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 board.

The schematic:

Click here to download the BFA-555 MK1 schematic.
Click here to download the BFA-555 MK2 schematic.
The circuit is true to the original schematic, with the addition of the local supply bypass capacitors. Part numbers for the MK1 have had a prefix of 1xx or 2xx appended to differentiate left and right channels. (Thus R1 becomes R101 for the left, and R201 for the right.)

Errors in GFA-555 MK2 Service Manual

  • R117 and R167 are listed as 381 Ohms, they should be 681 Ohms.
  • C103 and C153 are listed as 0.1uF, they should be 1.0uF. (Kit supplies Kemet F461 1.0uF/160V.)
  • C106 and C156 are listed as 50pF, they should be 33pF.


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.)

Install into the amp!

Connections are labeled the same as original. For MK1 amps, the brown wire for the “-V” line must be replaced with the slightly longer one included with the kit. (The power distribution rails were moved to the center of the board, to accommodate the local supply bypass capacitors, to keep the layout tight, and to reduce the local loop area.)

Why two ground leads?

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 two 16ga or 14ga wires with ring terminals on the end.

Star-grounding: It’s important that the signal ground be kept away from any charging currents. Thus, the signal ground ring terminal should be positioned at the very top of the star-ground bolt, with the speaker ground just below that, then the buss bar, then below the buss bar, connect the transformer center tap, and the chassis ground lead. If you have added bypass capacitors to the output modules, their ground leads can connect anywhere below the buss bar.

*If you have an older revision of the Hoppe’s Brain GFA-555 Soft-Start power supply, you will find that there is no connection for the input board’s “Bypass GND” connection. No problem, we can adapt. It simply needs to go to the terminal labeled “Bypass GND” on the right-hand side of the board. This is also where local power supply bypass caps for the output modules connect. Since we can’t stuff two wires into one terminal, make a short “Y” connector to combine the ground leads from the input board bypass caps, and the bypass caps from the right-hand output module. (Assuming you have bypasses installed on your output modules.)

Y-connection to bring input board bypass cap, and right-side output module bypass caps together. Then they plug into the right-side bypass cap terminal. Later revisions of the Soft-start board will have an additional terminal for this.

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:

MK1: Bias spec is 16mV across the emitter resistors.
MK2: Bias spec is 5mV across the emitter resistors, or 10mV across the test points TP201 and TP301, which spans two emitter resistors in series. I prefer not to use TP201 and 301, and measure each emitter resistor separately.

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 12mV. This will climb as the amp warms up a bit. After 15 minutes or so, I’ll then re-adjust to 16mV. 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.

That should be it!