Some notes about restoring a GFA-5802

…just some thoughts and pictures from this Adcom GFA-5802 I’ve just restored for Jeff in Washington.

This was Adcom’s biggest and most expensive amplifier. A 300WPC monster, with 16 HEXFET output devices per channel. Minimalist in circuit design, with only three gain stages, but maximalist in execution.

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These HEXFET output transistors are interesting beasts, and have a bit of cult following. They have a wonderfully flat curve if they are biased heavily enough. Indeed, the GFA-5802 sits idle at about 450VA, at temperatures around 130F on the outside of the heatsinks. The heat ages the capacitors prematurely, so I have installed high-endurance 105C types throughout. In particular, the 4700uF 16V power-supply filters for the high-voltage supply are known to die and cause a loss of bass response. I’ve replaced them with 105C type Panasonic TS-HA’s with a slightly higher capacitance of 5600uF, and higher voltage rating of 25V.  These should hold up much better.

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Also, the white cube is a new power relay, rated at 16A versus the original 12A. I always replace power switches and relays just in case they are corroded over time.

Here are the input boards, which contain the RCA and XLR input connectors, along with the circuitry for the first stage of the amp. It’s a long-tail pair, with cascodes to minimize voltage distortion.

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There are four MOSFETs on the input board (IRFD210) and they need to be matched for VGS of less than 10mV between them. These were found to be off by quite a lot, perhaps due to heat stress.
Fortunately, these devices are still available, so I bought 200 of them, built a jig to match them, and sorted out pairs that match to less than 1mV. (Service manual says 10mV) There’s very little trimming of the DC offset when they are matched well.

And they run hot enough to burn your finger, so I’ve equipped them with small aluminum heatsinks. Some heat is good, so I didn’t go overboard.

I’ve replaced the input capacitors with Nichicon FGs, and bypassed them with WIMA MKS4. There are four small mosfets on this board, which run hot enough to burn your finger, so I’ve equipped them with aluminum heatsinks. Two of these mosfets had actually lost some gain, possibly due to heat stress, so I replaced them. The symptom was that I needed to turn the DC bias pot all the way to one side to get it to zero.

Here’s the un-modified board. I don’t like all these unreliable pin-connectors, and have removed most of them from the amp and soldered the wires instead. Notice the pin-connector on the RCA input!

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It would be ironic to use fancy, expensive RCA cables, when there is this weedy little push-pin connector on the inside. I’ve soldered it permanently.

The output boards. Nice quality fiberglass through-holes.

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It sounds amazing! I’d say it has kind of effortless quality to it. Like a motor with a huge flywheel. Smooth sounds came out of the studio monitors. Hooked to my subwoofer, it was utterly unconcerned with the large current demands, and delivered tight, effortless bass as a matter-of-fact.

I measured 335WPC with both channels driven into 8 ohms, just before clipping.

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