New Adcom GFA-555 MK1 Input boards!

By popular demand, I have created new input circuit boards for the GFA-555 MK1! These boards integrate all of the improvements I normally make when I refurbish a GFA-555.

Buy one here.


  • 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 jumpers for LEDs and thermal breakers.
  • Heatsinks are fitted to the TO-126 driver transistors. This is an occasional failure point for the GFA-555. Broken solder joints can develop due to heat cycling. The heatsinks keep the transistors quite comfortable.
  • Local power supply bypass capacitors built in. These additional capacitors reduce noise on the power rails, and provide faster current delivery to the input board than can 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.
  • Full kit also includes two matched pairs of input transistors. Matched transistors pairs that make up the input stage should be thermally bonded with paste and heat-shrink. These transistors have been located 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 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.

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.

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