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 the newer Mark 2, “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”)

*Does not exactly fit the earliest versions of the 555 with the vertically mounted input board. It’s possible to adapt, but that would be up to you.

Legend has it that the “GFA” in GFA-555 stands for “Good effing amplifier”. Ahem… With this improved board, 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. These transistors are thermally bonded copper foil tape, paste and heat-shrink. They are positioned face-to-face to make this easy.
  • Additional parts are included to address common reliability issues with the 555. The kit is intended to make the amplifier reliable again for another 30 years or so.
    • The original driver transistors on the output modules are known to blow out on occasion, due to insufficient reverse breakdown voltage. To correct this, the kit includes four On Semi driver transistors, MJE15032/33, 250V, 8A.
    • Also included is a new power switch, exactly the same one as original. Due to the 555’s large power supply, this switch is known to spark on power-up, and eventually burns out or welds shut.  The Hoppe’s Brain GFA-555 soft-start power supply resolves this issue permanently as well as improving amplifier performance.

Note: This is an intermediate-level electronics project. You should have some experience repairing audio amplifiers. I can provide only 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($60/hr) DIYAUDIO.COM is a great resource, and there are many threads about the GFA-555. Before purchasing a kit, please review the instructions to be sure this is a project you’re comfortable with.

You might be wondering, “Couldn’t I just order a bare board, and buy the parts myself and save money? Or re-use some of my original parts?” Yes, absolutely! In the spirit of DIY, I offer this option for people on a budget for whatever reason. Buying parts from me at a markup supports Hoppe’s Brain, and I thank you. I am not getting rich on this. :^)

Parts list here.

Additional information

Weight N/A
Dimensions N/A

Complete Kit, Board Only

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