$80.00 – $380.00
ADCOM GFA-555 MK1 VERTICAL Input Board
Improved input board for early-model Adcom GFA-555 Mark 1 amplifiers with a vertically-mounted circuit board. (Later and more common models use a horizontal board, which I also sell kits for.)
**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.
Which kit should I get? There are TWO VERSIONS of the “Vertical board” GFA-555, and thus two versions of the kit.
If your amp looks like this in the back…
…then you need the “MEZZANINE board” version of the kit. This mezzanine board holds the original bridge switch in place, and allows installation of your own chassis-mount RCA jacks.
If your amp looks like this on the inside, and has two separate, widely-spaced, chassis-mount RCA jacks…
…then you need the NON-MEZZANINE version of the kit. The RCA jacks mount straight to the chassis, and the bridge switch is hard-wired into the input board, so the mezzanine board isn’t needed.
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 intermediate-level electronics project. You should have relevant 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. ($60/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.