GFA 555II is an audio amplifier made by ADCOM at mid of 80s. Its use 4 pair of power transistors and uses symmetrical 60 until 65 VDC Volt.
Beberapa reviewer mengatakan bahwa amplifier ini mampu menghasilkan daya audio until 200 Watts at 8 Ohm and 325 Watts at 4 Ohm load.
SCHEMATIC DIAGRAM
Here is the cropped and edited schematic diagram:
Dari skema di atas nampak bahwa Power Amplifier ini terdiri dari beberapa blok:
- Preamp
- Limiter input
- Penguat tegangan
- Servo Driver
- Driver Amplifier
- Power Amplifier
Firstly is Preamp. By using Transistor Q151 and Q152 ( all 2SC2362 ) with current stabillizer Q153 ( 2SC2240 ). The gain of the opamp is among R173: R174 or within 22x of GAIN. Use no filter like elco or mylar.
Secondly, The Limiter Input that its using IC with custom number ADCOM3A. An audio Forum saying that the OPAMP is equal with number AD711 or AD820. But however, some people say that its could be change with the OPAMp number OP97 ( READ HERE ).
Voltage Amplifier Stages. Its using Q157 ( 2SA1210 ) with Q158 ( 2SC2912 ) as the Current Mirror.
Servo driver are using the 2SC2240 transistor and its located on Power Transistor Heatsink. The transistor acting as bias stabillizer for the Final Power Transistor.
Q201 are Driver transistor in this circuit diagram. Its uses 2Sc2912 and act as darlington circuit with Q202 2SD1047. Its also same function with other Q301 2SA1210 and Q302 2SB817.
Lastly is the Final Power Transistor. Its uses number 2SD424.
NOTE
Inside the manual book, the schematic diagram are not specify the number of Voltage to Supply the Amplifier. But if you looks the Working Voltage of the Elco C802 and C803 at 100V WV, its mean the Ampli use between 60 to 80 Volt DC.
Datasheets said that the transistor could reaching the power up to 150 Watts or 100 WRMS ( DATASHEETS ).
