Adcom GFA-555 MK1 Input Board


Adcom GFA-555 MK1 Circuit Board Kit

SKU: gfa-555-mk1-input-board Categories: , ,


Improved input board for the original Adcom GFA-555 Mark 1.

**Not for currently produced GFA-555 “SE” or “MS” models. 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.

Does not fit the earliest versions of the 555 with the vertically mounted input board. MK1 “Vertical board” is available here.

Also does not fit some early rack-mount versions of the 555 that have an oval cutout around the RCA input jacks. Early Rack-Mount GFA-555 boards are 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.

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 Assembly Notes

Parts list

BFA-555 MK1 Schematic Diagram

Additional information

Weight N/A
Dimensions N/A

2 thoughts on “Adcom GFA-555 MK1 Input Board

  1. I am looking forward to order One unassymbled Input Board gfa555 from you .I’m not sure if it’s a Mk1 or 2 have to peak inside I used to be stationed in Germany
    88 to 92 and stayed after I got out . I am owner of 2 gfa555 So this will be a transAtlantic order . I do have PayPal but no popular Credit card., I hope that’s fine with you. Hope to hear /read from soon.
    SY Denis

    1. Hi Denis! Yes, have a look inside and keep in mind that if you have a MK1, there are actually three different versions of that board and I sell them all.

      Yes I can do international shipping. Please send me your shipping address here and I will determine the shipping cost. Don’t order from my store, I’ll send you an invoice instead, and yes I take PayPal.

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