Class A power amplifier using SAP15N/P audio pair tubes

Class A transistor power amplifiers have a warm and sweet tone that makes people fall in love with them. However, Class A power amplifiers have a high temperature rise. If ordinary bipolar transistors are used, the temperature stability is poor, debugging is difficult, and it is also worrying during use. This article introduces a Class A power amplifier with a Sanken product SAP15N/P audio pair tube in the output stage. SAP15N/P has an output current of 15A, a withstand voltage of 160V, a power of 150W, and a junction temperature of 150℃. The transistor has a built-in temperature compensation diode array, which solves the technical problem of temperature compensation delay in the power amplifier circuit. Its temperature compensation diode is placed in the center of the transistor chip, which can quickly and synchronously detect the change of transistor temperature and correct the bias current to keep it constant. Class A power amplifiers made of ordinary bipolar transistors take a long time to stabilize due to temperature changes. It often takes half an hour for the amplifier to stabilize, and the bias current will change with the temperature. In actual use, it takes half an hour to burn in the machine before you can hear the real Class A tone. The SAP15N/P pair of tubes, due to the consistency of its internal temperature rise and compensation, the amplifier enters a stable state after a few minutes of power-on, with high efficiency, stable and safe operation. Therefore, it can be said that the SAP15N/P pair of tubes is the best partner for Class A power amplifiers. In addition, SAP15N/P also has many advantages such as good working linearity, small saturation voltage drop, and high power utilization. In addition to the temperature compensation diode, SAP15N/P also has its own final front-stage driving transistor and emitter 0.22Ω resistor, which has a high cost performance. Figure 1 is the appearance diagram and internal electrical schematic diagram of SAP15N/P. It has 5 pins. There is a PN junction diode inside its NPN side and five Schottky diodes inside its PNP side for temperature compensation. When in use, add a variable resistor between the diode of the NPN tube and the Schottky diode of the PNP tube to adjust the no-load current.

Figure 1

Figure 2

How it works

　　Figure 2 shows a power amplifier circuit using a Class A monophonic design. The power amplifier has temperature detection and protection functions, as well as features such as DC midpoint servo. Its input stage uses a common source-common base circuit composed of field effect twin tubes V1/V2 (K389/J109) and transistors V3~V6. The circuit has low high-frequency distortion, large dynamic range, and high signal-to-noise ratio. In the second voltage amplifier stage, V15 and V16 form a two-stage push-pull cascade circuit, V15/V7 and V16/V8 form ideal current sources, current amplification is completed by V7 and V8, and voltage amplification is completed by V15/V16. The speaker protection circuit uses Toshiba speaker protection integrated circuit TA7317 (IC3), in which VD6 is the shutdown noise power supply detection, connected to the AC end of the transformer. V12 and VD8 constitute the relay working power supply. IC1A in the field effect integrated dual op amp IC1 (LF353) performs DC midpoint servo to stabilize the midpoint potential; IC1B, V13, temperature detection switch SW2 and photocoupler IC2 form a temperature detection and protection action circuit. The temperature switch contact is a normally closed contact with an action temperature of 90°C. When in use, it is fixed to the heat sink with screws to detect the temperature. Under normal circumstances, the contact of the temperature switch SW3 is closed, the voltage of the ⑤ pin of IC1B is higher than the ⑥ pin, the ⑦ pin outputs a high level, V13 is turned on, its collector is low, and no current passes through the photocoupler IC2; when the heat sink temperature reaches 90°C, SW3 is disconnected, the voltage of the ⑤ pin of IC1B is lower than the voltage of the ⑥ pin, the ⑦ pin outputs a low level, V13 is turned off, its collector is high, and the current flows through the ① and ② terminals of IC2, causing the ③ and ④ terminals to be turned on. At this time, the resistor R51 shunts the excitation signal, thereby reducing the static current of the final stage, and the working state is changed from Class A to Class A and B, thereby reducing the working temperature of the transistor to ensure its safety. IC1 and VD7 also form a self-holding circuit. When the temperature protection circuit is activated, the comparison voltage of the ⑤ pin of IC1B is clamped at a low level, so that the power amplifier maintains the Class A and Class B working state. After the temperature protection is activated, the Class A working state is automatically restored after the power is turned off and then turned on. SW1 is a manual selection switch for Class A or Class A and Class B working mode, which can be set in advance. SW1 is closed for Class A amplification, and SW1 is disconnected for Class A and Class B amplification. When the light-emitting diode LED is on, it indicates that the power amplifier is working in Class A and Class B state. The temperature of the Class A power amplifier is relatively high. With temperature detection and protection, there will be no worries about "unpredictable weather". In addition, switch SW3 can select whether the final stage adopts large loop negative feedback. The power supply of the power amplifier excitation stage is provided by the ±50V power supply on the circuit board. In order to save volume and facilitate the circuit board (250×80mm) to be installed vertically close to the radiator, the final stage rectifier filter device is not on the power amplifier board, and the electrical schematic diagram of the final stage power supply is omitted in Figure 2.

Circuit debugging

　　In order to obtain a larger dynamic range, the input stage output current is 2.5mA, and the potentiometer R1 is adjusted to reduce the voltage on R8 to 3V; at this time, the second stage
voltage

　Class A transistor power amplifiers have a warm and sweet tone that makes people fall in love with them. However, Class A power amplifiers have a high temperature rise. If ordinary bipolar transistors are used, the temperature stability is poor, debugging is difficult, and it is also worrying during use. This article introduces a Class A power amplifier with a Sanken product SAP15N/P audio pair tube in the output stage. SAP15N/P has an output current of 15A, a withstand voltage of 160V, a power of 150W, and a junction temperature of 150℃. The transistor has a built-in temperature compensation diode array, which solves the technical problem of temperature compensation delay in the power amplifier circuit. Its temperature compensation diode is placed in the center of the transistor chip, which can quickly and synchronously detect the change of transistor temperature and correct the bias current to keep it constant. Class A power amplifiers made of ordinary bipolar transistors take a long time to stabilize due to temperature changes. It often takes half an hour for the amplifier to stabilize, and the bias current will change with the temperature. In actual use, it takes half an hour to burn in the machine before you can hear the real Class A tone. The SAP15N/P pair of tubes, due to the consistency of its internal temperature rise and compensation, the amplifier enters a stable state after a few minutes of power-on, with high efficiency, stable and safe operation. Therefore, it can be said that the SAP15N/P pair of tubes is the best partner for Class A power amplifiers. In addition, SAP15N/P also has many advantages such as good working linearity, small saturation voltage drop, and high power utilization. In addition to the temperature compensation diode, SAP15N/P also has its own final front-stage driving transistor and emitter 0.22Ω resistor, which has a high cost performance. Figure 1 is the appearance diagram and internal electrical schematic diagram of SAP15N/P. It has 5 pins. There is a PN junction diode inside its NPN side and five Schottky diodes inside its PNP side for temperature compensation. When in use, add a variable resistor between the diode of the NPN tube and the Schottky diode of the PNP tube to adjust the no-load current.

How it works

　　Figure 2 shows a power amplifier circuit using a Class A monophonic design. The power amplifier has temperature detection and protection functions, as well as features such as DC midpoint servo. Its input stage uses a common source-common base circuit composed of field effect twin tubes V1/V2 (K389/J109) and transistors V3~V6. The circuit has low high-frequency distortion, large dynamic range, and high signal-to-noise ratio. In the second voltage amplifier stage, V15 and V16 form a two-stage push-pull cascade circuit, V15/V7 and V16/V8 form ideal current sources, current amplification is completed by V7 and V8, and voltage amplification is completed by V15/V16. The speaker protection circuit uses Toshiba speaker protection integrated circuit TA7317 (IC3), in which VD6 is the shutdown noise power supply detection, connected to the AC end of the transformer. V12 and VD8 constitute the relay working power supply. IC1A in the field effect integrated dual op amp IC1 (LF353) performs DC midpoint servo to stabilize the midpoint potential; IC1B, V13, temperature detection switch SW2 and photocoupler IC2 form a temperature detection and protection action circuit. The temperature switch contact is a normally closed contact with an action temperature of 90°C. When in use, it is fixed to the heat sink with screws to detect the temperature. Under normal circumstances, the contact of the temperature switch SW3 is closed, the voltage of the ⑤ pin of IC1B is higher than the ⑥ pin, the ⑦ pin outputs a high level, V13 is turned on, its collector is low, and no current passes through the photocoupler IC2; when the heat sink temperature reaches 90°C, SW3 is disconnected, the voltage of the ⑤ pin of IC1B is lower than the voltage of the ⑥ pin, the ⑦ pin outputs a low level, V13 is turned off, its collector is high, and the current flows through the ① and ② terminals of IC2, causing the ③ and ④ terminals to be turned on. At this time, the resistor R51 shunts the excitation signal, thereby reducing the static current of the final stage, and the working state is changed from Class A to Class A and B, thereby reducing the working temperature of the transistor to ensure its safety. IC1 and VD7 also form a self-holding circuit. When the temperature protection circuit is activated, the comparison voltage of the ⑤ pin of IC1B is clamped at a low level, so that the power amplifier maintains the Class A and Class B working state. After the temperature protection is activated, the Class A working state is automatically restored after the power is turned off and then turned on. SW1 is a manual selection switch for Class A or Class A and Class B working mode, which can be set in advance. SW1 is closed for Class A amplification, and SW1 is disconnected for Class A and Class B amplification. When the light-emitting diode LED is on, it indicates that the power amplifier is working in Class A and Class B state. The temperature of the Class A power amplifier is relatively high. With temperature detection and protection, there will be no worries about "unpredictable weather". In addition, switch SW3 can select whether the final stage adopts large loop negative feedback. The power supply of the power amplifier excitation stage is provided by the ±50V power supply on the circuit board. In order to save volume and facilitate the circuit board (250×80mm) to be installed vertically close to the radiator, the final stage rectifier filter device is not on the power amplifier board, and the electrical schematic diagram of the final stage power supply is omitted in Figure 2.

Circuit debugging

　　To obtain a larger dynamic range, the input stage output current is 2.5mA, and the potentiometer R1 is adjusted to reduce the voltage on R8 to 3V; at this time, the second stage voltage

The output current of the amplifier
　　stage is about 6mA: SAP15N/P requires a current of 2.5mA flowing through the internal diode, and this 6mA current is used by two pairs of SAP15N/P (R51 and R52 have a shunt of several hundred microamperes). Due to the shunt relationship of the current flowing through the temperature compensation diodes inside the two pairs of SAP15N/P tubes, if one side is adjusted alone, a "seesaw" phenomenon will occur. When adjusting, the static current should be adjusted up or down in small amounts in turn, not all at once, and gradually make the static current of the two pairs of power tubes approach the same. The current of each pair of tubes is 800mA, which is the current required for a Class A output of 50W (50W or more is Class B), or the size of the static current can be determined according to one's own situation and the size of the radiator. After the Class A quiescent current is adjusted, disconnect SW1 and set the Class A and B working mode manually. At this time, the final stage quiescent current should drop. Adjust the resistance of the shunt resistor R51 to obtain the Class A and B bias current you need, such as 200mA, which is shared by each pair of tubes at 100mA. After the adjustment is completed and stabilized, you can actually listen to it. Everyone has a consensus on the warm tone of the Class A power amplifier. I will not render it here. I will only compare the listening with different negative feedback choices. The final stage chooses no large loop negative feedback. Because it avoids the influence of the speaker's back electromotive force on the front stage, the bass sounds heavier, very soft, and elastic, but the clarity of the mid-high details seems to be slightly inferior to that with large loop negative feedback, but it is not very obvious to distinguish. Therefore, in actual use, you can choose according to your personal preferences and different styles of music. In my opinion, music with rich bass and large dynamics is better served by using the final stage without large-loop negative feedback; string music, classical music, symphony and solo can use the final stage with large-loop negative feedback.