Yet another variation of the Adcom GFA-555 discovered in the wild!

GFA-555 Rack Mount

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.

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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!

Nope, not the same.

Apparently, these very early rack-mount 555’s were a little different. I thought I had all models of the GFA-555 covered. I sell the most common type; the horizontal GFA-555 input board, and the vertical style board seen in some early-models.

But this early rack-mount 555 board has mounting holes that are more widely spaced, and the bridge switch is larger and in a different spot. Also, if you remove the original plastic RCA jacks you are left with a big oval hole in the chassis that you can’t mount RCA jacks to.

Nowhere to mount new RCA jacks.

I don’t think there’s many of these early models out there, but I want to support them, so I sat down to design new input boards to fit these models, as well as a little adapter plate for the RCA jacks. The board design is derived from my original GFA-555 board, so that part wasn’t too difficult. The hard part was getting the dimensions just right on the switch and adapter plate.

RCA Jack Adapter Plate

The adapter plate has a gold-plated ENIG finish, so as to be galvanically neutral with the gold-plated RCA jacks. A surface-mount ground connection is provided.

Nailed it! Perfect fit!

The modifications as I found them

There were a number modifications to the amplifier, and it would have been interesting to fix the amp, listen to it, and take some measurements. But ultimately I decided not to bother, as I wanted to install one of my boards in it anyways.

But this is what I found that the modifications consisted of. Again, this is not something that Wavetrace Technologies did.

  • Input Stage
    • 2N6661 FETs replace BJT transistors in the input stage.
    • A CR220 Current regulator diode, 2ma, replaces the current source transistor
    • There is a 2N2907 connected from C-E across the collectors of the input transistors.
  • VAS Stage
    • A IRFF9221 HEXFET replaces the VAS transistor
    • A 2N3440 BJT replaces the VAS current mirror, (Yes, that means the NPN side of the amp is driven by a FET and the PNP side is driven by a BJT. Weird?)
  • 42,000uF capacitors installed to replace original 15,000uF.
  • A soft-start circuit comprising a solid-state relay triggered on time delay by an R-C network off the power supply caps.
  • Local power supply bypass caps were installed on a separate board mounted near the input board. The input board has its own supply lines fed direct from the filter capacitors. Curiously, these caps had two series 1N4007 diodes, which would have caused the supply voltage to the input board to be about 1.2V less than the output sections see. I would think this would cause early clipping, but I suppose that might not matter when the amp is under transient loads, and the main caps drain faster than these small local supply caps. It does have the advantage of de-coupling the input board’s power supply from the output section.

Previous mods. **NOT MY WORK or that of Wavetrace Technologies**

So all this work is getting torn out, and the amp is getting re-done my way. Here’s how far I’ve gotten… More later when it’s finished.

P.S. I am keeping this amp for myself! I have a pair of classic B&W Matrix 805’s in the bedroom, and this amp is perfect for them. I really like this version of the 555. The sheet metal is super thick, and it’s held together with machine screws into threaded inserts. Later models used self-tapping sheet metal screws, which works fine, but this is just nicer.

2 thoughts on “Yet another variation of the Adcom GFA-555 discovered in the wild!

    1. Hiya Nelson! Thank you so very very much! People really love their Adcoms, thanks to you and your brilliant design that hit every single point that people were dying for. High-end sound without the snobbery! My Adcom costumers are a refreshing bunch in the world of audio, in that they don’t care too much for status objects, they just want a transcendent musical experience. In fact I have long wondered if I should be directing my talents towards more profitable and expensive high-end equipment and charging more for my services…because I am insanely meticulous about my work, and I could certainly be getting paid more… I could be working on expensive collectors items, for people who park their excess money in high-end audio, like they do unthinkingly with expensive art and exotic cars… But I like the Adcom customers! They are not luxury consumers. They’re music lovers! The 5×5 is not a complicated amp, but I continue to learn new things about it, how it was implemented, and ways I can improve on things. I don’t like to second guess your design. My approach to mods is very conservative and I am careful to research the ramifications of any small changes I might make. Anyways, thank you again, I am no where near bored with these amps, thank you!!

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