Thanks for buying a board from me! Have fun, and go slow, it’s faster.

This document is called “Assembly Notes” and not “Assembly Instructions” because every amp is a little different, and every technician’s approach to this work is going to vary. This is an advanced-level electronics project. It is assumed that you are experienced and comfortable working with audio amplifiers.

Thus, I present to you, vaguely sequential bullet points to review, not step-by-step instructions.

The board.

And the RCA jack adapter plate.

The schematic:

Click here to download the BFA-555 MK1 schematic.
The circuit is true to the original schematic, with the addition of the local supply bypass capacitors. Note: Early 555’s used a different part-numbering scheme versus later models. I am using part numbers based on the later versions. Refer to the new schematic provided above. Here is the old schematic, just for reference.


Download the parts list here.


  • I recommend a regulated temperature soldering station or at least a powerful hand-held iron like the TS-80 or TS-100. This is a 2-layer board with plated through-holes, and large ground and power planes that wick away heat. No problem with a good iron, but a cheap pencil iron will be frustrating.
  • Getting the solder to flow to both sides of the joint: I use a cotton swab to apply a little paste flux to each and every solder pad on the top of the board. It’s messy, but it helps the solder wick through to the top sides of the solder pads. I touch-up from above the ones that did not flow through. Having both sides of the pads soldered not only looks nice, but is more reliable and serves as a double-check on each solder connection.
  • Solder. Most people use 60/40 or 63/37 “no-clean” solder, like Kester 44. That’s fine, but if you are going to be cleaning the board, then this type of flux is very difficult to remove. Instead, use a solder with water-soluble flux.

Populating the board:

  • Start with the resistors and diodes, because they lie flat.
    • Resistor values are printed on the board using the convention of replacing the decimal point with the SI prefix, “M” K” or “R”. (Mega-ohms, Kilo-ohms and Ohms) For example: 47.5 Kilo-ohm = 47K5 or 68.1 Ohms = 68R1
    • Watch your decimal places! There are a lot of samey-samey values like 68R1 and 681R, or 47K5 and 475R.
    • Dale resistors are marked using at least two different naming conventions, that may differ from how they are printed on the board.
    • No shame; use your meter to check resistances if you are unsure.
    • Install the Dale resistors so that their values can be read from the top.
    • High-precision 0.1% tolerance resistors: There are (5) 22.1K resistors, and (4) 1K. Their locations on the board are marked with a “0.1%” next to the resistor value.
  • Matched transistors:
    • Transistor gain varies with heat, so the two transistors that make up the differential long-tail-pair on the input should be thermally bonded. Copper foil tape and heat shrink is provided for this. You’ll need some heat sink compound to stick between the faces, or thermal epoxy. JBWeld original formula also works well, as it is non-conductive and tolerates high heat. (Even though these transistors only get a little warm.)

Transistors with Heat Sinks:

Populate the rest of the board:

From this point forward, it doesn’t really matter what order you install components, but it’s easier to install the components from smallest to largest.

Don’t forget to attach the thermal breakers, and wires for the RCA inputs. (You can also re-use the original RCA jacks; they solder directly to the board.)

RCA Adapter plate installation:

Install the RCA jacks into the adapter plate, and then mount to the chassis using the two supplied black M3 screws and lock washers.
Be careful to center the jacks in the oval before tightening the screws down. There is a little bit of play. The barrels of the RCA jacks should not touch the metal of the chassis, or they could possibly introduce noise due to a ground loop.

Output module stuff:

Later model GFA-555’s use 68pf ceramic disc frequency compensation capacitors that go from the base to collector of the TO-222 driver transistors. Early models used a 22pF capacitor. I’m not sure why this change was made, but the 68pF value works well, so I recommend the 68pF capacitor. If you bought a full kit from me, it includes a really nice (and expensive) 68pF C0G/NP0 ceramic capacitor.

Also, the driver transistors are a common failure point for the GFA-555, due to insufficient reverse voltage breakdown voltage. I recommend swapping these for ON Semi MJE15032 and MJE15033, which are 250V parts.

IMPORTANT: Testing your output modules

Please do not skip this step! You must be certain your output modules are good before applying power to the amp. This GFA-555 is direct-coupled from one stage to the next. If you try to power up the amp with blown transistors on the output modules, the “Domino Effect” may ripple backwards through your amp, blowing up everything all the way back to the input stage. It’s a big job to repair the damage.

All transistors on each output modules should be tested with the diode check function on your multimeter. Check out this excellent video on the subject by Mr. Carlson, and this discussion thread at AudioKarma, which contains an excellent guide written by user ‘Echowars’.

The output transistors are wired in parallel, so if you want to test each one individually, they will need to be taken out of circuit. You don’t need to de-solder the transistors entirely. Just remove their bolts, which disconnects their collectors, and then lift one leg of each emitter resistor.

If you find a bad transistor or transistors: When power output transistors fail, they tend to do so very obviously, and will usually either blow out completely shorted, or completely open. If yours test OK on the diode check function, and show the usual 0.5-0.6V voltage drops, and no shorts, they are probably good. That is no guarantee. The most important thing is that none are shorted, because that’s how input boards get smoked.

If you find that you just have one or two blown transistors, it is recommended that you replace the whole set of four NPN or PNP, as they need to match. I recommend On Semi MJ21193/94, but there are many good substitutes. If you order from a reputable source like Mouser or Digikey, you will probably receive a whole set with the same batch code. Manufacturing tolerances are so good these days that a set from the same batch code will likely be decent matches.

Install into the amp

Connect all the wires.

  • Output modules. Use the supplied, longer brown wires for V-.
  • Thermal breakers
  • Feedback wires. (Thin twisted pair that comes from the binding posts.)
  • LEDs
  • Input wires
  • Ground wires, see below.

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

Testing the amp:

Bring the amp up on a variac if you have one, or use a “Dim bulb tester“. Apply a 0.1VAC sine wave to the input. Monitor the output on a scope. If the amp is working, you should be looking at a nice clean sine wave of approximately 2.2VAC. If not, you have some troubleshooting to do.

DC Offset:

Short the RCA inputs to the amp. You should read less than 25mV at the speaker terminals. If you have more than this, something is wrong.

Bias Adjustment:

Early models of the GFA-555 used 0.47R emitter resistors and later ones with the horizontal boards use 0.82R. This affects the bias adjustment, so if you have 0.47R emitter resistors, IGNORE what is in the service manual, which refers to later models.

If you have 0.47R emitter resistors—all vertical board GFA-555’s AFAIK—your bias target is 9mV across each emitter resistor.

If you have 0.82R emitter resistors—most common—then your bias target is 16mv.

I recommend checking all 8 resistors on each side. Take an average, it doesn’t need to be exact. Connect the negative probe of your meter to the speaker output and the probe each emitter resistor. Beware of probe slippage! I’ve blown way too many expensive output transistors that way. Now I use these Fluke extended fine point tips, and it hasn’t happened since.

The GFA-555 MK1’s bias has a rather pronounced positive thermal coefficient. That is, when it gets hot, the bias goes up. It can get as high as 80mV when the amp is hot, and this is perfectly normal. It makes bias adjustment kind of fussy though, as the bias will fluctuate with the temperature of the room, and air circulation.

What I do is let the amp sit on the bench, with no clutter around it, with the cover on. From stone cold, I power-on and adjust bias to about 7mV. This will climb as the amp warms up a bit. After 15 minutes or so, I’ll then re-adjust to 9mV. (Assuming 0.47R emitter resistors) Then I put the cover back on and let it sit for about an hour. Check, adjust, check, adjust…. lather, rinse, and repeat as many times as needed.

What I’m looking for is that the bias hangs around the target, at room temperature with good ventilation.

Once adjusted properly, the heatsinks should feel just a little warm to the touch, about 100F. You can use an infrared thermometer to make sure both sides are approximately the same.

That should be it!