Description

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. Send me a message if you’re not sure which board you need.

**Not for currently produced GFA-555 “SE” or “MS” models, or newer GFA-555 MKII models from the early 90s. MKI 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 MKII is labeled “model GFA-555 II”.  MKII board is available here.

Later rack-mount 555’s used either the most common horizontal board, or the vertical board, both of which I sell.

The oval cut-out left in the chassis from the removal of the original RCA jacks, leaves a technician without anywhere to mount the RCA jacks, so this kit also includes an RCA jack adapter plate, and a pair of gold-plated RCA jacks. Thus, the price is a little higher than the regular GFA-555 board.

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?

Features:

• 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.

Support:

This is an advanced electronics project. You should have some experience repairing audio amplifiers. Please read through the documentation before ordering.

30 minutes free tech support is included with every purchase. Time beyond 30 minutes is billed at $75/hour. This includes time spent researching your issue, writing emails and talking on the phone.

I am happy to provide technical support, but please anticipate paying for for my time as part of the cost of your project. Tech support time is always billed, whether the issue is yours, mine or one of my suppliers.

Alternately, consider asking your question on a forum such as DIYAUDIO.COM. Send me a link and I may comment on the thread.

DOCUMENTATION:

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

Parts List

BFA-555 MK1 Input Board Schematic Diagram