Sold! Beautiful ADCOM GFA-555 II with all the mods

Posted on by hoppesbrain

UPDATE: Sold to David in BC.

Folks, this is the nicest GFA-555 II I’ve ever restored. What a beauty! The previous owner was a cellist for the Milwaukee Symphony Orchestra, who took excellent care of it. Cosmetically, it’s about as close to mint condition as they come. There are a few small ticks in the finish here and there, which I’ve touched-up, and are hard to notice, and the face is totally perfect! Click on the photos to see it in high-resolution.

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Since this one is so nice, I pulled out all the stops and installed every enhancement I know for Adcom amps. This is the best one I’ve ever done! It’s got the 14ga power cord, new supply capacitors, and the improved power-supply layout. As you can see, I got really fussy about the neatness of the wiring in this one.

This is a thorough refurbishing! I want all my amps to last another 30 years. All electrolytic capacitors have been replaced with audio-grade types from Nichicon, WIMA or Panasonic. Even the big power-supply filter capacitors are new, something many restorers leave out, or charge extra for, because they are expensive.

Features:

    • Improved power-supply layout shortens wires and reduces clutter and radiated noise.
    • Transistors in the critical input stage are matched for gain. This step is important to assure low DC offset at the output, and low distortion.
    • Local power-supply bypass capacitors added to the input board for greater power-supply smoothing.
    • AC noise-snubber capacitors added to bridge rectifiers.
    • Driver transistors changed to 180V 2SA1492/3856 for greater voltage headroom. The originals are 140V parts, and are known to fail when driven hard at high-frequency, due to the power supply swing of 170V.
    • Output transistors are original; the very excellent Toshiba 2SD424/554. This is a particularly well-matched set from Adcom.
    • Circuit boards are cleaned and all solder joints are re-flowed and inspected.
    • New power switch and spark-supressor capacitor. (Most Adcoms do not have soft-start circuits, and are hard on power switches.)
    • New, sealed Bournes bias trim-pots.
    • Heat sinks added to the Class-A driver transistors for greater reliability.
    • Dale LVR 1% 5W emitter resistors replace the original 5% sand-cast resistors.
    • 50A bridge rectifiers.
    • High-temperature and high-endurance capacitors used throughout.
    • 14ga Power cord with heavy-duty plug.
    • New gold-plated RCA input jacks and binding posts.

I measured 260WPC at 8 ohms and 340WPC at 4 ohms just before clipping.

Sound quality is clean and smooth as expected with the studio monitors, and it drives my subwoofer like a beast! Amazingly strong and tight bass.

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Pretty typical as they come to me. Dusty and tired. Issues found: Loose chassis ground, cold solder joint on power supply drain resistor, slight gain mis-match in left channel input, 47uF bypass capacitors on output modules deteriorated with high ESR.
Compare to the original layout. (Not this actual amp.)
New power supply layout is much cleaner, with less spaghetti running through the chassis.
New power supply layout is much cleaner, with less spaghetti running through the chassis.
Input board. The four Panasonic FC capacitors in front are local power-supply bypasses. These should improve performance at high frequencies. Many amplifiers use something like this. My guess is that Adcom left them out because audio reviewers don't like to see electrolytic capacitors, as they are wrongly perceived to
Input board. The four Panasonic FC capacitors in front are local power-supply bypasses. These should improve performance at high frequencies. Many amplifiers use something like this. My guess is that Adcom left them out because audio reviewers don’t like to see electrolytic capacitors, as they are wrongly perceived to “sound bad”. But these are not in the signal path, and only serve to smooth the power supply.
Heatsinks are attached to driver transistors, as these otherwise run extremely hot and are known to fail.
Bournes sealed trim-pots for bias.
Also, I’ve removed the 8 wire-wrapping posts and soldered the wires directly.
Check out these bus-bars! I made them out of solid 6ga copper wire, and soldered large terminals to them. The new supply capacitors are physically smaller than the originals, so I was able to improve the layout of the power supply. Originally, the bridge rectifier is mounted on a heatsink on top of the capacitors, where they dump their heat into the capacitors. Moving the bridge to the traditional location means better heat-sinking.
Check out these bus-bars!
I made them out of solid 6ga copper wire, and soldered large terminals to them.
The new supply capacitors are physically smaller than the originals, so I was able to improve the layout of the power supply. Originally, the bridge rectifier is mounted on a heatsink on top of the capacitors, where they dump their heat into the capacitors. Moving the bridge to the traditional location means better heat-sinking.
Output modules. Dale 1% LVR emitter resistors. Supply bypasses enhanced with WIMA MKS4 0.1uF poly capacitors in parallel with Panasonic ED 47uF. I often discover that the original electrolytics in this position are exhausted, and show high ESR. The Panasonic ED is a terrific replacement. It's a high-temperature, high ripple-current and high-endurance capacitor, and I am using a 250V capacitor in an 85V application for even more reliability.
Output modules. Dale 1% LVR emitter resistors. Supply bypasses enhanced with WIMA MKS4 0.1uF poly capacitors in parallel with Panasonic ED 47uF.
I often discover that the original electrolytics in this position are exhausted, and show high ESR. The Panasonic ED is a terrific replacement. It’s a high-temperature, high ripple-current and high-endurance capacitor, and I am using a 250V capacitor in an 85V application for even more reliability.

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Original AC wiring block. Long wires snake through the amp chassis to this terminal block.
Original AC wiring block. Long wires snake through the amp chassis to this terminal block.
This improved terminal block makes for much shorter transformer primary wires than the stock arrangement, fewer mechanical connections, and much less spaghetti-wiring inside the amp.
This improved terminal block makes for much shorter transformer primary wires than the stock arrangement, fewer mechanical connections, and much less spaghetti-wiring inside the amp.

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14ga power cord with heavy-duty plug. If you prefer, I can install an IEC or Neutrik Power-Con power inlet for $125. Personally, I prefer hard-wired power cords, as it's one less physical connection.
14ga power cord with heavy-duty plug.
If you prefer, I can install an IEC or Neutrik Power-Con power inlet for $125.
Personally, I prefer hard-wired power cords, as it’s one less physical connection.

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Contact me here if you’re interested. Thanks!

One thought on “Sold! Beautiful ADCOM GFA-555 II with all the mods”

  1. Just received my 555MKII Hunka Hunka Burnin’ Love. What a sexy beast!
    Thanks Brian

    David
    Salmon Arm

    Reply

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