Adcom GFA-555 PRO pre-production prototype mod/upgraded 300WPC please read

  

This is a pre-production prototype of the Adcom GFA-555 PRO. This is, essentially THE FIRST GFA-555 PRO and was built in-house some years before I started working at Adcom in 1993. The 555 PRO does all of the things a 555 does, but features balanced inputs, external circuit breakers and input level controls on the front panel. This particular unit looks and operates as a highly tuned consumer GFA-555; the breakers, input controls and balanced input were all removed or disabled before I brought it home from "the factory". I also installed GFA-5500 filter capacitors (the 5500 had just come on the market) and the very last 19 555 faceplate from stock.

Incidentally, this never had a proper PRO faceplate because it did not exist when this was built. I'd really like to mention all the little things that I have done to this amp over the years, but it would be too much to read here. I will just call out the main points: Circuit breakers and input level controls removed (for lower noise floor, direct signal path) Completly disabled balanced inputs and bridging switch (the balanced inputs [opamps] were noisy and harsh sounding) Bigger filter capacitors Matched input transistor pairs to within 1% Upgraded with Cardas, annealed silver wire, Caddok, Dale, and Black Gate capacitors in critical locations. Replaced original 650W power transformer with an 800W transformer... I need to tell you about this one: This 800W toroid brought the output power to 300WPC into 8 ohms (both channels driven)!

This power supply is so stiff that it did not sag when operating at the rated 200WPC. I cannot test into 4 ohms, but it should be close to 600WPC, and again, 1200W into 2 ohms. This super-stiff supply puts some of the parts at or near their maximum ratings, so this amp would best be enjoyed in the hands of an experienced audiophile who knows how to handle high-power equipment properly.

I will include the original transformer as well. One of a kind Adcom GFA-555 PRO without the PRO. Highly tuned and modified by former Adcom Service Tech/Product Development Asst.

Owned by same for 25+ years. This amp is in very good to excellent cosmetic shape. The faceplate and handles are in excellent condition with only a few small nicks on the edges and the top cover and sides have minor scratches.

The left heatsink has a small dent that was difficult to photograph. The functional condition is fantastic, this amplifier functions better than new (better, meaning lower distortion, lower noise [pretty much silent], and higher power). The offset (non-adjustable) has drifted a bit over the years, but is still within spec. This amplifier has not been heavily modified to sound like something other than a GFA-555 - this amplifier has been lightly modified only to allow it to be the best GFA-555 it can be. In other words, this sounds and behaves like a 555, just more refined and effortless.

Thanks for looking and good luck! The item "Adcom GFA-555 PRO pre-production prototype mod/upgraded 300WPC please read" is in sale since Monday, December 7, 2020. This item is in the category "Consumer Electronics\TV, Video & Home Audio\Home Audio\Receivers & Amplifiers\Amplifiers & Preamps". The seller is "jlwoodley" and is located in Annandale, New Jersey. This item can be shipped to United States.

1. Number of Channels: 2
2. Model: GFA-555 PRO
3. Type: Power Amplifier
4. Features: Analog
5. Number of Inputs: 2
6. Power: 300 W
7. Brand: ADCOM