is connected to the output end of the high-pass filter to form the output end of the frequency selection network. Specifically, these two sets of filters can attenuate and filter part of the signal as needed, further increasing the anti-interference ability of the signal.

[0056] Optionally, the low-pass filter includes a resistor R5, a resistor R6 and a capacitor C19; the first end of the resistor R5 is the input end of the low-pass filter, the second end of the resistor R5 is connected to the first end of the capacitor C19 and the first end of the resistor R6, the second end of the capacitor C19 is grounded, and the second end of the resistor R6 is the output end of the low-pass filter. The low-pass filter is formed by the above structure to filter the high-frequency signal.

[0057] Optionally, the high-pass filter includes a resistor R4, a resistor R7, a capacitor C6 and a capacitor C7; the first end of the resistor R4 is the input end of the high-pass filter, the second end of the resistor R4 is connected to the first end of the capacitor C6, the second end of the capacitor C6 is connected to the first end of the resistor R7 and the first end of the capacitor C7, the second end of the resistor R7 is grounded, and the second end of the capacitor C7 is the output end of the high-pass filter. The low-pass filter is formed by the above structure to filter the low-frequency signal.

[0058] Optionally, the first common-mode filter circuit includes a common-mode inductor L1, a capacitor C1, a capacitor C2, a capacitor C3, a capacitor C4, a capacitor EC1 and a resistor R1; the first end of the common-mode inductor L1 is connected to the first end of the capacitor C1 and serves as the first input end of the first common-mode filter circuit; the third end of the common-mode inductor L1 is connected to the first end of the capacitor C2 and serves as the second input end of the first common-mode filter circuit; the second end of the common-mode inductor L1 is connected to the first end of the capacitor C3 and the first end of the resistor R1 and serves as the second output end of the first common-mode filter circuit; the fourth end of the common-mode inductor L1 is connected to the first end of the capacitor C4 and the second end of the capacitor EC1, the second end of the capacitor EC1 is connected to the second end of the resistor R1 and serves as the first output end of the first common-mode filter circuit; the second end of the capacitor C1, the second end of the capacitor C2, the second end of the capacitor C3 and the second end of the capacitor C4 are grounded. That is, the first common-mode filter circuit is composed of the common-mode inductor L1 and its peripheral circuits, and is used to filter the audio signal input from the microphone to improve the anti-interference ability of the input signal.

[0059] Optionally, the signal output circuit 130 includes: a resistor R13, a capacitor EC6, an audio transformer T1, and a second common-mode filter circuit; the first end of the audio transformer T1 is connected to the first end of the capacitor EC6, the second end of the capacitor EC6 is connected to the first end of the resistor R13, and the second end of the resistor R13 is connected to the output end of the signal input circuit; the second end of the audio transformer T1 is grounded, the third section of the audio transformer T1 is connected to the first input end of the second common-mode filter circuit, the fourth end of the audio transformer T1 is powered, and the fifth end of the audio transformer T1 is connected to the second input end of the second common-mode filter circuit; the first output end of the second common-mode filter circuit and the second output end of the second common-mode filter circuit are the output ends of the signal output circuit 130. In the signal output path, the single-ended output is converted into a balanced output by the audio transformer, which can effectively isolate the front and rear stage circuits, reduce interference, and improve the purity of the audio signal. Finally, it is achieved that the quality and clarity of the sound are improved while ensuring the stable transmission of the signal when the sound signal is transmitted over a long distance to prevent signal distortion.

[0060] Optionally, the second common-mode filter circuit includes a common-mode inductor L2, a capacitor C15, a capacitor C16, a capacitor C17 and a capacitor C18; the first end of the common-mode inductor L2 is connected to the first end of the capacitor C16 and serves as the first input end of the second common-mode filter circuit; the fourth end of the common-mode inductor L2 is connected to the first end of the capacitor C15 and serves as the second input end of the second common-mode filter circuit; the second end of the common-mode inductor L2 is connected to the first end of the capacitor C17 and serves as the first output end of the second common-mode filter circuit; the third end of the common-mode inductor L2 is connected to the first end of the capacitor C18 and serves as the second output end of the second common-mode filter circuit; the second end of the capacitor C15, the second end of the capacitor C16, the second end of the capacitor C17 and the second end of the capacitor C18 are grounded. That is, the second common-mode filter circuit is composed of the common-mode inductor L2 and its peripheral circuits. It is used to filter the audio signal input to the audio processing circuit to improve the anti-interference ability during the entire signal transmission process.

[0061] It can be understood that the above embodiments only express the preferred implementation methods of the utility model, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the patent scope of the utility model. It should be pointed out that, for ordinary technicians in this field, without departing from the concept of the utility model, the above-mentioned technical features can be freely combined, and several deformations and improvements can be made, which all belong to the protection scope of the utility model. Therefore, all equivalent changes and modifications made to the scope of the claims of the utility model should belong to the scope covered by the claims of the utility model.

Claims

1. A microphone preamplifier circuit, characterized in that: include: A signal input circuit, a first switch unit, a second switch unit and a signal output circuit; The signal input circuit receives an audio input and a power supply input of a microphone, and is used to convert the audio input into a single-ended signal; The first end of the first switch unit is connected to the power supply input, and the second end of the first switch unit is connected to the signal output circuit; the control end of the first switch unit is connected to the signal input circuit, and is used to be turned on when the single-ended signal is positive; The first end of the second switch unit is connected to the signal output circuit, and the second end of the first switch unit is grounded; the control end of the first switch unit is connected to the signal input circuit, and is used to be turned on when the single-ended signal is negative; The signal output circuit is connected to the second end of the first switch unit and the first end of the second switch unit, and is used for converting the received signal into a balanced signal and outputting the balanced signal to the audio processing circuit.

2. The microphone preamplifier circuit according to claim 1, characterized in that: The signal input circuit includes a capacitor EC3, a capacitor EC4, a resistor R8, a resistor R9 and a resistor R10; The control end of the first switch unit is connected to the first end of the capacitor EC3 and the first end of the resistor R8, and the control end of the second switch unit is connected to the first end of the capacitor EC4 and the second end of the resistor R9; The second end of the capacitor EC3 and the second end of the capacitor EC4 are used to input the single-ended signal; The second end of the resistor R8 is connected to the first end of the resistor R9 and the first end of the resistor R10 , and the second end of the resistor R10 is connected to the input end of the signal output circuit.

3. The microphone preamplifier circuit according to claim 2, characterized in that: The first switch unit includes a transistor Q2, a resistor R11 and a capacitor C10; the base of the transistor Q2 is connected to the first end of the capacitor EC3, the first end of the capacitor C10 and the first end of the resistor R11, the collector of the transistor Q2 and the second end of the resistor R11 are connected to the power supply input, and the emitter of the transistor Q2 and the second end of the capacitor C10 are connected to the input end of the signal output circuit; and / or The second switch unit includes a transistor Q3, a resistor R12 and a capacitor C9; the base of the transistor Q3 is connected to the first end of the capacitor EC4, the first end of the capacitor C9 and the first end of the resistor R12, the emitter of the transistor Q3 and the second end of the capacitor C9 are connected to the input end of the signal output circuit, and the collector of the transistor Q3 and the second end of the resistor R12 are grounded.

4. The microphone preamplifier circuit according to claim 2, characterized in that: The signal input circuit also includes a first common mode filter circuit, a balanced-to-unbalanced circuit and a frequency selection network; The first input terminal and the second input terminal of the first common mode filter circuit are respectively connected to the audio input, the first output terminal of the first common mode filter circuit is connected to the power supply input and the input terminal of the frequency selection network, and the second output terminal of the first common mode filter circuit is connected to the control terminal of the balanced-to-unbalanced circuit; The output end of the frequency selection network is connected to the first end of the capacitor EC3 and the first end of the capacitor EC4.

5. The microphone preamplifier circuit according to claim 4, characterized in that: The balanced-to-unbalanced circuit includes a transistor Q1, a resistor R3, a resistor R2, a capacitor C5, and a capacitor EC2; The base of the transistor Q1 is connected to the first end of the resistor R2, the first end of the resistor R3, the first end of the capacitor C5 and the first end of the capacitor EC2, the emitter of the transistor Q1, the second end of the capacitor C5 and the second end of the resistor R3 are connected to the input end of the frequency selection network; the emitter of the transistor Q1 and the second end of the resistor R2 are grounded, and the second end of the capacitor EC2 is connected to the second output end of the first common-mode filter circuit.

6. The microphone preamplifier circuit according to claim 4, characterized in that: The frequency selection network includes a low-pass filter and a high-pass filter; The input end of the low-pass filter is connected to the input end of the high-pass filter and forms the input end of the frequency selection network; The output end of the low-pass filter is connected to the output end of the high-pass filter and forms the output end of the frequency selection network.

7. The microphone preamplifier circuit according to claim 6, characterized in that: The low-pass filter comprises a resistor R5, a resistor R6 and a capacitor C19; the first end of the resistor R5 is the input end of the low-pass filter, the second end of the resistor R5 is connected to the first end of the capacitor C19 and the first end of the resistor R6, the second end of the capacitor C19 is grounded, and the second end of the resistor R6 is the output end of the low-pass filter; and / or The high-pass filter includes a resistor R4, a resistor R7, a capacitor C6 and a capacitor C7; the first end of the resistor R4 is the input end of the high-pass filter, the second end of the resistor R4 is connected to the first end of the capacitor C6, the second end of the capacitor C6 is connected to the first end of the resistor R7 and the first end of the capacitor C7, the second end of the resistor R7 is grounded, and the second end of the capacitor C7 is the output end of the high-pass filter.

8. The microphone preamplifier circuit according to claim 4, characterized in that: The first common-mode filtering circuit includes a common-mode inductor L1, a capacitor C1, a capacitor C2, a capacitor C3, a capacitor C4, a capacitor EC1 and a resistor R1; The first end of the common-mode inductor L1 is connected to the first end of the capacitor C1 and serves as the first input end of the first common-mode filter circuit; The third end of the common-mode inductor L1 is connected to the first end of the capacitor C2 and serves as the second input end of the first common-mode filter circuit; The second end of the common-mode inductor L1 is connected to the first end of the capacitor C3 and the first end of the resistor R1, and serves as the second output end of the first common-mode filter circuit; The fourth end of the common-mode inductor L1 is connected to the first end of the capacitor C4 and the second end of the capacitor EC1, and the second end of the capacitor EC1 is connected to the second end of the resistor R1 and serves as the first output end of the first common-mode filter circuit; The second end of the capacitor C1 , the second end of the capacitor C2 , the second end of the capacitor C3 , and the second end of the capacitor C4 are grounded.

9. The microphone preamplifier circuit according to claim 1, characterized in that: The signal output circuit includes: a resistor R13, a capacitor EC6, an audio transformer T1 and a second common mode filter circuit; The first end of the audio transformer T1 is connected to the first end of the capacitor EC6, the second end of the capacitor EC6 is connected to the first end of the resistor R13, and the second end of the resistor R13 is connected to the output end of the signal input circuit; The second end of the audio transformer T1 is grounded, the third end of the audio transformer T1 is connected to the first input end of the second common-mode filter circuit, the fourth end of the audio transformer T1 is the power supply input, and the fifth end of the audio transformer T1 is connected to the second input end of the second common-mode filter circuit; The first output end of the second common-mode filter circuit and the second output end of the second common-mode filter circuit are output ends of the signal output circuit.

10. The microphone preamplifier circuit according to claim 9, characterized in that: The second common-mode filtering circuit includes a common-mode inductor L2, a capacitor C15, a capacitor C16, a capacitor C17 and a capacitor C18; The first end of the common-mode inductor L2 is connected to the first end of the capacitor C16 and serves as the first input end of the second common-mode filter circuit; The fourth end of the common-mode inductor L2 is connected to the first end of the capacitor C15 and serves as the second input end of the second common-mode filter circuit; The second end of the common-mode inductor L2 is connected to the first end of the capacitor C17 and serves as the first output end of the second common-mode filter circuit; The third end of the common-mode inductor L2 is connected to the first end of the capacitor C18 and serves as the second output end of the second common-mode filter circuit; A second end of the capacitor C15, a second end of the capacitor C16, a second end of the capacitor C17, and a second end of the capacitor C18 are grounded.

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