Adcom GFA-555 MK1 and MK2 Soft-Start Power Supply Upgrade Kit


Available on backorder

SKU: adcom-gfa-555-mk1-and-mk2-soft-start-power-supply-upgrade-kit Categories: , , ,


Power supply upgrade kit for Adcom GFA-555, MK1 and MK2. (Not for recent “SE” or “MS” models)

More details here at this blog post.

Kit arrives mostly assembled, as seen in the first picture. You’ll install the big capacitors in a later step.

*Please allow up to 7 days for shipping. They are hand-built to exacting standards, and I don’t always have one ready on the shelf.

Installation requires the amplifier to be fully torn down to the bare chassis and re-wired. It’s a sizable project with a rewarding result!


  • Soft-start circuit reduces stress on components like switches and capacitors.
  • Improves the amplifier’s signal-to-noise ratio, reducing hum, buzz and noise from the amplifier’s speaker outputs. (Especially on original MK1 amps, due to relocation of transformer.)
  • Capacitance upgrade from 4×15,000uF to 4×27,000uF.
  • WAGO Cage-Clamp terminal blocks provide a spring-loaded, gas-tight connection that will never come loose.
  • Earth-loop breaker with diode clamp safety shunt. (Reduces ground loop hum and buzz often caused by connecting multiple 3-wire appliances.)
  • Amp is converted to a 3-wire, earth-grounded chassis appliance, for safety and better noise rejection.
  • Surge protection MOV
  • Snubber capacitors across AC side of bridge rectifiers
  • Compact layout. Overall wiring lengths reduced, fewer mechanical connections, and a much cleaner internal layout.
  • Designed to cope with fault conditions such as a brown-out. Soft-start circuit will operate correctly down to 75VAC without relay chattering.
  • This kit may be used with other amplifiers, besides the Adcom GFA-555, but it is up to you to figure out if it will work.

The soft-start circuit consists of a 10-ohm NTC thermistor in series with the incoming AC. After 1/2 second, a high-current relay shorts across the thermistor.

There are no cost-saving shortcuts in this design. The GFA-555 is a famously reliable amplifier. Many examples are still working basically fine after 33 years, so the soft-start relay driver circuitry must also be this reliable. Thus, the board features a traditional transformer power supply, feeding a 12V linear regulator, instead of a cheap off-the-shelf switch-mode power supply module. The relay is driven by a good-old NE555 timer IC, and the drive current of the 555 is isolated from the relay load by an open-collector switching transistor. This circuit should out-live the amplifier, and probably yours truly.

Please read these assembly instructions before you order, so that you are comfortable with the scope of the project. This involves working with AC mains voltages, so please know what you’re doing, and be safe!

Tools required:

You’ll need a decently equipped electronics laboratory, with the usual array of test gear like an oscilloscope, multimeter, signal generator, and maybe a distortion analyzer. There is some drilling of the chassis.

Specifically, you’ll need:

  • A really good, regulated temperature soldering station. The board uses very wide copper traces that suck heat away from the iron tip. A weak soldering iron will not be able to fully wet and flow solder into the the capacitor terminal pads. Use a large tip for soldering the big caps.I recommend a soldering station as least as good as the Hakko FX-888.
  • 5.5mm nut-driver or socket for the bridge rectifier mounting bolts
  • 1/8″ drill bit for mounting board stand-offs
  • MK1 only: Stepper bit to drill a 7/16″ hole, for re-locating transformer mounting bolt.
  • MK2 only: 3/16″ drill bit for re-locating four transformer mounting bolts.
  • Small blade screwdriver for pushing levers on the WAGO cage-clamp terminals.
  • A variac to slowly ramp up power supply voltage is highly recommended. It prevents expensive mistakes.

Additional information

Weight 4 oz
Dimensions 13 × 9 × 9 in

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