KA550D is an audio amplifier of KENWOOD in the 80’s which is able to produce an audio output until 35 Watts at 8 Ohm load.
The Power Section of this amplifier uses 1 pair of power transistors and uses a symmetrical supply voltage of 50 VDC.
SCHEMATIC DIAGRAM
Below is a redrawn schematic of the original schematic:
From the schematic diagram, I can explain you that the circuit is consists of several circuit blocks, namely:
- Initial amplifier or preamp.
- Preamp bias regulator and DC preventer on initial start
- Voltage amplifier
- Servo Driver
- Power Amplifier
The explanation is as follows:
Preamplifier are using components Q1 and Q2 ( 2SA992 ).
Q10 acting as a preamp circuit switch.
It will delay the electric current flow to Q1 and Q2 by waiting for the DC current discharging by C5.
When the power start , a DC current going to charging to C5.
Its also discharges and flow through the base of Q10 to then conducting a current from the collector to the emitter of Q10.
The signal from the amplification of Q1 will enter Q4 for amplification of the voltage.
Meanwhile, the signal from Q2 which is a current mirror of the input signal is also amplified by Q12 through Q11.
Then Q5 is a transistor whose job is to maintain the IDLE current on the power transistor.
You can set this idle current by turning RV1.
Q6 and Q7 are transistors that serve as Driver transistors.
It amplifies the output currents of Q4 and Q5.
Because it is felt that it is still not strong enough, the current output from this driver transistor will be further strengthened by the Power Q8 and Q9 transistors.