Hybrid 25W Hi-Fi amplifier with tone control

Although the use of electron tubes is less in modern electronic technology applications, they are still favored by people in some fields, especially audio circuits, because they have some superior characteristics that are irreplaceable for transistors. This is a hybrid amplifier composed of a "beautiful" sound tube and an audio integrated circuit. The amplifier uses a vacuum tube as the front stage and audio-specific integrated circuits AD711 and LM1875 as the rear stage. The circuit has small distortion, low output impedance, and large dynamic range, ensuring good sound quality. Because it is combined with integrated circuits, the circuit is simple and suitable for enthusiasts, especially "enthusiasts" to make their own. It can also be used as a reference for relevant technical personnel of audio companies when designing audio products.

1. Circuit working principle
The circuit principle is shown in the figure.

Only the left channel part of this circuit is drawn, and the right channel is omitted. The circuit uses a double three-pole 6N2 type tube to form a line input amplifier (half of the 6N2, VE1L, is used for the left channel, and the other half, VE1R, is used for the right channel). R2 is the DC bias resistor of the input stage. When the screen current Iao flows through R2, a DC voltage Eg of about 1.5V is generated, which is added to the gate of VE1L through the gate-drain resistor R1 to form the negative gate voltage of the line amplifier. At this time, VE1L is working in Class A status and has good linearity. Another function of R2 is to produce appropriate communication feedback to the audio source signal, further reducing distortion and further improving stability; the third function of R2 is to form pitch feedback. This input stage has outstanding advantages such as high input impedance of hundreds of kiloohms, large dynamic range, and good transient response, which is exactly what a Hi-Fi preamplifier must have.
The attenuated tone control network (TCN) is inserted between the front and rear stages. The audio signal output from the cathode K of VE2La on the SRPP circuit is sent to pin 3 of the subsequent integrated circuit IC1 through the volume potentiometer VR3; the other is fed to the cathode of the line amplifier VE1L through the TCN network. This combination can effectively suppress noise and distortion while maintaining the adjustment characteristics of the attenuated TCN.
The signal is amplified by VE1L and output from the anode, coupled through capacitor C2 to the power amplifier exciter VE2 composed of a tube 6N3 with excellent high-frequency characteristics. The two internal transistors are connected into a parallel regulated push-pull circuit SRPP. This circuit is characterized by small distortion, low output impedance, and large dynamic range, and is fully adaptable to various power amplifiers composed of IC, FET, TR, VAL, etc.
In Figure (a), capacitors C4, C5, resistors R7, R8 and potentiometer VR1 form a bass tone control network. When VR1 is raised, the negative feedback of the network composed of C5 and C4 to the low audio signal increases, and the bass is relatively weakened; conversely, when VR1 is lowered, the bass will be relatively enhanced.
Capacitors C9, C10, resistors R9, R10 and potentiometer VR2 form a high tone control network. When VR2 is adjusted upward, the amount of negative feedback of the high audio signal increases, and the treble is relatively weakened; conversely, when VR2 is adjusted downward, the treble is relatively enhanced.
In power amplifier circuits, we hope to obtain high-fidelity and high-power output. It is not difficult for general power operational amplifiers to provide larger power to the load, but most of them have the disadvantages of large distortion and poor linearity. If a precision operational amplifier IC1 with good linearity and small distortion is inserted at the front end of a high-power IC, and the power amplifier IC2 is in the feedback link of IC1, the effect of maximizing strengths and avoiding weaknesses can be achieved. This type of connection is called "turbocharged combination" (TCC). Integrated circuits IC1 (AD711) and IC2 (LM1875) form the post-stage of the TCC power amplifier. In the TCC network, C11, R12, and R13 form an RC network, which provides moderate phase compensation for the audio signal and stabilizes the frequency response areas of IC1 and IC2.
Figure (b) is the power supply circuit diagram of the whole machine. The high voltage in the front stage of the electronic tube is provided by 280V DC directly generated by the rectifier of the mains. The three filaments of the two electronic tubes are connected in series and powered by a set of AC 18V, which makes the circuit much simpler. Another set of AC 18V is rectified by bridge, C15, C16, C17, and filtered to generate ±25V to power IC1 and IC2.

 

 

2. Selection of components:
choose 6N2 for VE1, 6N3 for VE2, AD711 for IC1, LM1875 for IC2, 70VW series for low-voltage filter capacitors C16 and C17, CD17H series for high-voltage filter capacitor C19, polyester capacitor for C15, and LM1875 for C18. Polypropylene capacitors, select tantalum electrolytic capacitors for C6 and C8, and select CD03HV high-voltage electrolytic capacitors for C12 and C13. All resistors use metal film series. The potentiometer is selected from the KK210 series. Component parameters are selected based on the circuit diagram annotations.

3. Production and debugging methods:
Select components as required and install them correctly, and you can achieve success in one go without debugging. The electronic tube should be installed using an electronic tube holder, and the integrated circuit should be as far away from the electronic tube as possible to avoid overheating of the integrated circuit. After the circuit is installed, it should be installed in a chassis with heat dissipation holes, and the volume potentiometer and high and low tone potentiometers should be placed on the chassis panel for easy use and adjustment.