High-fidelity power amplifier quasi-complementary symmetry (OCL) circuit

▲ Circuit diagram principle

Circuit structure: T1 and T2 are differential amplifier input stages, T3 is a common emitter amplifier stage and T4~T7 form a quasi-complementary power output stage.

R7, D1, and D2 provide bias voltage for T4~T7 tubes to overcome crossover distortion.

R1 and R6 form the base current loop of T1 and T2 tubes respectively, and R6 forms DC negative feedback to stabilize the static operating point of the entire circuit. R6, C3, and R5 form AC voltage series negative feedback, which stabilizes the voltage amplification, increases the input resistance, decreases the output resistance, and reduces the nonlinear distortion.

C6 and R9 are bootstrap circuits, which are designed to fully utilize the final stage complementary symmetrical tube in order to increase the amplitude of the collector output voltage of the T3 push stage.

C5 plays the role of frequency compensation and eliminating self-oscillation; R14 and C7 are used to overcome the influence of the inductive load in the speaker and make it close to pure resistance to protect the output power tube and also help to avoid self-excitation.

In order to get the output stage to a suitable operating point, R10 and R11 simultaneously reduce the penetration current of T6 and T7 respectively, and increase their breakdown voltage value.

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▲Circuit adjustment

The static potential at point A at the output end of the OCL circuit should be zero.

If VA≠0, R2 should be adjusted. If VA>0, R2 should be adjusted smaller.

When adjusting the static potential of the output end, the speaker should be disconnected from the circuit and replaced with a dummy load to avoid burning out the speaker when VA≠0

When a sinusoidal signal is added to the input end, if the output waveform produces cross distortion, the R7 potentiometer should be adjusted to the minimum first, and then gradually increased to make the cross distortion just disappear. If R7 is too large, the current of T6 and T7 may be too large and the tubes may be burned out. When VA=0, adjust R7 to make VA≠0, so R7 and R2 should be adjusted alternately and repeatedly until VA=0 and the cross distortion is just eliminated.