The Adcom GFA-555 Mark One-and-a-half.

I’ve rescued this beautiful Adcom GFA-555 Mark II from an amateurish repair attempt. Evidently, the amp blew its output transistors, (rare with these amps actually) and it was repaired with non-matching parts, and some counterfeit output transistors! These were mixed with good original transistors. This repair no doubt worked, but it blew out again shortly after.

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So, I tore down the amp completely to the bare chassis, and I have inspected every last component, and replaced components with either original Adcom spares, or newer, superior options. All electrolytic capacitors have been replaced with audio-grade types from Nichicon 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 are working pulls from another 555, the classic Toshiba 2SD424/2SB554. They are excellent parts. Drivers changed to ON Semiconductor NJW3281G and NJW1302G for their higher breakdown voltage. The originals are 140V parts, B817/D1047, which is actually under-spec’d for the rail-to-rail voltage of 170V. Supposedly, Adcom hand-picked these for higher breakdown voltage, but these new ones are 250V, so no problem. I’ve never seen these transistors blown out, but some people say they do.

New sealed trim pots, AD820 Op-amps for the servo. The ones it came with were LT1006, and I suspect they were not original. The AD820 is recommended by expert sources in this application. It’s made for differential amp duty, and has a very well matched impedance between inverting and non-inverting input. Here, it’s job is to nullify any DC output offset by nudging the differential input pair with a little DC one way or the other. Contrary to popular belief, the audio signal does not pass through this op-amp.
The input board lacked local bypass capacitors, but I found room to install them, and so this should provide better supply decoupling and possibly lower noise.

This is a COMPLETE refurbishment. I intend it to last another 30 years.

And beyond a simple refurbishment, I have made significant improvements to the layout of the power supply.

People have differing opinions about which version of the GFA-555 is better; the Mark I or Mark II. There are valid points in either direction, so I have taken some of the best traits of the GFA-555 Mark I, and re-incorporated them into this Mark II.

So, maybe you could call it a GFA-555 Mark One-and-a-half.

One thing the Mark I had going for it was very short power leads from the capacitor storage bank to the banks of output transistors. There were fuses mounted straight onto the output boards, and so the wires are kept short for low inductance and excellent high-frequency performance. However, should you blow a fuse, you need to take the cover off to replace them.
So the Mark II moved the fuses to the back panel, where they can be swapped by the customer without tampering with the inside. However, this makes for long power supply wires, running from the capacitors to the back panel fuses, then up to the output modules. My modification restores the short power supply leads configuration of the Mark I, putting the fuses back on the output modules.

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Also, I moved all the AC transformer connections to where they were the Mark I. The Mark II snakes the transformer wires through the middle of the chassis to a terminal block. With my modification, the AC and transformer leads are extremely short and should result in less radiated noise.

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.
Much cleaner! Short wires radiate less noise.

And because of the smaller physical size of the new filter capacitors, I was able to mount them side by side and much closer to the output modules, so I was able to shorten those leads too.

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On the bench, I measured 260WPC into 8 ohms before clipping! The oscilloscope shows no signs of oscillation with sine or square waves, and the clipping behavior is nice and clean, as is typical for Adcoms. There are no nasty sounds when it clips. Very clean!

Hooked to speakers, this amp is DEAD quiet. Great sound!

Update: Sold to Doug in Cincinnati! He’ll be driving a pair of the classic Klipsch Cornwall II’s.

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