Folks, I have another refurbished Adcom GFA-555 II ready for a new home! This is a terrific amp. Not only is it 110% electrically, but as you can see, it’s in excellent cosmetic condition. The face is nearly perfect, and there are just a few minor ticks here and there. Click on the photos to see it in high-resolution.
This is a thorough refurbishing! 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 refurbishers leave out, or charge extra for, because they are expensive. Output transistors changed to the excellent On Semicondutor MJ21193/94. Drivers changed to ON Semiconductor NJW3281G and NJW1302G for greater voltage headroom.
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.
50A High-current bridge rectifiers are installed and a heat-sink affixed. This helps prevents power supply droop while the amp is being run hard.
Sealed Bournes trim-pot installed for precise bias adjustment and better reliability.
Circuit boards are cleaned and solder joints inspected and touched-up wherever needed.
New power switch and spark-supressor capacitor. (Most Adcoms do not have soft-start circuits, and so are hard on power switches.)
The four capacitors in the foreground are added for extra smoothing of the power supply to the input board.
New, sealed trim-pots for bias, and small heat sinks added to the Class-A driver transistors for greater reliability. Some refurbishers put large heat sinks on these, but I believe that is a mistake. They should run a little hot for improved gain.
Update Aug 16, 2015 – SOLD to Richard in Georgia, who will be using this 555 to power speakers for his houseboat. Sounds like a floating party house! Rock. This is exactly the kind of thing I am trying to facilitate!