Pure Class A final stage non-feedback power amplifier

This article introduces a MOS-FET output stage, a pure Class A final stage, no negative feedback power amplifier, and the output power of the power amplifier is 100W/4Ω.

Figure 1 is the circuit diagram of the amplifier, and Figure 2 is the circuit diagram of the power supply. Since the feedback circuit from the driver stage to the input stage adopts a resistor voltage divider, it is a genuine DC amplifier. In order to improve the DC stability, the amplifier increases the amount of negative feedback from the driver stage to the input stage. The negative feedback circuit consists of two resistors, 1.1kΩ and 10kΩ, and the gain of the voltage amplifier stage is about 10 times. Compared with the AC amplifier with a capacitor to form a negative feedback circuit, the DC amplifier with a resistor to form a negative feedback circuit has a better auditory effect.

1. Volume Control

In order to be able to connect directly with sound sources such as CD players, this amplifier has a volume potentiometer added to the input end. The usual volume potentiometer connection method is shown in Figure 3 (a), but this machine uses the current transmission type connection method shown in Figure 3 (b). In the circuit of Figure 3 (b), since R1 is selected to be large, the signal source including R1 can be regarded as a constant current source, and the output level can be changed by adjusting R2. This connection method is beneficial to improving the signal-to-noise ratio because the sliding point is grounded.

However, this connection of the volume potentiometer also has its disadvantages. For the signal source, the load of the signal source will change when the volume potentiometer is adjusted, and the output impedance of the amplifier will increase when the volume is increased. When the volume is reduced to zero, the input impedance of the amplifier is equal to R1. It can be seen that R1 is an important parameter and its value cannot be too small.

A smaller value of R1 is beneficial to the signal-to-noise ratio and frequency characteristics, but it may increase the load of the signal source. A larger value of R1 is beneficial to reducing the load of the signal source. After comprehensive consideration, the manufacturer of the amplifier circuit believes that it is more appropriate to choose between 5kΩ and 20kΩ. Therefore, R1 is set to 5kΩ in the circuit.

Since the volume potentiometer also uses a 5kΩ potentiometer (a larger one can also be used), the voltage gain of the amplifier itself is 10 times. After adding the sound wall potentiometer, the total voltage gain is up to 5 times.

2. Input stage and voltage amplifier stage

The input stage uses the latest low-noise pair of tubes 2SK389/2SJ109 from Toshiba. The circuit uses GR-grade tubes. BL-grade tubes can also be used, but the drain quiescent current should be appropriately increased to about 1.5mA. The drain quiescent current of this machine is 1mA.

Since the static operating current of each output tube is designed to be 0.44A, the transfer characteristics of the output tubes 2SK722 and 2SJ131 show that the bias voltage between the gate and source of the FET should be around 2.6V, and the total voltage of the upper and lower tubes is about 5V. If the voltage drop on the source resistance of the output tube of 0.22Ω is taken into account, the bias voltage of the final stage should be above 5V.

The estimation steps of bias circuit parameters are as follows: the drain current of 2SK213/2SJ76 is 25mA, and its bias voltage is about 1V. The collector current of 2SC2856/2SA1191 is 2mA. It is calculated that the source resistance of 2SK213/2SJ76 is 100Ω. For the convenience of adjustment, the gate resistance of 2SK213/2SJ76 is composed of 2kΩ, semi-variable resistor and 2.2kΩ fixed resistor in series.

3. Final MOS-FET

The final MOS-FET in the circuit was originally intended to use 2SK405/2SJ115 produced by Toshiba, but it was not available, so Sony's 2SK722/2SJ131 was used instead. You can also use Toshiba's 2SK1530/2SJ201 (the pins are the same as Sony's), or Hitachi's 2SK1056/2S1160 (the pins are G, S, D from left to right). If Hitachi's 2SK1056/2SJ160 is used, the bias voltage of the circuit is too large, and the fixed resistor between the gates of 2SK213/2SJ76 should be replaced with 1kΩ.

By the way, in order to reduce noise and distortion, the collector current flowing through Q7 and Q8 was finally changed to 1.87mA.

4. Production

The amplifier's printed circuit board is shown in Figure 4.

The input resistance can be selected between 5kΩ and 20kΩ, and the volume potentiometer can also be selected between 5kΩ and 20kΩ.

The series 1kΩ resistor of the input stage FET and gate on the printed circuit board can be selected from 100 to 1kΩ. The 220pF capacitor can be selected from 100 to 220pF. The 150kΩ can also be selected from 100kΩ to 470kΩ. If the positive and negative power supply lines are not long, it doesn't matter if the two 100uF/50V decoupling capacitors on the circuit board are not used.

The base voltage divider resistor of the common source-common base circuit has a better effect if it is larger. You can consider using 220kΩ and 100kΩ instead of 110kΩ and 50kΩ in the circuit for voltage division. In order to improve the signal-to-noise ratio, improve stability and improve CMRR, the circuit method of constant current diode and voltage regulator diode that was commonly used in the past is not adopted.

The resistor negative feedback circuit can be selected according to the needs. When 10kΩ and 1.1kΩ are selected, the amplifier gain is 10 times. When 10kΩ is replaced by 5kΩ, the gain is 5.5 times. When it is replaced by 20kΩ, the gain is 19 times. When the amplifier negative feedback is large and the gain is small, in order to prevent the amplifier from self-excitation, a 2-10pF ceramic capacitor should be connected at the position marked with * in Figure 1 for phase compensation. The amplifier gain of this machine is 10 times, and self-excitation will not occur even if the capacitor is not connected.

The 100Ω anti-vibration resistor connected in series to the gate of Q9 and Q10 can be selected from 47Ω to 220Ω. The .0047uF,

The 100Ω series circuit is used for vibration prevention, and can also be composed of 0.0022uF and 200Ω in series.

The 220Ω resistor in series in the output tube grid circuit is used to prevent self-oscillation and protect the grid. It can be selected between 100Ω and 470Ω. A larger resistance value will have a better effect, but it should not be too large, otherwise it will form an integration circuit with the grid input capacitance, resulting in deterioration of high-frequency characteristics.

The series circuit of 0.047uF and 30Ω at the output end is an integral phase compensation circuit, which can also be replaced by a circuit of 0.033uF and 10Ω in series.

The pin connection of semiconductor tubes is shown in Figure 1. 2SK146/2SJ73, 2SK147/2SJ72, 2SK170/2SJ74 can be used to directly replace 2SK389/2SJ109. Similarly, various low-noise small-signal transistors can be used to replace 2SC1775AE/2SA872AE and 2SC2856/2SA1191. 2SK213/2SJ76 can be replaced by 2SK214/2SJ77, 2SK215/2SJ78, 2SK216/2SJ79. These tubes have the same characteristics and higher voltage resistance.

In addition to the substitute tubes mentioned above, the final MOS-FET can also use Toshiba's 2SK1529/2SJ200, Hitachi's 2SK1057/2SJ161, 2SK1058/2SJ162. If Toshiba's 2SK1530/2SJ201 is used, after appropriately enlarging the heat sink, using two pairs of tubes can meet the output power requirements. Of course, the quiescent current of the tube should be 0.88A at this time.

5. Debugging

Perform debugging in the following order.

1. After soldering the printed circuit board, adjust all VRs to the middle position.

2. Check the no-load output voltage of the power circuit , which should be around ±35V.

3. Connect the power supply to the circuit board.

4. Turn on the power supply and use a multimeter to measure the voltage drop on the 750Ω resistor at the emitter of Q8. Adjust VR1 so that its voltage drop is 1.4v.

5. Connect the multimeter to terminals ④-⑦ and check whether the voltage displayed on the multimeter can change around 5V when VR3 is rotated. This voltage is the final bias voltage and should be able to change continuously between 2.5V and 6V.

6. If the previous step is normal, adjust the voltage between terminals ④-⑦ to the minimum. It is worth noting that adjusting VR1 can also change the voltage between terminals ④-⑦, but VR1 is used to adjust the static current of the second stage amplifier. VR3 should be used to adjust the final stage bias.

7. After all the above steps are completed, the final stage circuit can be connected. After the power is turned on, first adjust VR1 to make the voltage drop on the 750Ω resistor of Q8 emitter to 1.4V. Then adjust VR3 to make the voltage drop across the 0.22Ω source resistor of the final output tube to 97mV, and adjust VR2 to make the voltage at the output end of the amplifier zero volt. Since the above adjustments affect each other, they should be repeated several times. Due to the discreteness of the parameters of the final output tube, the source resistance voltage drop of the tube with the largest static working current should be 97mV during debugging, and the remaining tubes should be less than 97mV. The smallest resistance voltage drop when this machine is made is 52mV.

6. Characteristics

The residual noise of the amplifier does not change with the input short circuit or open circuit, and is about 0.4mV (flat) and 0.1mv (1HF-A curve). The frequency characteristics are shown in Figure 5, and the -3dB point is 300kHz. The damping coefficient is flat in the range of 10Hz to 100kHz, which is about 50. There is no problem with the capacitive load test using a 20kHz square wave , and no self-excitation phenomenon occurs. The harmonic distortion rate is shown in Figure 6.

Judging from the audition results, the final stage of this machine uses MOS-FET, but the sound is very similar to the final stage of the non-negative feedback power amplifier introduced earlier that uses a crystal triode. Without careful identification, it is impossible to distinguish them.