Class AB2 Amplifier-McIntosh MC80

The output stage uses two 1614 connected into a triode for AB2 class pp push-pull (see the attached diagram for the circuit). The design working parameters are board voltage 420V, grid negative voltage -42V, static board current is about 2×42.5mA, static board power consumption is 2x17.85W. When the load impedance is 4000n, the input signal is 52Vp-p, the output power is 80w, and the maximum signal board current reaches 220mA.

The plate current variation range is more than twice. In order to reduce the power variation of the power supply voltage, B+ adopts 2x5V4 parallel full-wave rectification, and the maximum output current can reach 350mA. Its large current and low internal resistance characteristics make the power supply have good load variation stability. 5V4 is a side-heated full-wave rectifier with a high-voltage output delay function when the power is turned on. It effectively avoids high-voltage shock when the 1614 cathode does not reach the radiation temperature. The filter is a voltage input filter, which greatly reduces the output voltage change rate with the load current. The plate current of the AB2 class amplifier changes greatly with the signal amplitude. Therefore, the cathode resistor self-supply gate negative voltage method cannot be used. Even if the gate negative voltage is fixed, a low internal resistance is required. MC80 stabilizes the gate negative voltage when the gate current changes. In the circuit, 12AU7 is connected to form an automatic adjustment circuit. The cathode of the transistor VIB is negative and the gate is positive. Adjusting the gate potential can cause the VIB plate current to change. As a result of the shunt, -42V is adjusted to a predetermined value, which is equivalent to a reduction in the internal resistance of the power supply. When the output stage generates a gate current. The cathode potential of VIB has an upward trend. Because the gate potential changes slightly, the VIB shunt is reduced, and the voltage drop of the resistor 4.7kΩ is reduced, maintaining -42V unchanged. Here, VIB is equivalent to the adjustment tube of the parallel regulator. The driving power of 1614AB2 class is only 4.06Mw. The gate current is also small, and it is sufficient to use a small power triode to adjust the shunt.

In order to make the driver stage have enough driving power, 2x6S4 is used to form a cathode output class A push-pull amplifier. Because it is a cathode output, the negative feedback coefficient β = 1. The output impedance of this stage is extremely low.

6S4 was originally a vertical scanning tube for 90° deflection angle CRT in the 1950s. When the plate voltage is 250V and the grid negative voltage is -8V, it can output nearly 1W of power as a class A output tube pp push-pull, which is sufficient to drive 1614 AB2 class. The transconductance of 6S4 is S=4.5mA/V. When used for cathode output, its output impedance Eo=l/S, that is, only 220Ω. It forms a cathode output device with a gain of about 0.9. If calculated based on an output of 80W, the symmetrical driver stage of 6S4 needs a 57.8Vp-p signal to meet the output requirement of 52Vp-p. This data can be used as the basis for the design of the pre-stage gain.

To meet the AB2 class drive requirements, the 6S4 cathode uses 1:

1's coupled transformer T1, the primary is driven by the 6S4 cathode output.

In order to output about 1W of power, the cathode output device is rich, and the primary inductance does not need to be strict, but the primary winding DC resistance is required to be 350Ωx2. The purpose is to make the 6S4 24mA plate current produce a voltage drop of -8V as a self-supplied gate negative voltage. The two sets of secondary of Tl are 1614 gate circuits, and the DC resistance is required to be as small as possible. Therefore, under the condition of 1:1 turns ratio of Tl, the secondary can use a slightly larger diameter wire. This is the purpose of winding T1 into two sets of primary and secondary. The wire diameters are slightly different, and the number of turns is the same. It can be considered that their inductive reactance is similar.

According to rough calculation, the total closed-loop gain of the MC80 VI, V2, and V3 three-pole amplifiers is about 120 times, and the input signal only needs 450mVp-p to achieve full power output.

To improve the effect, MC80 uses p=0.087 current negative feedback in the front stage to reduce large signal distortion. In addition to adding β =0.02 current negative feedback to the second stage V2, a 20dB large loop feedback is added from the cathode and the 8Ω end of the secondary of T2 (the cathode resistor of the symmetrical amplifier V3 is 4.7kΩ and the two tubes share it, the signal phases are opposite, and they cancel each other out without current negative feedback).