Electron tube power amplifier with external timbre selection function

Abstract

The utility model provides an electronic tube power amplifier with an external timbre selection function, which is connected with an external timbre selection circuit, a voltage amplification stage circuit is used for providing voltage gain and outputting a first driving signal to a voltage promotion stage circuit, a grid electrode of a third triode amplifier of the voltage promotion stage circuit inputs the first driving signal, and the third triode amplifier of the voltage promotion stage circuit outputs a second driving signal. The middle point of a filament resistor of the transformer output circuit is connected with the first bias circuit, the anode of the transformer output circuit outputs a second driving signal to the power amplification stage circuit, the power amplification stage circuit is used for performing power amplification on the second driving signal, and the output end of the transformer output circuit is connected with the loudspeaker and is used for outputting a third driving signal to drive the loudspeaker after impedance matching; and the tone selection circuit is a multi-stage large-loop negative feedback circuit for reducing the total gain of the electron tube power amplifier circuit. According to the utility model, the feedback quantity is changed through the design of the multi-stage large-loop negative feedback circuit, so that the circuit gain is controlled, and the effects of outputting audio signals with different timbres and having good circuit stability are achieved.

Description

Technical Field

[0001] This utility model relates to the field of audio playback devices, specifically to an external tube power amplifier with timbre selection function. Background Technology

[0002] In the field of audio playback, vacuum tube amplifiers, due to their non-linear characteristics, can generate rich harmonics, making the sound more three-dimensional, and thus have been widely used in the power circuits of audio playback equipment. Existing vacuum tube amplifiers mainly consist of vacuum tubes, transformers, and output transistors. Their working principle is to amplify the audio signal through the vacuum tube amplifier circuit, making the sound clearer and fuller. The vacuum tube, as a signal amplifier, controls the signal gain to achieve fine adjustment of the audio signal; the transformer is responsible for converting alternating current to direct current to provide energy to the vacuum tube; and the output transistor is responsible for amplifying and outputting the signal from the vacuum tube.

[0003] However, the tube amplifier circuits in existing music players offer a relatively limited range of tones. Even higher-end music players, while constantly pursuing high-fidelity sound quality, often neglect the listener's demand for tonal diversity. Furthermore, providing a variety of tonal options requires a redesign of the tube amplifier circuits in music players. How to design a tube-based power amplifier with good stability and improved output signal quality are also issues that need to be considered.

[0004] Therefore, in order to further enrich the tonal selection of audio playback devices, a tube power amplifier that can achieve different tonal selection functions and has good circuit stability is needed. Utility Model Content

[0005] This utility model provides an external tube power amplifier with tone selection function, which is mainly used to solve the problems of the limited tone selection of existing music players and the circuit stability design of power amplifiers with multiple tone selection functions, thereby achieving the effect of realizing different tone selection functions and good circuit stability.

[0006] This utility model achieves the above objectives through the following technical solutions:

[0007] An external tube power amplifier with tone selection function is connected to an external tone selection circuit. It includes a voltage amplification stage circuit, a voltage driver stage circuit, a power amplification stage circuit, and a transformer output circuit. The voltage amplification stage circuit receives a voltage signal at its input terminal to provide voltage gain and outputs a first drive signal to the voltage driver stage circuit. The voltage driver stage circuit includes a third transistor amplifier and a first bias circuit. The first drive signal is input to the gate of the third transistor amplifier, the midpoint of its filament resistor is connected to the first bias circuit, and its anode outputs a second drive signal to the power amplification stage circuit. The output terminal of the power amplification stage circuit is connected to the input terminal of the transformer output circuit to amplify the second drive signal. The output terminal of the transformer output circuit is connected to a speaker to output a third drive signal after impedance matching to drive the speaker. The input and output terminals of the tone selection circuit are connected to the output terminal of the transformer output circuit and the input terminal of the voltage amplification stage circuit, respectively, forming a multi-stage large-loop negative feedback circuit to reduce the overall gain of the tube power amplifier.

[0008] The timbre selection circuit includes at least a resistor topology and a timbre switch. The resistor topology is used to adjust the input negative feedback signal and output it to the input terminal of the voltage amplification stage circuit. The timbre switch is connected to the resistor topology and is used to adjust the feedback amount output by the resistor topology through gear switching.

[0009] A further embodiment is that the voltage amplification stage circuit is a two-stage voltage amplification circuit, including a first voltage amplification stage circuit and a second voltage amplification stage circuit. The voltage signal is input to the input terminal of the first voltage amplification stage circuit to provide voltage gain, and a first inverted signal with the opposite phase to the voltage signal is output to the second voltage amplification stage circuit. The second voltage amplification stage circuit is used to provide secondary voltage gain and outputs the first driving signal with the same phase as the voltage signal.

[0010] A further embodiment is that both the first voltage amplification stage circuit and the second voltage amplification stage circuit are common cathode phase inverting circuits. The first voltage amplification stage circuit includes a first transistor amplifier, a first RC coupling amplifier circuit, and a first resistor. The gate of the first transistor amplifier is connected to the voltage signal, its anode is connected to the first RC coupling amplifier circuit, and the first inverted signal is coupled to the second voltage amplification stage circuit through the first RC coupling amplifier circuit. Its cathode is grounded through the first resistor.

[0011] The second voltage amplification stage circuit includes a second transistor amplifier, a second RC-coupled amplifier circuit, and a third RC-coupled amplifier circuit. The gate of the second transistor amplifier is connected to the first inverted signal, its anode is connected to the second RC-coupled amplifier circuit, and the first drive signal is coupled to the voltage drive stage circuit through the second RC-coupled amplifier circuit. Its cathode is grounded through the third RC-coupled amplifier circuit.

[0012] A further option is that both the first and second transistor amplifiers use a 6SN7GTB side-heated medium-μ dual transistor.

[0013] A further option is that the third transistor amplifier uses a 300B directly heated transistor.

[0014] A further embodiment is that the first bias circuit is used to provide self-bias for the third transistor amplifier, and includes a first capacitor, a second capacitor, a second resistor, and a third resistor. The first capacitor is an electrolytic capacitor, the positive terminal of which is connected to the midpoint of the filament resistor of the third transistor amplifier, and the negative terminal of which is fed back to the gate of the third transistor amplifier through the second and third resistors connected in series. The second capacitor is used as a bypass capacitor, one end of which is connected to the midpoint of the filament resistor of the third transistor amplifier, and the other end is connected to the common connection terminal of the second and third resistors.

[0015] A further embodiment is that the power amplifier stage circuit includes a fourth transistor amplifier and a fourth resistor. The fourth transistor amplifier uses an 845SL directly heated transistor, whose gate receives the second drive signal, whose filament resistor has its midpoint grounded through the fourth resistor, and whose anode is connected to the input terminal of the transformer output circuit.

[0016] A further embodiment is that the transformer output circuit has four output terminals with output impedances of 8Ω, 6Ω, 4Ω and 0Ω, which are used to drive speakers with different rated impedance values ​​through impedance matching.

[0017] A further embodiment is that the resistor topology is a series resistor topology, which includes a fifth resistor and a sixth resistor. The fifth resistor and the sixth resistor are connected in series to form a first series resistor circuit. One end of the fifth resistor is connected to the output terminal of the transformer output circuit, and the other end is connected to the cathode of the first transistor amplifier through the sixth resistor.

[0018] The tone switch is connected in parallel with the sixth resistor. By actuating the tone switch, the fifth resistor or the series circuit of the first resistor is switched to the multi-stage large loop negative feedback circuit, thereby adjusting the circuit gain to achieve the selection of two tone circuits.

[0019] A further embodiment is that the tone selection circuit includes multiple resistor topologies and tone switches. The tone switches are multi-section contact changeover switches, with multiple sets of contacts connected to multiple resistor topologies respectively. By rotating the tone switch, the feedback input of the voltage amplification stage circuit is adjusted, thereby controlling the total gain of the circuit to achieve multi-tone switching of the tube power amplifier.

[0020] Therefore, this utility model has the following beneficial effects:

[0021] 1. This utility model of a vacuum tube power amplifier adopts a four-stage transistor amplification structure. It provides the main voltage gain to the tone circuit through two voltage amplification stages, a high-current voltage gain to the tone circuit through a driver stage, and output power to the tone circuit through a power amplification stage and a transformer, thereby enhancing the signal driving capability. A tone selection circuit forms a large-loop negative feedback with the aforementioned four-stage transistor amplification circuit. By changing the resistance value connected to the multi-stage large-loop negative feedback circuit, the feedback amount is adjusted, thereby changing the circuit gain to achieve audio signal adjustment. This alters the distribution and weighting of the harmonics in the output audio signal, resulting in different tonal experiences and enabling the selection of different tone circuits. Compared to existing vacuum tube amplifiers that can only provide a single tone, this new amplifier enables the selection of different tones, satisfying listeners' needs for tonal diversity.

[0022] 2. This utility model uses a tone selection circuit and a four-stage transistor amplification structure to form a large-loop negative feedback to control the total gain of the circuit. This reduces external interference and improves the stability of the circuit, as well as reduces the distortion generated during signal amplification, thereby improving the signal quality.

[0023] 3. This utility model achieves one-button switching of the resistance value of a multi-stage large-loop negative feedback circuit by using an external switch to switch between different tone circuits. The circuit principle is clear, the structure is simple, and the stability is good. It has high practical value for simplifying circuit design and improving circuit efficiency.

[0024] 4. This utility model uses two common-cathode transistors as voltage amplification stages to provide circuit voltage gain. The common-cathode phase inverter circuit formed by it has the advantages of low distortion, good frequency response and high fidelity. Moreover, the phase inverter circuit composed of dual transistors has good symmetry and high gain.

[0025] 5. The driver stage of this utility model adopts a common cathode transistor amplifier composed of 300B directly heated transistors, which has excellent linearity, low distortion, low internal resistance and strong driving force. It has excellent sound characteristics and sound quality performance in the mid-high frequency range, and can better drive high-sensitivity speakers.

[0026] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0027] Figure 1 This is a circuit diagram of the electron tube power amplifier according to an embodiment of the present invention.

[0028] Figure 2 This is a circuit diagram illustrating the switching between two timbres in an embodiment of this utility model.

[0029] Figure 3 This is a circuit diagram of the electron tube power amplifier according to an embodiment of the present invention.

[0030] Figure 4 This is a circuit diagram of an embodiment of the present invention applied to a left and right channel power amplifier. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of this utility model clearer, the technical solutions of the embodiments of this utility model will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this utility model. All other embodiments obtained by those skilled in the art based on the described embodiments of this utility model without creative effort are within the scope of protection of this utility model.

[0032] An embodiment of an external tube power amplifier with tone selection function

[0033] See Figure 1 This utility model relates to an external tube power amplifier with tone selection function, comprising a voltage amplification stage circuit 10, a voltage driver stage circuit 20, a power amplification stage circuit 30, a transformer output circuit 40, and a tone selection circuit 50. The voltage amplification stage circuit 10 receives a voltage signal at its input terminal to provide voltage gain and outputs a first drive signal to the voltage driver stage circuit 20. The voltage driver stage circuit 20 includes a third transistor amplifier V2 and a first bias circuit. The first drive signal is input to the gate of the third transistor amplifier V2, and the midpoint of its filament resistor is connected to the first bias circuit. The circuit is connected such that its anode outputs a second driving signal to the power amplifier stage circuit 30. The output terminal of the power amplifier stage circuit 30 is connected to the input terminal of the transformer output circuit 40 to amplify the power of the second driving signal. The output terminal of the transformer output circuit 40 is connected to the speaker to output a third driving signal after impedance matching to drive the speaker. The input and output terminals of the tone selection circuit 50 are connected to the output terminal of the transformer output circuit 40 and the input terminal of the voltage amplifier stage circuit 10, respectively, forming a multi-stage large-loop negative feedback circuit to reduce the total gain of the tube power amplifier.

[0034] The tone selection circuit 50 includes at least a resistor topology and a tone switch. The resistor topology is used to adjust the input negative feedback signal and output it to the input terminal of the voltage amplification stage circuit 10. The tone switch is connected to the resistor topology and is used to adjust the feedback amount output by the resistor topology through gear switching.

[0035] Specifically, in this embodiment, the input and output terminals of the resistor topology are connected to the output terminal of the transformer output circuit 40 and the input terminal of the voltage amplification stage circuit 10, respectively. This is used to change the total gain of the tube power amplifier circuit through impedance matching, thereby achieving sound production with a first timbre. The timbre switch is used to change the resistor topology structure, thereby changing the feedback amount of the multi-stage large-loop negative feedback circuit, and thus changing the total gain of the circuit to achieve multi-timbre switching of the tube power amplifier.

[0036] In this embodiment, the voltage amplification stage circuit 10 is a two-stage voltage amplification circuit, including a first voltage amplification stage circuit 11 and a first voltage amplification stage circuit 12. The voltage signal is input to the input terminal of the first voltage amplification stage circuit 11 to provide voltage gain, and a first inverted signal with the opposite phase to the voltage signal is output to the first voltage amplification stage circuit 10. The first voltage amplification stage circuit 10 is used to provide secondary voltage gain and outputs a first driving signal with the same phase as the voltage signal.

[0037] See Figure 3 In this embodiment, both the first voltage amplification stage circuit 11 and the first voltage amplification stage circuit 12 are common cathode phase inverting circuits. The first voltage amplification stage circuit 11 includes a first transistor amplifier V1A, a first RC coupling amplifier circuit, and a first resistor R103. The gate of the first transistor amplifier V1A is connected to the voltage signal, its anode is connected to the first RC coupling amplifier circuit, and the first inverted signal is coupled to the first voltage amplification stage circuit 10 through the first RC coupling amplifier circuit. Its cathode is grounded through the first resistor R103.

[0038] Specifically, the first RC-coupled amplifier circuit in this embodiment includes a resistor R104, a capacitor C100, a resistor R105, and a capacitor C101. One end of capacitor C100 is grounded, and the other end is connected to a 132V DC power supply. It is also connected to the anode of the first transistor amplifier V1A through the resistor R104. One end of capacitor C101 is connected to a 132V DC power supply, and the other end is connected to the anode of the first transistor amplifier V1A through the resistor R105. The anode of the first transistor amplifier V1A is connected to the gate of the second transistor amplifier V1B.

[0039] The first voltage amplifier stage circuit 10 includes a second transistor amplifier V1B, a second RC-coupled amplifier circuit, and a third RC-coupled amplifier circuit. The gate of the second transistor amplifier V1B is connected to the first inverted signal, its anode is connected to the second RC-coupled amplifier circuit, and the first drive signal is coupled to the voltage driver stage circuit 20 through the second RC-coupled amplifier circuit. Its cathode is grounded through the third RC-coupled amplifier circuit.

[0040] Specifically, the second RC-coupled amplifier circuit in this embodiment includes a resistor R107 and a capacitor C104. One end of the resistor R107 is connected to a 415V DC power supply, and the other end is connected to the anode of the second transistor amplifier V1B, with an anode voltage of 270V-265V. One end of the capacitor C104 is connected to the anode of the second transistor amplifier V1B, and the other end is connected to the gate of the third transistor amplifier V2.

[0041] Specifically, the third RC-coupled amplifier circuit in this embodiment includes a capacitor C102, a resistor R106, and an electrolytic capacitor C103. The electrolytic capacitor C103 is connected in parallel with the capacitor C102 and the resistor R106, respectively. Its positive terminal is connected to the cathode of the second transistor amplifier V1B, and its negative terminal is grounded.

[0042] In this embodiment, both the first transistor amplifier V1A and the second transistor amplifier V1B use a 6SN7GTB side-heated medium-μ dual transistor.

[0043] In this embodiment, the third transistor amplifier V2 uses a 300B directly heated transistor.

[0044] Specifically, in this embodiment, the third transistor amplifier V2 is used to obtain a high-current, low-gain inverted signal, and the phase of the second driving signal it outputs is opposite to that of the first driving signal.

[0045] In this embodiment, the first bias circuit is used to provide a self-bias voltage for the third transistor amplifier V2, including a first capacitor C106, a second capacitor C105, a second resistor R110, and a third resistor R109. The first capacitor C106 is an electrolytic capacitor, whose positive terminal is connected to the midpoint of the filament resistor of the third transistor amplifier V2, and whose negative terminal is fed back to the gate of the third transistor amplifier V2 through the series-connected second resistor R110 and third resistor R109. The second capacitor C105 is used as a bypass capacitor, with one end connected to the midpoint of the filament resistor of the third transistor amplifier V2, and the other end connected to the common connection terminal of the second resistor R110 and the third resistor R109.

[0046] Specifically, in this embodiment, the voltage driver stage circuit 20 also includes a fourth RC coupling amplifier circuit, through which the second drive signal is coupled to the power amplifier stage circuit 30.

[0047] Specifically, the fourth RC-coupled amplifier circuit described in this embodiment includes resistors R114a and R114b, and capacitor C108. Resistors R114a and R114b are connected in parallel, with one common terminal connected to a 475V DC power supply and the other common terminal connected to the anode of the third transistor amplifier V2, with an anode voltage of 326V. One end of capacitor C108 is connected to the anode of the third transistor amplifier V2, and the other end is connected to the gate of the fourth transistor amplifier A1 through resistor R116.

[0048] In this embodiment, the power amplifier stage circuit 30 includes a fourth transistor amplifier A1 and a fourth resistor R117. The fourth transistor amplifier A1 is an 845SL directly heated transistor. Its gate is input with the second drive signal, the midpoint of its filament resistor is grounded through the fourth resistor R117, and its anode is connected to the input terminal of the transformer output circuit 40.

[0049] Specifically, in this embodiment, the fourth transistor amplifier A1 is used to invert the second driving signal and output it, so that the third driving signal is in phase with the input voltage signal of the first transistor amplifier V1A.

[0050] Specifically, in this embodiment, the power amplifier stage circuit 30 can control the amplitude and phase of the output signal by adjusting the power supply, bias and gate parameters of the fourth transistor amplifier A1, thereby realizing signal transformation and enhancement.

[0051] In this embodiment, the transformer output circuit 40 has four output terminals with output impedances of 8Ω, 6Ω, 4Ω and 0Ω, which are used to drive speakers with different rated impedance values ​​through impedance matching.

[0052] Specifically, in this embodiment, the transformer output circuit 40 uses a multi-winding output transformer T1. The transformer T1 is used to drive a speaker with an impedance of 8Ω. Its 8Ω output terminal is connected to the speaker and connected to the tone selection circuit 50 to form a multi-stage large-loop negative feedback circuit. Its 0Ω output terminal is grounded.

[0053] See Figure 2 In this embodiment, the resistor topology is a series resistor topology, which includes a fifth resistor R118 and a sixth resistor R119. The fifth resistor R118 and the sixth resistor R119 are connected in series to form a series circuit of the first resistor R103. One end of the fifth resistor R118 is connected to the output terminal of the transformer output circuit 40, and the other end is connected to the cathode of the first transistor amplifier V1A through the sixth resistor R119.

[0054] In this circuit, the tone switch K2 is connected in parallel with the two ends of the sixth resistor R119. By operating the tone switch K2, the series circuit of the fifth resistor R118 or the first resistor R103 is switched to the multi-stage large loop negative feedback circuit, thereby changing the circuit gain to achieve the selection of two tone circuits.

[0055] Specifically, in this embodiment, the tone switch K2 is a double-control switch K2, with its common terminal connected to one end of the sixth resistor R119, its normally closed terminal K2A ​​connected to the other end of the sixth resistor R119, and its normally open terminal NFB left floating.

[0056] When the tone switch K2 is in the normally open state, the first resistor R103 is connected in series to the multi-stage large-loop negative feedback circuit. After the fifth resistor R118 and the sixth resistor R119 are connected in series to divide the voltage, the feedback signal level is reduced, and the feedback signal is input to the cathode of the first transistor, thereby reducing the total gain of the circuit.

[0057] When the tone switch K2 is in the normally closed state, the sixth resistor R119 is short-circuited, and only the fifth resistor R118 is connected to the multi-stage large loop negative feedback circuit. After the voltage is divided by the fifth resistor R118, the feedback signal level is reduced, and the feedback signal is input to the cathode of the first transistor, thereby reducing the total gain of the circuit.

[0058] The total gain of the circuit is determined by the sum of the gains of each stage of the circuit and the multi-stage large-loop negative feedback circuit.

[0059] Specifically, this embodiment provides two levels of tone circuitry by switching the resistance value of the multi-stage large-loop negative feedback circuit, namely NFB.0dB tone circuit and NFB. -3dB tone circuit.

[0060] When the tone switch K2 is in the normally open state, the setting is switched to the NFB.0dB tone circuit. At this time, the negative feedback depth of the multi-stage large-loop negative feedback circuit is relatively shallow, and the distribution spectrum of each harmonic distortion characteristic gives the tone a unique flavor. This is especially evident in the directly heated transistor, where even-order harmonic components dominate, presenting a delicate, sweet, and rich sound with a distinct "tube sound".

[0061] When tone switch K2 is normally closed, the circuit is switched to NFB -3dB tone circuit. At this time, the negative feedback depth of the loop negative feedback circuit is deeper, resulting in better consistency in amplifier circuit performance and a significant reduction in total harmonic distortion (THD). Under the same THD conditions, its output power is greater. Conversely, its even-order harmonic components are slightly weaker than in the NFB 0dB tone circuit. The distribution spectrum of harmonic distortion characteristics results in slightly fewer overtones in the perceived tone, presenting a clearer sound, better dynamics, and a slightly less tube-like sound.

[0062] Specifically, the vacuum tube amplifier with a large-loop negative feedback structure provided in this embodiment is used to determine the main tonal characteristics. By adjusting the static current and operating voltage of each stage of the vacuum tube, the distribution and proportional weight of each harmonic of the output audio signal can be changed, thereby obtaining different tonal sensations and achieving fine-tuning and refinement of the tonal sensation.

[0063] See Figure 4 Specifically, this embodiment is applied to the left and right channel power amplifier circuits. The left channel power amplifier circuit 200 and the right channel power amplifier circuit 100 respectively adopt the external tube power amplifier with tone selection function. The circuit structure of the left and right channel power amplifier circuits is the same and they work independently. The left channel audio signal and the right channel audio signal are respectively input to the input terminal of the tube power amplifier, and the tone circuit is selected by switch K2 respectively.

[0064] Specifically, the circuit structure of the timbre selection circuit 50 described above in this embodiment is merely exemplary and not the only feedback method. Other methods may also be available:

[0065] The tone selection circuit 50 includes multiple resistor topologies and tone switches. The tone switches are multi-section contact changeover switches, with multiple sets of contacts connected to the two ends of the multiple resistor topologies respectively. By rotating the tone switch, the feedback amount of the multi-stage large-loop negative feedback circuit is changed, thereby changing the total gain of the circuit to realize the multi-tone switching of the tube power amplifier.

[0066] Specifically, the structure of the resistor topology in this embodiment can be designed according to the feedback method required by the actual circuit. For example, using voltage series negative feedback can significantly reduce distortion and improve sound quality; using voltage parallel negative feedback can significantly reduce noise and improve the accuracy and stability of the tube output; using current series negative feedback can improve the stability and efficiency of current output, and is suitable for applications requiring high current stability and linearity; using current parallel negative feedback can reduce noise and distortion while ensuring gain.

[0067] The above embodiments are merely preferred embodiments of this utility model and should not be construed as limiting the scope of protection of this utility model. Any non-substantial changes and substitutions made by those skilled in the art based on this utility model shall fall within the scope of protection claimed by this utility model.

Claims

1. An external tube power amplifier with tone selection function, characterized in that, Connects to an external tone selection circuit, including: The system comprises a voltage amplification stage circuit, a voltage driver stage circuit, a power amplification stage circuit, and a transformer output circuit. The voltage amplification stage circuit receives a voltage signal at its input terminal to provide voltage gain and outputs a first drive signal to the voltage driver stage circuit. The voltage driver stage circuit includes a third transistor amplifier and a first bias circuit. The first drive signal is input to the gate of the third transistor amplifier, and the midpoint of its filament resistor is connected to the first bias circuit. Its anode outputs a second drive signal to the power amplification stage circuit. The output terminal of the power amplification stage circuit is connected to the input terminal of the transformer output circuit to amplify the second drive signal. The output terminal of the transformer output circuit is connected to a speaker to output a third drive signal after impedance matching to drive the speaker. The input and output terminals of the tone selection circuit are connected to the output terminal of the transformer output circuit and the input terminal of the voltage amplification stage circuit, respectively, forming a multi-stage large-loop negative feedback circuit to reduce the overall gain of the tube power amplifier. The timbre selection circuit includes at least a resistor topology and a timbre switch. The resistor topology is used to adjust the input negative feedback signal and output it to the input terminal of the voltage amplification stage circuit. The timbre switch is connected to the resistor topology and is used to adjust the feedback amount output by the resistor topology through gear switching.

2. The external tube power amplifier with tone selection function according to claim 1, characterized in that: The voltage amplification stage circuit is a two-stage voltage amplification circuit, including a first voltage amplification stage circuit and a second voltage amplification stage circuit. The voltage signal is input to the input terminal of the first voltage amplification stage circuit to provide voltage gain, and a first inverted signal with the opposite phase to the voltage signal is output to the second voltage amplification stage circuit. The second voltage amplification stage circuit is used to provide secondary voltage gain and outputs a first driving signal with the same phase as the voltage signal.

3. The external tube power amplifier with tone selection function according to claim 2, characterized in that: Both the first voltage amplification stage circuit and the second voltage amplification stage circuit are common cathode phase inverting circuits. The first voltage amplification stage circuit includes a first transistor amplifier, a first RC coupling amplifier circuit, and a first resistor. The gate of the first transistor amplifier is connected to the voltage signal, its anode is connected to the first RC coupling amplifier circuit, and the first inverted signal is coupled to the second voltage amplification stage circuit through the first RC coupling amplifier circuit. Its cathode is grounded through the first resistor. The second voltage amplification stage circuit includes a second transistor amplifier, a second RC-coupled amplifier circuit, and a third RC-coupled amplifier circuit. The gate of the second transistor amplifier is connected to the first inverted signal, its anode is connected to the second RC-coupled amplifier circuit, and the first drive signal is coupled to the voltage drive stage circuit through the second RC-coupled amplifier circuit. Its cathode is grounded through the third RC-coupled amplifier circuit.

4. The external tube power amplifier with tone selection function according to claim 3, characterized in that: Both the first and second transistor amplifiers use a 6SN7GTB side-heated medium-μ dual transistor.

5. The external tube power amplifier with tone selection function according to claim 1, characterized in that: The third transistor amplifier uses a 300B directly heated transistor.

6. The external tube power amplifier with tone selection function according to claim 5, characterized in that: The first bias circuit is used to provide self-bias for the third transistor amplifier, and includes a first capacitor, a second capacitor, a second resistor, and a third resistor. The first capacitor is an electrolytic capacitor, the positive terminal of which is connected to the midpoint of the filament resistor of the third transistor amplifier, and the negative terminal of which is fed back to the gate of the third transistor amplifier through the second and third resistors connected in series. The second capacitor is used as a bypass capacitor, one end of which is connected to the midpoint of the filament resistor of the third transistor amplifier, and the other end is connected to the common connection terminal of the second and third resistors.

7. The external tube power amplifier with tone selection function according to claim 1, characterized in that: The power amplifier stage circuit includes a fourth transistor amplifier and a fourth resistor. The fourth transistor amplifier uses an 845SL directly heated transistor, whose gate receives the second drive signal, whose filament resistor has its midpoint grounded through the fourth resistor, and whose anode is connected to the input terminal of the transformer output circuit.

8. The external tube power amplifier with tone selection function according to claim 1, characterized in that: The transformer output circuit has four output terminals with output impedances of 8Ω, 6Ω, 4Ω and 0Ω, which are used to drive speakers with different rated impedance values ​​through impedance matching.

9. The external tube power amplifier with tone selection function according to claim 3, characterized in that: The resistor topology is a series resistor topology, which includes a fifth resistor and a sixth resistor. The fifth resistor and the sixth resistor are connected in series to form a first series resistor circuit. One end of the fifth resistor is connected to the output terminal of the transformer output circuit, and the other end is connected to the cathode of the first transistor amplifier through the sixth resistor. The tone switch is connected in parallel with the sixth resistor. By actuating the tone switch, the fifth resistor or the series circuit of the first resistor is switched to the multi-stage large loop negative feedback circuit, thereby adjusting the circuit gain to achieve the selection of two tone circuits.

10. The external tube power amplifier with tone selection function according to any one of claims 1-9, characterized in that: The tone selection circuit includes multiple resistor topologies and tone switches. The tone switch is a multi-section contact changeover switch, with multiple sets of contacts connected to multiple resistor topologies respectively. By rotating the tone switch, the feedback input of the voltage amplification stage circuit is adjusted, thereby controlling the overall gain of the circuit to achieve multi-tone switching of the tube power amplifier.

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