HK775 or HARMAN KARDON 775 commonly known as Monophonic Amplifier.
It’s produced in the 80’s, and he could produces 130 Watts of audio power at 8 Ohms and 260 Watts at 4 Ohm loads.
INFO
HARMAN KARDON in the 80’s is a luxury class amplifier because it uses more advanced technology than other similar amps. HK775 estimated production around 1980.
Dan berikut adalah penampakan isi dalam amplifier ini.
I got the file and save it HERE.
SCHEMATIC DIAGRAM
I just crop and edit this file, and only took the power amplifier section until it became a scheme like the following:
Pre-amp block in this article is a complex circuit because it contains 10 transistors supporting this circuit.
Also 1 transistor for the auto muting circuit and audio clip signal.
Its will be locked if there is a peak signal and DC voltage at the start of the amplifier when turning on.
The next level is the Voltage Amplifier Stage. Its using 2 levels of transistors ( Q409 and Q411 and Q410 and Q412 ).
This circuit also has a servo driver block, whose the component is Q413.
Its controls the power transistor ambient current by turning the trimpot VR402.
According to the service manual, the result of this trimpot rotation must be able to reach a voltage about of 36 milli Volt on the emitter resistor of the power transistor.
The amplifier driver circuit block in this circuit is unique and different from other amplifier driver circuits. It uses 8 interlocking and linking transistors.
CLICK TO ENLARGE
And finally, the final 4 pairs of transistors from Q427 to Q432 use the numbers 2SD845 and 2SB775.
NOTE
The supply voltage for this power amplifier circuit is symmetrical 58VDC.
By the voltage it its possible the amp to reach 300 Watts of power per channel.
Q420 is a PNP type transistor (Note: the schematic image is not very clear) and the number is 2SB649.
The power dissipation of the final output transistors D845 and B775 is 120 Watts. ( SOURCE ).
By the 4 pairs power transistors numbered D845 pairing with B775, it is possible for the amp to produce the audio until 480 Watts.
