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 intermediate-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 boards.

The schematic:

Click here to download the BFA-555 MK1 VERTICAL schematic.
The circuit is mostly 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 like Kester 331. (Which must be removed because of it’s aggressive flux action.) I wrote a whole thing about this.

Populating the board:

  • Start with the resistors and diodes, because they lie flat.
    • Resistor values are printed on the board and use the convention of replacing the decimal point with the SI prefix, “M” K” or “R”. (Megaohms, Kiloohms and Ohms) For example: 47.5 Kilo-ohm = 47K5 or 68.1 OHMS = 68R1
    • Be careful with 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 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 value 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.)

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 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.

Wires and stuff:

The local supply bypass capacitors have their own dedicated ground lead. This is to keep capacitor charging currents separate from the input board’s ground reference.

Install 16ga or larger wires with ring-terminals, to go from the “Bypass GND” and “Signal GND” terminals to star-ground. If you are using a Hoppe’s Brain Soft-Start Power Supply, connect the “Signal GND” wire to the “Input BRD” terminal on the power supply. If you are using the original star-ground bolt, the “Signal GND” wire should go on the top-most position on the bolt, for lowest noise. (Capacitor charging currents will not pass through this point. It is the “quiet point”.)

The way I do it, from top to bottom:

  1. Signal GND from input board
  2. Speaker grounds
  3. Bypass GND from input board
  4. Bypass grounds from output modules. (GFA-555 MK1 does not come with these caps, but I always install them. Refer to MK2 schematic and location if you want to add your own.)
  5. Transformer center tap
  6. Chassis GND wire
  7. Buss-bar

Notice the zobel cap and resistor are missing in these photos, and only the feedback terminal block is installed. That is just because I opted to mount the zobel cap and resistor straight to the binding posts in this install.

Install into the amp!


Install the mezzanine board as shown. Use the male-female standoffs to mount the mezzanine board, and use the female-female standoffs to fasten it. You should already have the thermal breakers and LED wires attached. Connect your RCA jacks.

If you have the NON-Mezzanine version of the board, you will need to hard-wire your bridge switch to the board. There is a diagram on the back of the board itself.

Mount the input board onto the mezzanine board, and connect the output modules using the WAGO cage-clamp terminal blocks. Use a small flat-blade screwdriver to slide the little white actuators to open the terminals. Insert the wires and slide the actuators back again to clamp the wires. Give the wires a good tug to be sure they are connected solidly.

Connect the speaker output and speaker ground wires to the terminals along the bottom of the board. (This connects the zobel network and feedback lines.)

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—none that I have heard of—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!