A gallstone hybrid power amplifier using a switching power supply

The preamplifier uses a European TESLA brand low-noise high-cheek dual triode PCC88. The tube has a moderate gain (μ=33), a large anode allowable current (Ia=15mA), a high transconductance (12.5mA/V), and a low anode voltage. It can be connected in SRPP mode to obtain a voltage gain of about 25 times.

In order to improve the sound quality, reduce interference and stabilize the working point, the following measures are taken for the PCC88 filament power supply: 1. Step-down power supply. The filament voltage is reduced from the standard 7V to 6.5V, which can greatly reduce the thermal noise of the tube and increase the stability of the circuit. 2. The filament power supply is obtained by the switching power supply through LM317, and a suspended structure is adopted to avoid affecting the working state of the electron tube and the sound quality. 3. The filament potential is raised, and the anode voltage is divided by two resistors to raise the filament potential to 1/3 of the positive voltage (these two resistors also serve as high-voltage vents), which reduces the potential difference between the filament and the anode, especially the grid and cathode of the U2 tube, and can effectively reduce noise interference. 4. Use a fixed grid bias method. The -48V power supply is stepped down by a 56k resistor and an RD2.7 voltage regulator to obtain a -2.7V voltage, which is isolated and connected to the grid by a 220k resistor. The cathode is directly grounded, eliminating the problems of frequency response and distortion index reduction caused by the large electrolytic capacitor connected in parallel to the cathode resistor in the self-supplied bias method.

The current amplifier stage uses a three-stage emitter follower, setting sufficient current gain to reduce the burden on the front-stage tube. The Darlington tube structure composed of two pairs of D669/B649 can eliminate the saturation clipping caused by signal overload. When the pre-stage tube approaches saturation, the current gain will decrease rapidly, so that the final stage tube will never enter the saturation zone, achieving a "soft landing" of the signal and avoiding harsh distortion. Negative feedback is completely eliminated, and push-pull circuits are used at all levels.

The final stage uses two pairs of Toshiba Hi-Fi tubes 2SC2564/2SA1094 for parallel output. The tube fT = 90MHz, which is three times the common 2SC3280/2SA1301 characteristic frequency, with a current of 12A and a power of 120W. The driver stage uses Hitachi fever tube 25D669/2SB649, which has an fT of up to 140MHz and a power dissipation of 20W. It can output a very warm and sweet tone when paired with 2SC2564/2SA1094.

The final stage is designed as a high-bias Class A and B output, and the tube static current is set to 360mA, so that the power amplifier tube always works in the linear amplification area. At the same time, it can also obtain about 8W of Class A power (8Ω). When it is used as Class B output, the maximum output power can reach 140W!

A high-frequency switching power supply is used as the energy supply for the entire power amplifier (see Figure 2). The electronic tube power supply uses the Sanyo 83P color TV switching power supply circuit, which is slightly modified and made by ourselves. The post-stage power supply uses the Meanwell brand industrial-grade S-320-48 switching power supply produced by two major manufacturers to form a ±48V, 6.5A power supply, with a power of 320W, a switching frequency of 167kHz, and is equipped with multiple protection functions such as overcurrent, overvoltage, and overheating, and a built-in automatic air cooling system.

The line layout follows the star-shaped one-point grounding rule. Except for the patient and careful repeated debugging of the static operating point of the final transistor, the whole machine can obtain a beautiful and pleasant playback effect without too much adjustment.