Sold: Fully Restored ADCOM GFA-555 II

UPDATE: 08-04-2017: Looking back, I never should have sold this so cheaply! I was seriously under-charging for my work; a common mistake with new businesses. This amp represents several full days of work and a high parts cost. I would now charge around $1300 for this same amp.

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Hey folks, I have another 555 II ready for a new home! It’s got the full package. All new capacitors throughout, including the power supply caps. Matched input transistors, local supply bypasses, etc etc etc.

This one is in excellent cosmetic condition. There are a couple of dings that I have touched up, and are hardly noticeable, but since the amp is not totally “mint”, I am selling it for $820 instead of the usual $850. (I am very picky about this stuff.) Please click on the hi-res photos to see for yourself.

This is a thorough refurbishing! I want my amps to last another 30 years. The entire amp has been stripped to the chassis and rebuilt from the ground up. Every component has been checked, every solder joint inspected, and every wire neatly routed. 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 with thick copper bus-bar shortens wires and reduces clutter and radiated noise. (New capacitors are smaller so I can mount them closer.)
  • Transistors in the critical input stage are carefully 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 250V NJW1302/1381 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.
  • 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 and bias transistors for greater reliability.
  • Dale LVR 1% 5W emitter resistors replace the original 5% sand-cast resistors.
  • 50A bridge rectifiers with snubber caps installed across AC input
  • High-temperature and high-endurance capacitors used throughout.
  • 14ga Power cord with heavy-duty plug.
  • New gold-plated and binding posts.

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

Listening tests were done over three days with a pair of JBL 4208 studio monitors. Sound is superb as expected.

A beautiful face. There is a small ding in the middle of the right side. It’s been touched up and is hard to see, but this is why I am taking $30 off the price.

Input board. Note the matched input transistors thermally bonded with heatsink grease.

The four large capacitors are local power supply bypasses, added for additional smoothing and reduced noise. You can also see that I’ve removed all wire-wrap posts and soldered the wires directly.

Output modules with Dale LVR emitter resistors, Panasonic bypass caps, and WIMA MKP bypassing the bypasses.

New power supply layout shortens wiring. Bridge rectifiers are mounted in the traditional spots instead of on heatsinks attached to the power supply caps, where they would formerly dump their heat into the caps. Power supply caps do not have bypasses. This is intentional, as there is a set of them on the output modules as well as the input board, which is a better location. (The don’t accomplish much across the main supply caps, due to the inductance of the wires.)

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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Thanks for looking! Please contact me here if you’re interested, or have any questions.