Pioneer SPEC-2 Service Manual - Page 6

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4. CIRCUIT DESCRIPTION

4 . 1 P O W E R A M P L I F I E R

The SPEC-2 is a high power design, employing a

cascoded differential amplifier first stage, push-pull

drive, and darlington triple push-pull circuit with

all stages direct coupled (OCL).

The circuit diagram is shown in Fig. 2. Q1 and Q2

form the differntial

amplifier. The input signal is

applied to Q1 and NFB from the output stage

applied to Q2 base. These two transistors are con-

nected in common base circuits with Q3 and Q4,

forming a cascode amplifier. Since the common

emitter circuit (Q1 & Q2) load becomes the input

of the following common base circuit stage, load

impedance is low and stable amplification can be

achieved.

Q3 and Q4 outputs drive Q5 and Q6 in opposite

phase. The Qb output via Q7 is applied to the

current mirror circuit formed by D2 and Q8, where

the phase is again inverted. Q6 and Q8 outputs

thus become signals of the same phase and perform

a pre-driver function.

The cunent mirror circuit is depicted in Fig. 1.

Current flowing through R2 and Q8 becomes equal

to that flowing through Rl and D2. In other words,

since R1 and R2 are equal, and the characteristics

of. D2 and Q8 base-emitter are also equal, the

current +BL reaching point A (in Fig. 1) through

R1 and D2 is equal to that reaching the same point

through R2 and Q8 base-emitter. This becomes

equivalent to having Q5 output directly driving

Q8.

I I I P U T H

By adopting this current mirror

circuit, when

muting relay RLb is OFF Q8 is cut off, Q10 - Q20

are switched OFF in succession, and operation

stops. The reversed phases of Q8 and Q6 outputs

cause them to cancel each other, reducing pop

noise and muting relay switching noise.

Q13 and Q14 in the following driver stage drive

the power transistors by direct coupling. The final

stage is a high output design with three power

transistors for each channel coupled in parallel.

This parallel coupling permits comparatively large

current (Ic) to be employed through each tran-

sistor, plus selection of an operating point with

good linearity and high amplification (hfe).

+ B l

l l : t 2

- llrirel

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Fig. 1 Current Mirror Circuit

Fig. 2 Power Amplifier Circuit