ADCOM GFA-555 MK2 Input Board


ADCOM GFA-555 MK2 Input Board

SKU: adcom-gfa-555-mk2-input-board


Improved input board for the Adcom GFA-555 Mark 2.

**Not for currently produced GFA-555 “SE” or “MS” models. MK2 amps are labeled “model GFA-555 II” above the power switch. MK1 amps can be identified by the lack of a “Thermal Protection” LED on the front panel. MK1 board is available here.

Legend has it that the “GFA” in GFA-555 stands for “Good effing amplifier”. Ahem… With this improved board, model BFA-555 II, you will have a “Better effing amplifier”. See what I did there?


  • Two-layer circuit board allows for shorter, wider traces, better spacing between conductors, and better component placement. A ground plane covers the entire top surface.
  • Clear component labeling. Lots of thought went into making component placement as confusion-free as possible. All components are labeled with their value and type of component. Inputs and outputs are labeled, as are terminals for LEDs and thermal breakers.
  • Heatsinks are fitted to the TO-126 driver transistors. This addresses an occasional failure point for the GFA-555. These transistors run a little hot, and broken solder joints can develop due to heat cycling. The heatsinks keep the transistors quite comfortable.
  • Local power supply bypass capacitors built in. These added capacitors reduce noise on the power rails, and deliver fast, high-frequency currents to the input board than cannot be provided by the main power supply capacitors, which reside at the end of long supply wires.
  • Top-shelf parts from On Semi, Vishay/Dale, TE, Kemet, Panasonic, WIMA and Cornell Dubilier.
  • 0.1% tolerance resistors are used in the signal path anywhere they would affect gain. Channel matching should be within 0.01db.
  • The complete kit also includes two pairs of matched input transistors. For best thermal tracking, these transistors are stuck together with thermally conductive epoxy, wrapped in copper foil tape and heat-shrink. They are positioned face-to-face to make this easy.
  • NEW: Includes new bias transistor cable assembles.

Note: This is an advanced-level electronics project. You should have some experience repairing audio amplifiers. I can only provide limited technical support, answering questions about the board itself, to clear up any confusion about what parts go where and such, but if you need help actually troubleshooting the amplifier, I need to charge my usual hourly rate($75/hr) DIYAUDIO.COM is a great resource, and there are many threads about the GFA-555. Before purchasing a kit, please review the assembly notes to be sure this is a project you’re comfortable with.


Hoppe’s Brain BFA-555 MK2 Assembly Notes

Parts List

BFA-555 MK2 Schematic Diagram

Additional information

Weight N/A
Dimensions N/A

2 thoughts on “ADCOM GFA-555 MK2 Input Board

    1. I wouldn’t recommend the GFA-555 for this task. It would probably survive, but the it’s just not designed for this kind of load. The power supply would sag like crazy. Better to have a lower voltage amp with more current, for the same power rating.
      The GFA-565 might fit the bill though! I had a customer who had a pair of Eminent Technology LFT-6’s, and these are also transformerless planars, with some crazy low-impedance dips, though not as extreme as the Scintilla’s. This customer hadn’t found an amp that could drive them decently until this pair of GFA-565’s I built. The 565 is totally unbothered by a 1 ohm impedance dip. There’s 20 Output transistors!

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