Dynamic Class A bias circuit with easy adjustment

Dynamic Class A with Easy Adjustment Bias Circuit Class A amplifiers are highly praised by audiophiles for their mellow and sweet sound, but their low efficiency, large heat sink and high cost make audiophiles stay away from them. Dynamic Class A amplifiers are closest to Class A amplifiers in terms of sound quality, high efficiency, low power consumption at small signals, and smaller heat sink than Class A. It should be said that dynamic Class A amplifiers are a good choice for audiophiles. The most critical circuit in dynamic Class A amplifiers is the bias circuit. Figures 1 and 2 are traditional dynamic bias circuits. In Figure 1, the circuit works in dynamic Class A only when the signal is in the positive half cycle, and the bias is only...

Dynamic Class A Bias Circuit with Easy AdjustmentClass A amplifiers are highly praised by audiophiles for their mellow and sweet sound, but their low efficiency, large heat sink and high cost make audiophiles stay away from them. Dynamic Class A amplifiers are closest to Class A amplifiers in terms of sound quality, high efficiency, low power consumption at small signals, and smaller heat sink than Class A. It should be said that dynamic Class A amplifiers are a good choice for audiophiles. The most critical circuit in the dynamic Class A amplifier is the bias circuit. Figures 1 and 2 are traditional dynamic bias circuits. In Figure 1, the circuit works in dynamic Class A only when the signal is in the positive half cycle, while the bias is only a constant voltage bias in the negative half cycle. In Figure 2, there are 4 resistors that need to be adjusted, which is difficult to adjust. The dynamic Class A bias of the Japanese JVC machine has a relatively complex circuit and is difficult to make. Figure 3 is a new dynamic Class A bias designed by the author based on Figures 1 and 2. It has only 2 adjustment resistors. After experiments, the circuit is easy to adjust and the effect is relatively ideal.

In the circuit, R5 is used to adjust the static working current of the final stage, and R6 is used to determine the signal current value I for the dynamic bias to take effect. Once the instantaneous signal current exceeds the I value, the dynamic bias takes effect immediately. In the positive half cycle of the signal, if the instantaneous signal current is less than the I value, the dynamic bias does not work; the same is true for the negative half cycle. If the instantaneous signal current is greater than I, part of the current is injected into the emitter of V3 through R1, V3 is turned on, the resistance between the e and c electrodes of V3 decreases, the voltage drop on R3 decreases, the voltage between the e and b electrodes of V1 decreases, the conductivity of V1 decreases, resulting in an increase in the voltage between C and D, and an increase in the final current. The same is true for the negative half cycle of the signal.

When making, V1, V2, V3, and V4 use C2240/A970 tubes or C1815/A1015 tubes, and R5 and R6 are precision adjustable resistors of 2kΩ and 330kΩ respectively, R1=R2=1kΩ?R3=R4=510Ω. The resistance value can be adjusted appropriately according to the specific situation. When testing the machine, first adjust R5 so that the final static current is about 100mA. Then connect the amplifier to the sound source, turn the volume to about 1/4, adjust R6 to the maximum, and then write down the current value range of the final stage; play the same piece of music, and slowly reduce R6 so that the current of the final stage is significantly larger than the resistance value range of the previous time. The current value of the latter time can be determined according to the size of the power amplifier heat sink. If it is too large, the dynamic current of the final stage will sometimes be very large, and the heat will increase dramatically. The author adjusted it to about 125mA. Readers with conditions can use a signal generator instead of a sound source for precise adjustment, and input a constant signal to the amplifier. Another point worth noting is that if the final stage uses a field effect tube for power amplification, care should be taken to prevent self-excitation.