Adcom GFA-545 MK2
Adcom GFA-545 MK2 Power Amplifier The GFA 545 MK2 is nothing special as far as the circuit goes. It uses all bi-polar transistors, not a Jfet in sight. The input differential is a conventional bi-polar pair driving a single transistor in the VAS (Red line). No darlingtons are used for this. After the VAS comes a Pre-driver followed by the main driver which in turn drives 3 Sanken output transistors, namely 2SC3856 and 2SA1492. This arrangement is commonly known as a 3-stage complementary output. The main advantage is it provides a very high current gain to drive the loudspeakers and it does this without “loading” the VAS. How does she compare to the GFA-545
Fig 2 – Adcom GFA-545 Amplifier Schematic Sound wise, they are the same.The GFA545 (Fig 2) and the GFA545 mk2 shares the same circuit. However, in the MK2, Adcom incorporated some refinements. Most notable is a op-amp Servo used for DC Offset. The other is a thermal controller using a transistor whereas the earlier 545 uses a thermistor at the base of the positive driver. in the MK2, the added a fuse after the thermostat which I believe is mounted onto the heatsink. When there’s a major failure, the fuse blows, thus shutting down the VAS and all others downstream. And lastly, the 545 uses only one transistor for driver. This is where the MK2 will likely sound better. Summary I have not had the chance to listen to my GFA 545 MK2 because one channel is down. There appear to be some burnt marks around the Servo op-amp. Worse than that, someone tried to repair the amplifier but failed miserably. I am hesitant to repair the amplifier because I really don’t want to waste my time sorting out other peoples problems. If I decide to work on my GFA 545 MK2, I will probably modify the circuit. A new pcb will obviously need to be fabricated but that’s not an issue. I may even change the outputs to the Sanken 2SC2922 and 2SA1216. That’s a tempting thought. |
August 1, 2025Amplifiers, Electronics
