Introducing the Hoppe’s Brain Adcom GFA-555 soft-start power supply

Update! This product has been superseded by a new power supply!
BFA-555 Smart Soft-Start Power Supply

Exciting news! Hoppe’s Brain power supply boards for the GFA-555 are here!


These boards feature a number of improvements over the stock arrangement with the old-style chassis-mount components:

  • Soft-start circuit reduces stress on components like switches and capacitors.
  • Capacitance upgrade from 4×15,000uF to 4×27,000uF. Nearly double!
  • Earth-loop breaker with diode clamp safety shunt.
  • 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.

The power supply is available as a complete kit, supplied mostly assembled, for $375, or as an installed option with a GFA-555 refurbishing for the same price. The board comes tested and assembled except for the big power supply capacitors and one of the AC snubber caps. (The bridge rectifiers need to be soldered in place after the standoffs are drilled, and the board is test-fitted.) Web documentation provided.

20171102_221756I was really pleased with my improved power supplies for the GFA-535 and GFA-545, so next I turned my attention to the GFA-555! I wanted to take everything I’ve learned about the GFA-555 power supply, and integrate it into one circuit board.


The soft-start:

I get a lot of people asking about soft-start power supply options for the GFA-555. The 555 has a very generous power supply, with a big toroidal transformer, and 60,000uF total capacitance. It draws a huge surge of current when first switched on. You may notice your room lights dip out for a moment. This in-rush current causes arcing across the power switch contacts, and I often get 555’s in the shop with their power switches arc-welded shut. I always replace the power switch, but it would be better if it the amp didn’t burn up the switch! Most large amps have a soft-start circuit these days. I’ve implemented mine in a time-honored fashion; with a 555 timer and a relay. The soft-start circuit has it’s own linear regulated power supply. I considered using an off-the-shelf PCB-mount switching power supply, instead of this old-fashioned transformer, caps and regulator arrangement, but I was concerned about the longevity of such devices. I’m aiming to make these amps last another 30 years or more.

Soft-start circuitry. Nothing new, just your basic 555 timer with a 30A relay. When the amp is first powered on, AC current flows through the 10-ohm NTC thermistor, which starts to heat rapidly while its resistance drops. After about 1/3 of a second, the capacitors have charged to about 80%, the relay clicks in and shorts the thermistor, bypassing it. The power switch and capacitors experience far less shock.

Which leads to another benefit…

Bigger caps!

Since the power switch isn’t under so much stress anymore, we can nearly double the size of the capacitors from 4×15,000uF to 4×27,000uF. Also, I get to use the excellent Cornell Dubilier 382LX capacitors.

Earth-loop breaker:

I convert all the amps I work on to properly safety-earth-grounded-chassis appliances with a three-prong plug. Some isolation of the amp’s ground reference and the chassis is needed to prevent noise from ground loops. A common method is to insert a small resistance between the amp’s ground reference and the chassis, with a small capacitor in parallel to conduct RF. This works great, but gives no protection from power supply faults, such as a power supply wire touching the chassis. So I’ve included a bridge rectifier, also in parallel, acting as a diode clamp for safety. It does nothing in normal operation, but clamps any faults above 1V to the chassis.

Earth loop breaker with diode shunt. This decouples the chassis from the amplifier’s ground, and still provides a low impedance path to ground in case of a fault.

More on the earth-loop breaker here…

The bridges:

The original bridge rectifiers were the old-style square types, bolted to the chassis for heat-sinking, and connected to the power supply with wire, spade and ring terminals. The newer, low-profile, flat-pack style bridge rectifiers allow me to mount them directly beneath the circuit board, bolted to the chassis for heat-sinking. Wire lengths are shortened, and 8 mechanical connections eliminated.

35A bridge rectifiers are bolted to the chassis for heat-sinking.


The soft-start circuitry is designed to cope with fault conditions such as a brown-out. The circuit will operate correctly down to 75VAC without relay chattering. Chattering is limited to 0.5Hz, the time-constant of the 555. If the relay should fail, then the thermistor will simply heat up and become nearly zero ohms, and the amp will continue to operate, though with an undesirable, variable, small resistance in series with the mains. If the thermistor should fail open-circuit, the soft-start circuit will not operate, and the relay will not attempt to close the connection, causing an arc. If the thermistor should fail short-circuit, then the power switch will take the brunt of the switching, and will arc and probably burn out after some number of cycles. But it will be obvious that the soft-start is not working, as your lights will be dimming much more than when the soft-start was operating. And if by some weird chance the thermistor were to fail half-way and become high-resistance but still conducting, then the soft-start circuit might still receive enough current to operate the relay, which would then be asked to take the full brunt of the in-rush current. It’s a 30A relay, so it should be able to tolerate that for some time.
So far, I haven’t heard of any failures in the field.

The Hoppe’s Brain GFA-555 power supply works in both versions of the GFA-555; Mark I or Mark II. (But not the current “SE” models.) Price is $375, either for a DIY kit, or installed in an amp along with a refurbishing.

Thanks for reading!


Update: 09-14-2017

Here’s a board installed in a GFA-555 II. I’ve switched to WAGO cage-clamp terminal blocks. These things are awesome! It makes a super tight spring-loaded connection that will not loosen over time as the typical rising-cage clamp types can. These give a high degree of confidence in the connection, and are easier to install as well. There are little levers you push to open the clamp and insert the wire. At first glance, a spring-loaded mechanism might seem like it would be weak, but actually, these things pinch really, really tight, and are designed in such a way that they self-tighten even further if you pull on them.

GFA-555 II with Hoppe’s Brain Soft-start power supply board.

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