Thanks for buying a power supply kit from me! I think you’ll find it’s a fun project with very satisfying results. Take your time, inspect your work as you go, and you’ll get a great result. As they say in woodworking, “Go slow, it’s faster!”

The board:

Tools required:

You should have a reasonably well equipped electronics lab, with the usual assortment of hand tools, meters, scopes etc. Specifically, you will need:

  • A powerful, regulated-temperature soldering station: 50W or better! e.g. Hakko FX-888. This board uses double-thick 2oz copper, and large polygon pours that rapidly wick away heat. Some of these polygons cover both layers. A weak soldering iron will make poor solder connections, and be frustrating to use. Components can be overheated due to the long time you need to hold heat to the work.
  • A large soldering tip for heat-sucking areas like terminal blocks, relay contacts and the power resistor.
  • A normal soldering tip for soldering SMD components.
  • A tweezers for placing SMD components.
  • Liquid or paste flux.
  • 5.5mm and 7mm hex nut drivers.
  • 2mm hex driver
  • Solvents and tools to clean circuit boards; Alcohol and a brush, plus compressed or canned air to blow the solvent away. I use an ultrasonic cleaner with alcohol. All components can be submerged except the power transformer.

The schematic:

Hoppe’s Brain BFA-555 Smart Soft-Start Schematic

The Board Layout:

Hoppe’s Brain BFA-555 Smart Soft-Start LAYOUT.

The Spice:

If you want to play around with the circuit and see how it works, here you go! Keep in mind, there is no need to adjust timing or voltage thresholds in this design. Even if you install absurdly large filter capacitor banks, the timing does not need to be adjusted, as the timer to close the inrush relay will not even start until the capacitors are approximately 70% charged, and then the relay doesn’t actually close for another second. The relay will under no circumstances, close unless the current through the inrush resistor is low.

Hoppe’s Brain Smart Soft-Start SPICE model.

You may copy and adapt this circuit for your own purposes, but please don’t try to copy my board design.

Power Supply Capacitor Selection:

The board is deigned to accept a wide variety of 2.5″ or 65mm capacitors. Use your existing, original capacitors, or select one of the recommended capacitors, or use something you have in your own parts bin!

Original Adcom Capacitors. Free!!
The original capacitors on a MK1 amplifier could be as old as 1985 and as old as 1990 in a MK2. Despite their age, most are actually just fine. I have only ever encountered a few bad power supply capacitors in GFA-555 amps. These capacitors, due to their large physical size, are actually under very little stress in an audio power amplifier. I would guess they are likely to last another decade or more, but you never know. The consequences of one going bad are not usually catastrophic anyway.

New Capacitors:

Available from Digikey, Mouser and others.

Kemet ALS70 36,000µF $37.33
Best value IMO.

Vishay BC Components MAL210119223E3 22,000µF
Formerly Mallory brand. Excellent, proven formulation.

Kemet ALS30A223MF100 22,000µF

United Chemicon U36D Series. (Part numbers starting with E36D.)
Mechanical clearance issue.
These are good quality, excellent value for money, and would work fine, except their terminal posts do not stick up out of the capacitor body far enough to give good clearance to the underside of the board. Pins and wires may poke into the plastic insulated wrapping. You could add spacer washers to the posts if you really wanted to make them work.

Kemet PEH Series: Same physical clearance issue.


If you have your own capacitors already, or want to use something
other than the options listed above, they must meet these physical criteria:

Capacitors must be tall enough so that the bottom of the board clears the top of the power transformer by 11mm. This gives enough room to install the board support feet, which should be adjusted to press gently on the transformer. Very tall capacitors may require some wires
be extended to reach the terminals.

GFA-555 internal case height is 166mm.
Height of MK1 transformer is 58mm.
Height of MK2 transformer is 76mm.
If you plan to utilize the internal fuseholders on GFA-555 MK2, they will be the highest point above the board at 48mm tall.
If you do not plan to utilize the internal fuseholders, or have a GFA-555 MK1. (Fuseholders unused as they are already internal as OEM.) then the bridge rectifier heatsinks will be the highest point at 39mm. 2mm is sufficient clearance with the top cover.

Ergo, capacitors heights, measured from the floor of the chassis to the tops of the screw terminals must be between these values:

MK1: 68-124mm
MK2 without internal fuseholders installed: 86-124mm
MK2 with internal fuseholders installed: 86-114mm

Capacitor body diameter should be between 63.5 and 66mm, or 2.5″.
Terminal spacing should be 28-28.6mm, or 1.125″
Capacitor terminal posts must be at least least 4mm high to allow clearance beneath the board. (Most are around 5-8mm.)

Component placement
Start with SMD:

Generally speaking, when building any sort of circuit board, you start with the smallest and lowest-profile components. So we’ll start with the SMD capacitors and bridge rectifier.

New to soldering SMD components? Check this tutorial video. Or this one that goes into more detail.

How I do it:

  • Tin one component pad, leaving a small blob of solder on it.
  • Apply flux liberally over the whole component outline
  • Grab the component with the tweezers
  • Melt the blob of solder and place the component down onto it
  • Solder the other pad.

Solder your small through-hole components next

Do the resistors, transistors, diodes, 556 timer IC, and the opto-couplers.

  • Pay careful attention to chip orientation! The Opto-Couplers have a dot next to pin 1 that corresponds to a dot on the board. The 556 timer chip has a notch cut by pin 1. The IRFD9020 MOSFET does not have a dot anywhere, but the dot should be on the gate pin. Orient the device so that the writing is right-side-up with the board in front of you as in the photo above.
  • Zener diodes: There are four zener diodes—two pairs—of two different voltages.
    • ZD1 and ZD2 are used to monitor the voltage—and thus current—across the inrush power resistor. If the voltage across the resistor is low enough, then the timer to close the soft-start bypass relay begins. If you have 120V mains AC as in North America, these diodes should be 56V. If you have different AC mains voltage where you live, install Zener Diodes with these voltage ratings:
      100VAC: 47V
      120VAC: 56V
      200VAC: 91V
      220VAC: 100V
      240VAC: 110V
      Use 1W or greater zener diodes.
    • ZD3 and ZD4 are 130V. These diodes detect if the incoming AC supply voltage is high enough to operate the relays with sufficient force. If AC supply voltage is greater than 95VAC, (134V peak) then the timer to close the power relay begins. Note these diodes should be 130V types regardless of AC mains voltage. (They sit across one of the 115V primaries of the soft-start power transformer, and so should see between 100-120VAC no matter what the mains voltage is.)

Place power regulator:

Apply thermal paste to the back of the regulator, and attach it to the heatsink with the supplied #6-32 screw with integrated lock-washer. There’s no need for a thermal pad. Solder the regulator and heatsink in place.

Place the bridge rectifier:

Apply thermal paste to the back of the bridge rectifier, and attach with the supplied M3x12mm screw and lock-washer. Solder the heatsink mounting pins. (photo shows the bridge snub cap already installed but you should install the rectifier first so that you can touch-up the solder connections from the top.)

Set the soft-start power transformer voltage:

The board supports mains voltage from 100VAC to 240VAC. Voltage selection for the soft-start power supply is performed by forming solder bridges across pads on the underside of the board. (Note this does not affect the main transformer primary wiring.)

It takes a lot of solder to bridge these pads! This is intentional, as I am maintaining a minimum 2mm creepage distance between live AC traces.

Install the soft-start bypass jumper.

Insert the two pins into the jumper first, so that they are held in alignment.

Then, solder in place.

Now, pull the jumper off and reinstall it on one pin only.

This is the normal position, and allows the soft-start to operate. If you are testing the amp on a variac, install this jumper to bypass the soft-start. The jumper shorts the incoming LINE terminal, directly to terminal 1 “Switched HOT OUT”.

Install the inrush power resistor:

First, attach the thermal cutoff switch to the inrush power resistor. If this resistor gets too hot, over 130F, the whole soft-start circuit is disabled until things cool down.

  • Bend the leads of the thermal cutoff 90 degrees as shown.
  • Apply thermal paste to the bottom of the thermal cutoff
  • Using a M3 hex drive machine screw, a lock washer, and the 10mm hex stand-off, attach the thermal cutoff to the bottom of the inrush resistor.
  • Attach the 11mm hex standoff to the other end of the resistor.

Make two short pieces of the supplied solid 16ga stranded wire. Strip 1/4″ insulation, insert the wire and fold it over sharply.

Solder in place.

Push shrink-wrap over exposed metal.

And apply hot air.
Strip insulation, leaving about 9mm from below the bottom of the resistor.

Attach the resistor from below with two M3 screws.

Don’t forget to solder the leads of the thermal cutoff! (Oops! They are not soldered yet in this picture.)

Solder the wires to the board, and to the resistor terminals.

Install the fuseholders… Or don’t!

If you have a GFA-555 MK1, then you don’t need the fuseholders on the board, and your kit did not come with them. The rail fuses on the MK1 are located on the output modules. Nice short supply wires come from the power supply capacitors, directly into the fuseholders on the output modules. This works great! The power wires are nice and short, and there is a small loop area.

However, you must bypass the fuseholder PCB pads, or no current will flow. Install a jumper wire in the pads provided. I’m using 14ga solid wire here.

If you have a GFA-555 MKII, you have a choice to make: The fuseholders on the MKII were moved to the back panel for ease of access. One does not need to remove the top cover of the amp to replace the fuses. I suspect Adcom made this change because they didn’t want people tampering with their amplifiers, and possibly getting shocked. (I don’t blame them.)

The problem with the rear-panel fuseholders, is the extra lengths of wire needed to accomplish it. Extra wire adds inductance to the power supply. Instead of the supply wires going from the capacitors, straight to the output modules as they do in the MKI, the supply wires in the MKII come off the capacitor terminals, through about 12 inches of 16ga wire to the back panel fuseholders, then back out to the output modules through another 12 inches of wire. It’s a lot of back-n-forth!

Utilizing the optional fuseholders on the Hoppe’s Brain board, you can eliminate literally FEET of wire from the amplifier, improving performance, and making for a neater installation.

Solder the fuseholders in place:

Install the terminal blocks:

Notice there are two different styles of terminal blocks. Some are “45 degree entry” and some are “Top entry”. The board is marked underneath with which style to be used in that position.

If you are not installing the fuseholders, then you only need one terminal block installed for the V+ and V- terminals.

If you are installing the fuseholders for a GFA-555 MKII, install two terminals as shown below. One terminal goes to the output modules, and the other goes to the input board power.

(Fuseholders have not yet been installed in this example.)

Install everything else:

If you will be ultrasonically cleaning the board, install everything except the power transformer. It should not be immersed in liquid and should be installed last. Otherwise, go ahead and install everything else!

Testing the board:

Before you attempt to install into the amp, test the soft-start circuit. Connect a fused power cord (1A is sufficient) to the LINE and NEUT terminals. Apply power. Beware of exposed live AC. Short the power switch terminals, (no worries the power switch circuit is only 12V and low current) and you should hear the power relay click in after about 1 second. The inrush relay should click in a second after that.

Further tests, If you want to be more thorough:

  • Power supply voltage as measured on the input to the power regulator should be about 23V. Probe between soft-start ground on the negative output of the bridge rectifier, and pin 1 of the regulator. (Power switch off, relays not activated.)
  • Regulated output should of course be 12VDC, +/- 0.4V. Probe pin 3 of the regulator. (Power switch on)
  • You can test the remote trigger function by applying 3.3-24VDC in either polarity.
  • If you have a variac, you can check that the relays will not engage until about 95VAC input. The relays should not chatter any faster than 0.7Hz, as you vary the input voltage up and down near 95VAC.
  • Test the inrush relay inhibit circuit by shorting pins 3&4 on OK3 with a screwdriver. The relay should click out as soon as you short these pins, and it should re-engage 1 second after you remove the short.

Build the right-side board:

No explanation needed: It’s simple compared to the left-side board you just built!

Installing the board in the amp:

DANGER!! Observe capacitor polarity! If you install one of those big power supply capacitors in backwards, it could explode violently!

  • For the GFA-555 MKI, you will need to rotate both of the left channel capacitors 180 degrees. Match the polarity markings on the capacitors to those on the board.
  • For the GFA-555 MKII, the capacitors should already be in the correct orientation. Please CHECK ANYWAYS.

Installing the supporting foot:

Note: New versions of the board use only only foot, as two were found to be unneeded.

The foot is meant to press gently down on the power transformer, to support the right side of the board. Several lengths of screws are provided, depending on how tall your capacitors happen to be. Assemble the foot as shown.

And loosely attach it to the board: (It’ll be tightened later)

Place the board down onto the capacitor terminals, and check the alignment of the mounting holes and the capacitor terminals. You may need to loosen the capacitor clamps and re-align the capacitors. Attach the ground buss bar on top of the circuit boards, and tighten the capacitor terminal screws.

Do the same for the right side.

Adjust the foot so it presses gently on top of the power transformer, and tighten the nuts.

Transformer Primary Connections:

The terminal block allows for connecting any one of several power transformer types that have been supplied with the GFA-555 over the years. The MKII is a simple matter; There is only one kind of transformer found in these models. The MKI had several variations in the early years of production, but there is one type used in later production runs that is by far the most common. Nearly all the 555 MKI’s with the striped front faceplate used this common type. The oddball transformers are usually found in rack-mount models.

Primary terminal block configuration:

  • The LINE terminal is for the hot AC input. (The narrow blade on a US plug) Wire directly to the AC fuseholder on the back panel. (There is no AC fuse on the board, you must use the original.)
  • Terminal 1 or “Switched HOT OUT” is where the switched power comes out of the soft-start circuit. Most 555 power transformers have a brown wire that goes out to a thermal fuse inside the power transformer, then connects to the high side of one of the primary coils. Connect the brown wire here. If you do not have a brown wire on your transformer, then this terminal should connect to the “high side” of your power transformer. If you don’t have a brown wire on your transformer, but need to provide switched power to two primary coils, such as in a 120V configuration where both coils are in parallel, you may split this output.
  • The NEUT terminal should connect directly to the neutral wire coming from your power wire, or power inlet. (The wide blade on a US plug.) It is tied together with terminals 2 and 3. The “low side” of your primaries go here.
  • Terminals 4 &5 are tied together but not connected to anything else. Use like a wire-nut to connect any two wires.
  • Terminals 6, 7 & 8 are open, no connection. Park any unused wires here.

GFA-555 MKII Primary Connections:

I’ll begin with the MKII because it’s simpler to explain, as all MKII transformers are the same. (As far as I know) Shown below are the color-coded connections for the GFA-555 MKII wired for 120VAC.

GFA-555 MKII primary connections for 120VAC. (NOT MKI!!)
GFA-555 MKII Primary connections

But the MKI is different!! The wire colors are different, and some early models without the brown wire have totally different color codes. Do not wire according to the above photo! There are a few different types of transformers found in MK1 amps. See the chart below.

GFA-555 MK1 Primary connections:
(Assuming you have a transformer with the brown wire.)

Most MKI Amplifiers have a transformer with a brown wire that goes to a thermal cutoff embedded in the transformer body. This thermal cutoff connects internally to the high side of one of the primaries. (And the yellow wire.)

GFA-555 MKI Primary connections, for the most common type of MKI transformer. (With the brown wire)

If you do not have a transformer with a brown wire: You’ll need to figure out how the transformer is supposed to connect. Examine how the original terminal block is wired to determine where the hot and neutral wires are going. You can email me to check that your plan is sound. I cannot be certain of the color codes of the wires, as there seems to have been quite a bit of variability in the early days of GFA-555 production.

There are two styles of primary coil configurations that you might have.
One type has two separate primary coils, and the other has a single, multi-tapped type.

  1. If you have two separate primary coils:
    1a. Wiring for 100V or 120V:
    You will need to split the switched AC line coming out of terminal 1, and connect it to the high side of both primaries. Then connect the low sides to terminals 2 and 3. (Neutral) Any unused wires can be parked on terminals 7 and 8, which have no connection.
    1b. Wiring for 200V, 220V or 240V:
    Connect the high side of one primary to terminal 1. (Switched out) and connect the low side of the other primary to terminal 2 or 3. (Neutral) Use terminals 4 and 5 to connect in series, the low side of the first primary, and the high side of the second.
    Any unused wires can be parked on terminals 7 and 8, which have no connection.
  2. If you have a single, multi-tapped primary coil.
    Connect the primary wire that corresponds to your incoming AC voltage to terminal 1. (i.e. 120V, 220V or 240V)
    Connect the low side of the primary to terminal 2. (Neutral)
    Any unused wires can be parked on terminals 7 and 8, which have no connection.

Secondary Connections:

Attach the orange and red secondary wires from the transformer to the “SEC1” and “SEC2” terminals on both sides. It doesn’t matter which way they go, but I like to put them both the same way. Red to SEC1 and Orange to SEC2.

Connect the black center-tap secondary wire to the star-ground point of the buss bar.

Testing your power supply:

Don’t hook up power to the rest of the amp yet! We should test the power supply first.

Again, are you REALLY REALLY SURE you have the capacitor polarity correct? I want my customers to keep all their fingers.

If you have a variac to test your supply, great! We can test things at low voltage before we commit to plugging it in full voltage. Short the BYPASS jumper. Plug the amp into the variac, and turn it up to about 5% or so. You should read a low DC voltage across all four capacitors. Verify the polarity is correct!

If you don’t have a variac, or a dim-bulb tester, you’ll have to just gun the throttle and hope. Make sure the bypass jumper is open. Be EXTRA CERTAIN you have capacitor polarity correct! Plug the power supply in, and short the power switch terminals. The power relay should click in after 1 second, the capacitors should charge up, and another second later, the inrush relay should click in. You should read approximately 83V on all four capacitors. None should explode and you should have the same number of fingers as when you began.

Remote trigger hookup:

An isolated 1/8″ phone jack is provided for remote trigger. It has a plastic insulated body so that it is fully isolated from the amp chassis, and no ground loops can be created when connected to your preamp. It is galvanically isolated by the optocoupler. The remote trigger can be anywhere from 3.3VDC to 24VDC, from 0.5 to 5ma, respectively, in either polarity. Higher trigger voltages are possible if one simply changes the current limiting resistor for the optocoupler LED.

Connect hookup wires to pins 1 & 2 of the remote jack. (Sleeve and tip.) (Wires not included, they can be anything you have lying around.)

Drill a 5/16″ or 8mm hole in the chassis where you want the remote jack to be. (I could have used a stepper bit instead of a regular drill bit and this hole would have been neater.)

Mount the jack and tighten the nut. Be careful not to over-tighten it; it’s plastic.

Affix the remote trigger sticker.

Connect the remote trigger wires to the small terminal block in the middle of the board.

Hook it all up!

You should now be ready to connect power to the rest of the amplifier! Connect your V+ and V- lines to the output modules. (Note the pictures below show a MKII without the onboard fuse option. With the onboard fuseholder option, the V+ and V- wires go straight to the output modules instead of to the fuseholders in back.)

Input board power wires; MKI

Do not connect any wires from the power supply boards to the input board. Power for the input board on the MK1 comes from the output modules.

Input board power wires; MKII:

If you have installed the optional onboard fuseholders, then also connect the V+ and V- supply lines from the power supply board to the input board. (The thinner orange and red wires.) If you have NOT installed the onboard fuseholder option, then the V+ and V- supply lines for the input board should come from the rear-panel fuseholders as original.

Star-Ground Connections:

The order in which the terminals are stacked onto the star-ground bolt makes a difference in noise! The most important thing, is that the signal ground does not share any conductive paths with charging currents between the rectifiers and the power supply and bypass capacitors.

Stack the star-ground bolt like this: From top to bottom

  • Signal Ground from input board
  • Speaker Grounds
  • Chassis ground wire
  • Bypass Capacitor ground from the input board. (Does not exist on original board. Hoppe’s Brain Input board adds local supply bypass capacitors, or you can add them to your original board.)
  • Bypass capacitor grounds from output modules. (MK2 has these capacitors installed, and they can be added to the MK1.)
  • Transformer center-tap
  • Buss-bar


That should be it! You are now ready to test your amp. Again, if you have a variac or dim bulb tester, this is a good time to use it.

Good luck!