ADCOM GFA-555 MK1 Input Board for EARLY Rack-Mount Models


ADCOM GFA-555 MK1 Early Rack-Mount Input Board

SKU: ADCOM GFA-555 MK1 Input Board for EARLY Rack-Mount Models


Improved input board for somewhat rare, very early-model Adcom GFA-555 Mark 1 amplifiers. If your back panel looks like this, with the oval RCA jack cutout, then this is the board you need.

Later rack-mount 555’s used either of the two other board styles I offer, the most common horizontal board, or the vertical board.

Because of the oval hole left in the chassis from the removal of the original RCA jacks, this kit also includes an RCA jack adapter plate and a pair of gold-plated RCA jacks. Thus, the price is higher than the regular GFA-555 board.

**Not for currently produced GFA-555 “SE” or “MS” models, or newer GFA-555 MK2 models from the early 90s. MK1 amps can be identified by the lack of a “Thermal Protection” LED on the front panel. They are also simply labeled “model GFA-555” whereas the MK2 is labeled “model GFA-555 II”.  MK2 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, 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 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.

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 linked below, to be sure this is a project you’re comfortable with.


Hoppe’s Brain BFA-555 MK1 EARLY Rack-Mount Assembly Notes

Parts List

BFA-555 MK1 Input Board Schematic Diagram


Additional information

Weight N/A
Dimensions N/A

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