Audio Research Reference 600 Power Amps Reviewed

Published On: January 11, 2009
Last Updated on: October 31, 2020
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Audio Research Reference 600 Power Amps Reviewed

This amp means serious business. If the massive size didn't give it away, maybe the VU meters did. It's a tube amp, but its rated at upwards of 600 watts, depending on the speakers. Interested yet? You should be because these tube amps are the state of the art performers.

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.

Additional Resources
• Read more stereo amplifier reviews on
• Find an AV receiver to pair with this amplifier.

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.


What's unusual about the Reference 1 is a ruse employed to prevent technofear. ARC rightly presupposes that its customers are used to rotary controls and may even prefer them, in the way that car manufacturers still haven't succeeded in convincing drivers who actually use their gauges that digital read-outs are better than analogue ones. So the '1 wears knobs, only they don't rotate. They're spring-loaded, self-centring intermittent types, like the ones on the Copland CD player and the Acurus ACT1 processor/pre-amp. They have small arcs of activity, and you jerk 'em left or right to raise or lower the volume, alter the balance or select sources. Hold them and they pan through the entire arc which surrounds them in the form of green LEDs. Invariably, you'll spend more time using the remote control, which provides every single function bar the choice between balanced and unbalanced input selection. And I must say that, as the remote arrived only the day before I had to surrender the system, you do get used to these intermittent controls very quickly.

Across the front, the '1 wears its gain, balance, record out and source selectors as the aforementioned faux-rotaries. In a channel below are press-press toggles with the same feel as the knobs. They always self-centre, so you push them up or down to choose on/off, balanced or single-ended input, normal or inverted polarity and mute/operate. Green LEDs all over the place tell you what mode you've chosen.

Is there a more jam-packed rear panel than the one on the Reference 1? Across the top are the right-hand phono and XLR inputs; across the bottom, the left-hand channel's inputs. In addition to these eight sets are three pairs of outputs, one for record and two for main, also in XLR and phono form. Lastly, there are easily accessible fuses, and ARC still uses captive mains leads. (Note that these products were the first I've used in '96 with the CE marks.)

Inquisitive readers might wonder how a pre-amp can weigh 30lb. Like the '600, this wears a 3/8in thick aluminium front panel. Inside are the eight Sovtek 6922 dual triodes, chosen for their reliability, while tube regulation is solid-state for maximum silence, via a combination of MOSFETs and JFETs. Part of the weight is also due to the inclusion of three separate toroidal power supplies, one each for DC regulation, high voltages and the digital section. Discrete components are used throughout, the Ref 1 being an IC-Free Zone.

Although only 21 LEDs surround the volume control, there are plenty of steps excess of 156 according to Bill, at 0.3dB per step. You'll soon develop the right flick of the wrist for turning the switch just enough to activate a single step. And you hold it for continuous adjustment. Simple. The balance control operates in an identical manner. Switch-on is accompanied by a 45 second delay, and Audio Research recommends lowering the volume even when you're using mute when changing sources.

Spec-wise, you're going to think I accidentally misplaced the sheet and picked up one from a tranny pre-amp. The frequency response is +/-0.5dB from 1Hz to 200kHz, with -3dB points at 0.3Hz and better than 400kHz. Distortion? Less than 0.01 percent at 2V RMS output. Gain is 12.3dB balanced, 6.3dB unbalanced, tape output is 0dB, and the input impedances are 220k ohm balanced and 110k ohms unbalanced. The output impedance is 400 ohms balanced, 200 ohms unbalanced, and maximum output is 7.0V RMS, or 3.5V RMS single-ended.

As the amplifier arrived long before the pre-amp, I had a chance to use it with a number of respected control units, but I shan't embarrass them: more than once I was told that I wouldn't know what these amplifiers could do until I heard them with the Reference 1 pre-amp, and these harbingers of bliss were right. And while I also tried the Reference 1 with other power amplifiers, I have every reason to believe that these items will sell in tandem; they're just so perfectly matched, as you'd expect of components designed to work together from the outset.

With products so close to perfect, so commanding, so coherent and so blatantly superior to anything else on the planet, it seems daft to break down the performance into those little categories which determine our priorities. So it is with a great sense of inconsistency that I, known as I am to be ambivalent about bass as a priority, must point you to the lower octave brilliance of this system. Consider that I have been using Wilson's System V ever since it was launched, that I know my room pretty well, and that I returned to
the usual source material. The CD player of choice was the Marantz CD-12/DA-12 in balanced mode. Wires were courtesy Harmonix for the speaker connections and Mandrake balanced for the rest - all familiar stuff.

And still I never knew just how much low-end information there was from the piano behind Lou Rawls on At Last, how much more bottom octave activity could be wrested from Willy DeVille's 'Assassin of Love'. More impressive still was the weight added to otherwise bass-free sessions like the a cappella of Bobby McFerrin or the Persuasions. Remember: I'm simply cannot be bothered with gut-churning, plaster-cracking macho-bullshit bass. I find it neither pleasing nor necessary. And yet here was a system which, while not changing my allegiance to the importance of the midband over all else, showed me what sins a component can commit in the sub-60Hz sector. So, no, I didn't spend too much time with my cherished LS3/5As in tandem with the Audio Research package. But I did rediscover a whole lot of material which I thought the Wilsons were being fed intact.

It's not dry, mechanical bass, as is the fruit of much lesser solid-state equipment. Rather, the Reference 600/Reference 1 combination emulates with ease the kind of bottom octave action many presume to be the sole preserve of the better solid-state amplifiers. It's bass without restrictions. Extension, quantity, speed, palpability, 'tunefulness' (gawd, I hate that word...) - pick a trait and I guarantee you that this system excels with it. More delightful still, and part of their overall supremacy as a purveyor of 3D images, is the way the Reference components prove that low frequency information is directional. Elder jazz recordings with unplugged bass were revealed to be so rich with localised bass sounds that I cannot imagine even the most venal of centre subwoofer vendors continuing to argue that bass is non-directional at best.

But the midband... How on earth do you juggle such clarity with so much warmth? A natural acoustic with almost heightened precision? When Editor Harris arrived for a brief listen (coward that I am, I refused to write the review until he confirmed my findings), I condensed the session by playing vocals of a highly textured nature. And SH knows the sound of Rawls, as well as the brilliant counterpoint of Dianne Reeves. Think of a juxtaposition that's better: Rawls' voice so lived-in that it makes Joe Cocker sound like McCauley Culkin, Reeves' voice so sweet, clear and pure that you worry about the absence of texture. And yet their duet on 'At Last' showed how two utterly disparate sounds can blend yet remain distinct, how a truly transparent, coherent system can handle both simultaneously, even when there's a minimum of ancillary instrumentation to cover up any minor cracks.

For vocal range, I played some Howard Tate, the remastered sessions released by Polydor in the USA as Get It While You Can. And SH finally understood why I rate Howard Tate as the only vocalist fit to follow Jackie Wilson. Better still, the sessions were mid-to-late Sixties East Coast recordings, where the studio musicians knew that rhythm was the antithesis of birth control. Sexy? You ain't never heard deep soul until you hear the way that the References plumb those depths. Emotional depths, way down past gut level, right to the core. I haven't been so moved by the sheer presence of music in my home since the first time I heard that Tate album, when the music had to transcend the system on which it was played.

At the risk of annoying my family and neighbours, I did rock out on occasion. I sampled Led Zeppelin in all its glory, got dusty with ZZ Top, trashed my imaginary Les Paul to Jeff Beck. I felt like a kid all over again, discovering hi-fi for the first time, even digging out the recordings which, back in '68, christened my first-ever system. And tried not to think about how much 9,900 and 32,000 equal...

Bill Johnson should retire. He has nothing left to prove because the Reference 1 pre-amp and Reference 600 power amps are, as far as I'm concerned, the finest audio amplification devices ever produced. Read my lips: the very best sound I've ever heard from a hi-fi system in my entire 43 years on the planet.

Now it's up to everyone else to take it from here.

Additional Resources
• Read more stereo amplifier reviews on
• Find an AV receiver to pair with this amplifier.

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