Audio Research Reference 600 Power Amps Reviewed

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Don't kid yourself: reviewers get nervous, too. At least this one does whenever he has to review a product which has the potential to rewrite the rules. Hell, the latest from Bill Johnson doesn't just rewrite 'em: he's thrown out the whole book, moved the goal-posts, started with a fresh page - pick your cliche'. Whatever your normal response to once-in-a-generation revelations, the Reference 600 monoblocks and matching Reference 1 pre-amp will render all who hear the system something akin to gob-smacked. This is where (as Barry Fox once told me I would) Ken Kessler finally exhausts his thesaurus of its superlatives.

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But what's so different about the first Audio Research products which the company feels are worthy of the name 'Reference'? It isn't the size of the tube complement, because earlier ARC models like the D-250 were fitted with valves by the dozen. It isn't price, because there are tube products from other companies which make the prices of these beauties seem positively affordable. The styling? No; the clues were there in the VT-series' special editions, and Reference 1 is an obvious descendant of the LS-range of pre-amps. Size? Aah - now we're getting somewhere...

Unless you found some installation where the owner managed to hide all but the Reference 600s' front panels, the first thing you'll notice is that a pair of these occupies almost as much floor space as half of a Mini-Cooper. (Don't laugh: I hear that there are Japanese car crazies who park their cars in their living rooms.) My wife noticed it with a terse, 'Do amplifiers have to be that large?', followed by a 'No sane people would want those in their homes...' as she left the room shaking her head. What she saw were two chassis measuring 483x267x749mm (WHD) each. That last measurement, in old money, is 29.5in deep, with an extra 1.5in for the handles. Only they look bigger. And then there's the weight, a back-breaking 77.2kg each. For those of you who buy by the pound, compare the �32,000 for a pair of Reference 600 monoblocks with certain single-ended triode amplifiers selling for �125,000.

That vast front panel is a knob-twiddling paranoid's birthday party and Christmas all rolled into one. The huge meters tell you the line voltage operating range and the power output; the latter also works with the bias controls. While the former meter cannot tell you anything about your AC mains quality, it sure was nice to note that Seeboard's juice levels into the Kessler household were well within spec. Directly below the meters is a row of 16 set screws for adjusting the bias on the 16 output tubes. Underneath these, from left to right, are the on/off rotary with a stand-by position, a fan speed adjuster, a knob for meter on/off and illumination intensity, and three knobs which select between the 16 valves for biasing and switching the meter from power output reading to bias level.

Aside from periodic valve adjustments, depending on just how audio/anal you might be, the only controls which you really need to address on a daily basis are the three rotaries at the lower left hand corner. On/off involves moving the control to the standby position, which lights the amber standby light between the meters. Leave it for at least five minutes if you're switching on from cold; only you and your bank manager can decide if you want to leave the '600 in standby between listening sessions, as it draws 800W idling and 300W in standby mode. Finally, you move the switch to 'on'. Eventually, the green light marked 'operate' will stop blinking and you're ready to go; this delay controls the warm-up of key components. Note that the standby position controls a circuit which also provides automatic shutdown in the event of a failure in the tube bias circuit.

Bias adjustment, which will be performed in situ by your dealer if he's worthy of your custom, is as simple as it gets. Switch the rotary to select which bank of valves you're going to work on first (V1-V8 or V9-V16), select a tube, adjust the set screw to position the meter in the green area so designated and, er, that's it. Do this 32 times and you've biased both amps.

But don't rush your connections: the back has a surprise or two as well. For one thing, this amplifier is balanced front to back, so it's XLR-only signal entry. Secondly, there are more stout binding posts at the back than you might need. Look closely and you'll see that you're offered not just a choice if impedances, but of unbalanced or balanced speaker connection. If you're connecting the '600 to speakers or switchboxes (such as certain headphone adapter boxes) which have a common ground system, you have to connect the negative speaker lead to the unbalanced terminal to avoid shorting the amplifier, and the positive to the 1, 2 or 4 ohm unbalanced position. Although I used the '600 with a half-dozen speaker types and models, all were able to operate from the balanced outputs.

Inside, where you absolutely must look if you're to understand where the �32,000 went, are the eight matched pairs 6550C output tubes split into two sets on facing sides of the amp. The rest of the valve complement consists of another quartet of 6550Cs acting as regulators, two 7233 regulators, two 6922 regulator amplifiers, eight 12AX7 drivers and two 6922 input tubes. Yep: 34 tubes in all, and you're gonna want factory replacements rather than any old stuff, eh? Still, Audio Research has built in a certain amount of flexibility, so the fastidious user can experiment with 6550s, 6550B, the aforementioned 6550C, KT88s (you wish...), KT90, KT91 or KT100. Note that the company's "current choice for reliability and long service" is the "Russian 6550C", with 2000 hours being a conservative estimate for tube life. What I'd love to hear is the impossible: a pair of '600s stuffed with genuine, M-O Valve KT88s...

As Bill Johnson said - with the kind of joy in his voice that you'd expect of someone who was just told he could thrash a Ferrari around the old Nurburgring and hang the caution - "This is the first time that the Marketing Department told me that I could design the product first, and then think about pricing." This is the kind of stuff that all designers dream about, when the bean-counters have a good day or maybe someone spiked the water cooler and they thought the decimal looked better one point over to the left. But if there's a temptation to assume that Johnson looked back to the earlier ARC classics like the D150 and D79, the kind of power amplifiers which achieved classic status and prices to match, think again. The '600 really is a 'clean slate' operation, the company's intention being to produce a model which lived up to a 'reference' tag, and which could drive any speakers on the market. To create a 500W masterpiece, Johnson opted for 16 tubes producing a little over 30W each, for absolute reliability. Compare this with a number of amplifiers on the market deriving 70W from a pair of 6550s, with no reports of tube failures, and you can bet that the '600 will not be consuming glassware with the regularity of popcorn in a microwave.

Much is made of the stability factor, the '600 using tube regulation throughout for the output screen grids, the front-end circuitry's B+ voltages and the driver and output stage's B- voltages. The 16 output tubes combine with 50 percent partial cathode coupling to allow ARC to design and use a highly efficient, tightly coupled output transformer. The 50 percent cathode coupling and the low impedances involved preclude the need for constant tube re-biasing, and the company even suggests that users needn't worry about 'pin-point' bias matching. (Stop smirking: I've seen audio casualties spend hours trying to match the readings on their A-V-O meters to two decimal points...)

Specified as delivering 500W at 16 ohms from 20-20kHz, the '600 is also described by ARC as capable of producing 550W under average circumstances and 600W under ideal conditions. As this is still less than 38W per tube, you can relax. The '600's power bandwidth's -3dB points are 12Hz to 80kHz, frequency response is 2Hz to 200kHz, and overall negative feedback is 12Hz. Although the matching preamp offers both balanced and unbalanced inputs and outputs, the Reference 600 only accepts balanced signals, via XLRs. The input sensitivity is 2.0V RMS for rated output, while the impedance is 200k ohm. It's entirely direct-coupled to the outputs, all stages are constant current, and this is about as far as you're gonna get from the current craze for 8W out of a lone 300B without going solid-state.

Equally devoid of compromise is the quite breathtaking Reference 1 line-level-only preamplifier. What separates it from previous Audio Research designs is the microprocessor control of volume, source selection and balance, all of this taking place outside of the signal path. The microprocessor provides memory so the source selector remembers whether you're using balanced or single-ended inputs - inportant as every single one of the eight inputs - three auxiliary, tape, tuner, CD, video and one marked phono in anticipation of an external phono stage - has duplicated RCAs and XLRs, though you can't have both connected at the same time. So, no, this isn't actually a 16-input pre-amp, but an eight-input with choices.

Read more about the Reference 600 amps on Page 2.

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