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!
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.
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.
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.
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.