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

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

Hoppe’s Brain GFA-555 power supply installed in GFA-555 MK1. Compact layout allows the toroidal power transformer to be moved to the right side of the chassis, where it induces less hum, buzz and noise into the amp’s circuitry.

These boards feature a number of improvements over the stock arrangement:

  • Soft-start circuit reduces stress on components like switches and capacitors.
  • Reduced hum, buzz and noise from the amplifier’s speaker outputs.
  • Capacitance upgrade from 4×15,000uF to 4×22,000uF.
  • Earth-loop breaker with diode clamp safety shunt.
  • Surge protection MOV
  • Snubber capacitors across AC side of bridge rectifiers
  • Compact layout. Overall wiring and clutter reduced.

Hoppe’s Brain GFA-555 power supply. The toroidal power transformer is moved from the center to the right side of the chassis, where it induces less noise into the input board. Hum, buzz and noise at the speaker outputs is reduced dramatically!

I was really pleased with my improved power supplies for the GFA-535 and GFA-545, so I thought I would turn 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.

Like the GFA-535 MKI, the GFA-555 MKI is not the quietest-running amplifier in the world. You can hear a buzz if you put your ear close to the speaker, or if the room is very quiet. This is worse with efficient speakers, or for people using the GFA-555 MKI for near-field studio monitoring. I know this is pretty nit-picky, but I suspect the noise has an effect on the amp’s overall sound quality, so I think worth picking the nits.

The source of the noise is no secret, and not unique to Adcom. It’s due to the proximity of the power transformer’s magnetic field, to the sensitive circuitry on the input board. Here’s the original layout, with the old-style chassis-mount capacitors and bus-bars.

Adcom GFA-555 OEM power supply layout. Toroidal power transformer is located near the input board, where it induces noise into the amp.

Interestingly, the transformer can be rotated in such a way that one channel can be made dead quiet, while the other channel gets really buzzy. Rotate the transformer the other way, and the effect is reversed. This is due to a null point in the transformer’s magnetic field, and its alignment with sensitive components on the input board.

So, the transformer must be rotated into a compromised position, where each channel buzzes just a little. It comes adjusted this way from the factory. (Sometimes not so well adjusted.)

It’s always bugged me that I couldn’t make both channels nice and quiet.

With the compact layout of the new board, I can move the transformer to the right, and further away from the input board. And both channels go dead quiet!

Hoppe’s Brain GFA-555 power supply.

Check out these FFT’s of the noise coming out the speaker terminals. These are the “before and after” results of the the same amp; First with the OEM power supply layout, (But with new capacitors) versus the Hoppe’s Brain power supply.
The Hoppe’s Brain power supply is shown in green, overlayed and shifted to the right of the OEM supply shown in white.

Caveat: Even though the only physical variable in these measurements is the OEM power supply versus Hoppe’s Brain power supply, the measurements were taken two days apart, so there will be considerable margin of error, and values should not be interpreted as highly precise. However, the improvement is so vast and obvious, even to the naked ear, that it swamps any possible deviations in my power line.

Noise at LEFT speaker output, inputs shorted. OEM in white, Hoppe’s Brain in green.

It’s a little hard to see detail with my messy overlay, so here’s the individual graphs.


Noise is way down across the board! At 60Hz it’s down 10db in the left channel and 14db in the right. The improvement is even more marked at the higher harmonics, well into the high 20db’s!

Hooked to speakers, the difference is obvious. This amp is dead silent! I have to put my ear right up against the cone of the B&W 602’s at my workbench to hear even the faintest noise.

Note: The GFA-555 MKII has a metal can shielded toroidal power transformer, and is super quiet to begin with, but all the other benefits of this board apply.

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 huge amounts 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 welded shut. I replace the power switch on all the amps I refurbish, 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 regulated power supply.

Soft-start circuitry. Nothing new, just your basic 555 timer with a 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%, and then 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 up-size the capacitors from 4×15,000uF to 4×22,000uF. Also, I get to use CDM 382LX capacitors, which are excellent!

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 Hoppe’s Brain GFA-555 power supply is available for both versions of the GFA-555; Mark I or Mark II. Price is $375 along with a refurbishment. For the DIY’er, a mostly-assembled kit with all parts and instructions is $300.

Thanks for reading!


Update: 09-14-2017

Here’s a board installed in a GFA-555 II.



2 thoughts on “Introducing the Hoppe’s Brain Adcom GFA-555 soft-start power supply

  1. Can this be done as with the other 555 mkI where you made it a dual mono?
    Would it require two boards along with and two stacked torroids?

    so cool.


    1. Sure can! It would still use one power supply board. The two transformers are simply wired up with common center taps to both channels. Each transformer still has it’s own bridge rectifier and caps. All they share in common is that one star ground point.


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