Hi everyone! I don’t often have amps for sale; I am too busy with customer refurbs, and my meticulous nature isn’t conducive to high production throughput. :^/
I now have input boards for the very first GFA-555’s that came out in 1985. Judging from the serial numbers I’ve seen, there are perhaps less than 8000 of these amplifiers in existence. But they deserve to be restored!
At first glance, they look just like the boards in the more common 1986 version, but they are not physically compatible. The mounting brackets are spaced wider, the RCA input jacks are different, and the bridge switch is a bit chunkier.
CORRECTION and APOLOGY: I’ve been contacted by a friend of the original owner of Wavetrace Technologies, and was informed that the the mods I discovered in this amp are not actually a product of Wavetrace Technologies. They were done after the fact, and do not reflect on the quality of Wavetrace Technologies work. My apologies for any confusion. The article has been corrected.
A kind person called Bob contacted me out of the blue and offered me this cool old rack-mount GFA-555 for free. Of course I accepted, and I sent him a really nice custom Wisconsin cheese box as a thank-you.
The amp had been modified by Wavetrace Technologies back in 1988, and someone else did some further modifications.
It was blown out and passing DC at the speaker terminals. I didn’t want to spend a lot of time trying to repair and return it to FET operation, as I planned to install one of my input boards anyway.
I happened to have one of my replacement input boards for the GFA-555 all built up and ready to go…
But nope! The board doesn’t fit!
One issue that vintage Adcom amplifiers occasionally suffer from, is corrosion on the snap-in board headers and plugs that connect the bias compensation transistors to the input board on models GFA-565/585 and GFA-555 MK2. The corrosion can be caused by high humidity, salty sea air, or in the case of the GFA-565, leaked capacitor electrolyte. Failure of this connection can cause a DC offset to to appear at the output.
I have a matched pair of GFA-565 boards in stock! Fully tested. Near aerospace-grade construction. 😸
Some early models of the GFA-555 MK1 used these vertically-mounted input boards. My existing horizontal board kits could be adapted to work with these amps, but it required fabricating brackets, drilling holes, and adapting the wiring. It was kind of a pain and a bit confusing.
The biggest design challenge for the vertical board model, is the way the original bridge switch is mounted to a small circuit board that is held in place by the RCA jacks. These are not great RCA jacks, and most people will want to swap them for nice chassis-mount types. So I came up with this “Mezzanine board” arrangement with two PCBs stacked together. The first board holds the bridge switch in place, and houses connections for the RCA jacks, LEDs and thermal breakers. Then the input board stacks on top of the mezzanine board, and uses WAGO cage-clamp terminal blocks to connect to the output modules. The input board can be removed and serviced without soldering!
My GFA-555 MK2 boards are ready to go! Thanks to Jon Morris of Morris Audio in Wyoming for helping me work out the bugs and validate the design. He’s already restored a few customer’s GFA-555 MK2’s with my boards.
NEW – I am now offering these boards fully assembled and tested for $380. All you need to do is make sure your power supply and output section are good.
Do you have one of the first batch of ADCOM GFA-555 amps ever made? My GFA-555 MK1 boards work great in these amps, but they must be physically modified to work with the horizontal board.
Until now! I have designed new boards for these early “Vertical” 555’s.
I really like how this layout turned out. Very pleasing symmetry.
This is the original board:
Dang, I kinda want to keep this one, but here it is for sale!
Hoppe’s Brain Refurbished ADCOM GFA-555 MK1
This particular GFA-555 came to me as a customer trade-in, and was very well loved and cared for, so I thought it deserved a new life.
I went all-out on this one. It’s got all the options, including an entirely new input circuit board, and a soft-start power supply with (4) 27,000uF capacitors.
By popular demand, I have created new input circuit boards for the GFA-555 MK1! These boards integrate all of the improvements I normally make when I refurbish a GFA-555.