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Adcom GFA-7805 five-channel power amplifier Associated Equipment

Sidebar 3: Associated Equipment

Two-Channel System
Digital sources: California Audio Labs CL-20 DVD-Video/CD player, Linn Unidisk 1.1 universal player, Orpheus 0 CD player; Theta Digital Generation VIII, Orpheus One, Nagra DACs.
Preamplification: Sonic Frontiers Line-3 preamplifier.
Power amplifiers: Sonic Frontiers Power-3, Classé CAM-350.
Loudspeakers: Revel Ultima Studio.
Cables: Digital: i2Digital X-60 BNC. Interconnect (all balanced): AudioQuest Anaconda, Python; JPS Super-Conductor 2. Speaker: AudioQuest Granite. AC: JPS Aluminata.

Multichannel System
Digital…

Adcom GFA-7805 five-channel power amplifier Measurements

Sidebar 4: Measurements

Although the GFA-7805 is a five-channel amplifier, in general I measured pairs of channels at a time, with spot checks on the other channels. The Adcom is specified as being able to supply a maximum power of 300Wpc into 8 ohms with all channels driven. Before I performed any measurements, I performed the usual preconditioning by driving the amplifier (two channels only) at approximately 1/3 power (115W) into 8 ohms for an hour. (A solid-state amplifier with a conventional class-AB output stage undergoes the maximum thermal stress at 1/3 power.) Or rather, I drove…

Audio Research Classic 60 power amplifier

"The only tubes that I want to see in my household are...the picture tube in my TV and the magnetron in the microwave oven," a Glendale, CA, reader recently wrote, and I guess his feelings reflect those of many when confronted by a supposedly "obsolete" audio technology. Forty years after the invention of the transistor and 20 after the widespread introduction of solid-state amplifiers (footnote 1), it must come as a shock to readers of the mass-market "slicks" that not only do a number of American manufacturers manufacture amplifiers and preamplifiers using tubes, but some of those…

Audio Research Classic 60 power amplifier Page 2

My study of the Classic 60's schematic leaves me with a great deal of respect for its designers. Far from being based on an "obsolete" technology, the Classic 60 appears thoroughly modern in concept. Its Minnesota pedigree shines forth, its circuit being as painstakingly elegant and as thoroughly thought out as if the amplifier had been designed by Lake Wobegon's famous team of conscientious Norwegian bachelor farmers.

Listening
Though the SP-10 was my preamplifier of choice for many years—and still resides in my closet awaiting a new complement of tubes—the last Audio Research…

Audio Research Classic 60 power amplifier Page 3

Adding to this high degree of musical realism was a soundstage that verged on being the most transparent I have ever heard, particularly with the Avalon loudspeakers, which far outdo the Celestions in this respect. Now don't get me wrong: the venerable JGH has several times accused me of giving a system's soundstage and imaging performance a higher priority than even tonal neutrality. "And who hears that kind of precision in real life anyway?" he has often snorted down his nose at me before returning his attention to a Tanqueray Martini, straight up, with two twists.

Well, Gordon, old…

Audio Research Classic 60 power amplifier Page 4

The Classic 60 definitely opened up the soundstage in a more expansive nature than the Levinson, there being a superb presentation of depth. This is one of the areas where I have always found the No.20.5 to lack a little finesse compared to the best tube designs, but in the bass, it was no contest. As musically acceptable as the ARC sounded in this region, the Levinson controlled the SL700s with an iron fist, offering control and definition. "There's no substitute for current," a wise old man once said. (Actually, I think it was Martin Colloms.) He was right.

The final set of comparisons…

Audio Research Classic 60 power amplifier Specifications

Sidebar 1: Specifications

Description: stereo, hybrid tube/FET power amplifier. Tube complement: four matched pairs of low-gas 6550s, four 6FQ7/6CG7s. Maximum power output: at least 60W into 16 ohms (20.8dBW), 15Hz-20kHz, with less than 1% THD; typically 62W at 1kHz with 120V AC line. Power bandwidth (-3dB): 12Hz-40kHz. Input sensitivity: 550mV RMS for rated output. Input impedance: 100k ohms. Output regulation: approximately 0.3dB, 16 ohms to open circuit (equivalent to an output impedance of 0.56 ohms—damping factor quoted as approximately 30). Negative feedback: 20dB. Slew rate: 20V/…

Audio Research Classic 60 power amplifier Reference System

Sidebar 2: Reference System

The Classic 60 was used over a period of four months with a variety of ancillary components. Preamplifier was mainly a Mark Levinson No.26, first used with its outboard No.25 phono preamplifier, then with the latter replaced by the excellent External Power Supply version of the Mod Squad's Phono Drive. I also used both the sample of the Audio Research SP14 reviewed by Guy Lemcoe in June, as well as an SP14 loaned by California dealer Howard Rodgers (RogerSound Labs, Upscale Audio). My reference Mod Squad Line Drive Deluxe (soon to be updated to the latest 1990…

Audio Research Classic 60 power amplifier Measurements

Sidebar 3: Measurements

The Classic 60 measured as having a flat response across the audio band (fig.1), apart from a slight rolloff above 10kHz, reaching -0.32dB at 20kHz. This will be just audible as a very slightly soft top octave, but is otherwise inconsequential. Crosstalk was also inconsequential: fig.2 shows the leakage from Channel 1 into Channel 2 with the Channel 2 input shorted, below -120dB across the band, which is effectively buried in the amplifier's noise floor. Looking at the amplifier's response with an input signal consisting of a 200µs, positive-going, rectangular…

Audio Research Classic 60 power amplifier John Atkinson compares

John Atkinson compared the Classic 60 with the Mark Levinson No.23.5 in September 1991 (Vol.14 No.9):

The final set of comparisons was with the Audio Research Classic 60. Now it's true that the significantly lower-powered and less expensive tubed amplifier doesn't necessarily compete with the No.23.5 in the market, a more relevant comparison perhaps being between the solid-state amplifier and a pair of ARC Classic 120 monoblocks. But I'm very familiar with the Classic 60, and felt that as long as I didn't fall into the trap of describing differences that are purely due to the tube…