Adcom GFA-565 monoblock power amplifier Tom Norton

Thomas J. Norton reviewed the Adcom GFA-565 in June 1991 (Vol.14 No.6):

It was not the best of times. Aside from a major repair job on the car, it was National IRS week. This year, I was "asked" to make a major supplementary contribution to support the festivities leading up to April 15. At least I was able to use the EZ form this year. That's the one that asks you to list your income and send in a check for 80% of it.

Not that I expect any sympathy. It's just that this time of year reminds most of us of the mortality of our own personal finances. Here today, gone tomorrow. You not only can't take it with you, it's nearly impossible even to hold on to it along the way. For a music lover and audiophile, indulging in one's passion can involve more than a bit of creative fund-raising. Those who can satisfy their dreams in equipment (not to mention program material) without a second thought are rare. Though the "high end" of the high end includes products that often (but not always) produce stunning performance, these same products can also perform a stunning disappearing act with your bank balance. Many of us simply cannot afford them.

Fortunately, there are alternatives. While the manufacturers who make the priciest gear still have to engage in a certain juggling of cost-benefit tradeoffs, they have considerable flexibility in their choices. When in doubt, put it in. As the cost of the active parts escalates, the "package" must be upgraded. Not many buyers will consider Krell-level circuitry and parts built into the cosmetic equivalent of a brown paper bag. Would you? Probably not. Fortunately there are other manufacturers who court the potentially bigger market for more "affordable" products.

While Adcom can be accused of running a fire sale, it is operating a long way from the top-dollar high end. It would argue that it substitutes clever planning and well-considered tradeoffs for the brute-force megabuck megablocks produced by some of its pricier competitors.

Sam Tellig's piece on the Adcom monoblock amplifiers in his April 1991 column (Vol.14 No.4) certainly stirred up the pot—the groans from the competition could be heard through our fax machine. Sam covered the waterfront pretty well, but since we also had a pair here in Santa Fe it seemed like a useful idea to publish a follow-up of the Adcom, with lab tests.

Description
Adcom has long since stopped surprising us with this sort of thing. They've specialized in low-income penthouses since the original GFA-555. Looking very much like the latter, the GFA-565 is in actuality a monoblock amplifier with enough available output to roll back the rug and start your loudspeakers tap-dancing if you let it. At $1700 or so for the pair of them required for stereo, they would appear to offer a real value to those who need (or think they need) reams of power.

The toughest test of an amplifier (matters of sound quality per se aside) is its ability to drive a wide range of loudspeakers without suffering a nervous breakdown somewhere along the line. Static amplifier bench tests, ours included, are made into loads which are, effectively, purely resistive, a situation not typical of real-world loudspeakers (footnote 1). Most speakers have significant capacitive or (more often) inductive components, complicating an amplifier's job. And while any competently designed modern amplifier will not actively and audibly misbehave into any reasonable load, Adcom makes a special point of the GFA-565's ability to deliver, across the full audio bandwidth, high current into such complex loads—even low-impedance ones which dip below 1 ohm. Apogee Scintilla and early-model Wilson WATT owners should take notes here.

To accomplish this, the GFA-565 begins with a large toroidal power transformer (1.25kVA) followed by 70,000µF of power-supply filter capacitance. It finishes up with ten pairs of TO-3 type, metal-case, bipolar output transistors. For those into the private lives of amplifiers, the latter are configured in a "triple Darlington" array. No protection circuitry is used. Special "anti-sticking" circuitry—to provide for fast recovery from overload—is incorporated. The amplifier is direct-coupled throughout, using servo-circuits to minimize DC offset. The output stages, as well as the preceding driver stages (also triple-Darlington in configuration), use thermal and dynamic bias tracking to maintain the optimum operating point of each device. All internal point-to-point wiring is oxygen-free copper.

Our samples of the GFA-565 were furnished with the optional balanced-line, common-mode input. A switch is provided to choose between this input (with several levels of gain) and the standard, unbalanced input. At the output end, two sets of five-way binding posts to facilitate bi-wiring are also mounted on the rear panel—though the spacing around the terminals is definitely limited by the flanking heatsinks. You may have to resort to banana plugs rather than spade-type connectors if your loudspeaker cables are stiff and/or heavy. Fuse holders are also mounted on the rear. Our Santa Fe GFA-565s also incorporated the optional, top-mounted cooling fan, which is recommended for use in demanding circumstances (very high power, very low-impedance loads). It apparently only comes on when required—ours never engaged. The amps, in fact, never operated more than slightly warm to the touch during the listening tests. Nor did I ever trigger the Instantaneous Distortion Alert LED located on the front panel, which is designed to indicate total distortions above 1%.

With the power available from the Adcoms (or any other amplifier even approaching it in output), special precautions are needed, most of which are addressed in Adcom's owner's manual. One that may prove problematical is the recommendation that each of the amplifiers in a stereo pair be plugged into a separate circuit in the listening room. Otherwise, Adcom says, the large current draw may result in tripped circuit breakers. If your room is wired so that you can satisfy this requirement, you have no problem. If you don't, and wish to add a pair of these amplifiers to your system, I'd recommend a home trial to insure that they don't generate any private blackouts. You may not—my experience with the Adcoms and other ampere-hungry amplifiers has never resulted in the need to grope around in the dark for the main circuit box. But then, I don't often use my reference system to survey neighborhood opinions on new-age heavy-metal rap music.

Fit and finish
The Adcom is built to good commercial standards, though it won't exactly make you want to replace the top chassis panel with a plexiglass cover—that type of showcase can be had at much higher prices. I noted push-on internal connectors inside the Adcom, though the only visible ones were in the power supply (the output circuit-board connections were hidden from view). While I understand (and to a degree agree with) Corey Greenberg's preference for hard-wired connections in place of these not-too-reassuring-looking terminals, various types of push-on and screw-on fasteners are used in even considerably more expensive amplifiers. They do have the undeniable advantages of simplifying assembly and maintenance.

The sound
Clean, solid, secure. Those were my initial reactions on hearing the Adcom monoblocks through the Snell C/IV loudspeakers. There was nothing at all tentative about the sound of these amplifiers, and there was no mistaking their power capabilities. Bam! Zap! Crunch! It was the audio equivalent of an old "Batman" episode. Only this time, instead of the Joker, Penguin, and Cat Woman on the receiving end, it was my loudspeakers. I admit to a certain degree of intimidation when using amplifiers of this power capability, but I never blew out any drivers. A certain amount of caution and common sense go a long way here, however. I'm certain I never used more than a fraction of the GFA-565s' continuous power-output capabilities, and the loudspeakers chosen for the listening tests were all pretty decent power handlers with prodigious low-end responses. The Adcoms should definitely be combined with minimonitors only when accompanied by a parent or guardian.



Footnote 1: Resistive loads are used for bench tests because they are at least standardized. The "typical" loudspeaker load does not really exist.
Share | |
Site Map / Direct Links