Audio Research Reference 6SE line preamplifier Measurements

Sidebar 3: Measurements

From the outside, the Audio Research Reference 6SE looks identical to the Reference 6 preamplifier that Michael Fremer reviewed in December 2016. I measured the REF 6SE's performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Like the earlier preamplifier, the REF 6SE's volume control operates in approximately 0.3dB steps. The gain with the volume control set to its maximum—"103" on the front-panel display—was 12.2dB, balanced or unbalanced input to balanced output, and 6.1dB, unbalanced input to unbalanced output. The unity-gain setting was "62." Both sets of inputs and outputs preserved absolute polarity (ie, were noninverting). The unbalanced input impedance was 41k ohms at 20Hz and 1kHz, dropping slightly to a still high 34k ohms at 20kHz. As usual, the balanced input impedances were twice the unbalanced values.

The unbalanced output impedance was 308 ohms at 1kHz and 20kHz, rising to 592 ohms at 20Hz. The balanced output impedance was 669 ohms at 1kHz and 20kHz and 1394 ohms at 20Hz. The higher output impedance in the bass led to a low-frequency rolloff into the demanding 600 ohm load, which reached –3dB at 25Hz (fig.1, cyan and magenta traces). Into a high impedance (fig.1, blue and red traces), the output was flat down to 10Hz and was just –1.5dB at 200kHz. This graph was taken with the volume control set to its maximum; note the superb matching between the channels in this graph. (The match was equally good at lower volume-control settings.) However, as with the Reference 6, the Reference 6SE's ultrasonic bandwidth decreased at lower volume control settings. At unity gain, the output was down by 3dB at 90kHz, but it was still flat almost up to 20kHz.

1020ARC6SEfig1

Fig.1 Audio Research Reference 6SE, balanced frequency response with volume control set to maximum gain at 1V, into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (2dB/vertical div.).

The REF 6SE's channel separation was superb, at >100dB below 3kHz and still 83dB in both directions at 20kHz. In balanced mode, the REF 6SE was very quiet. Even with the volume control set to its maximum ("103"), the wideband, unweighted signal/noise ratio, ref. 1V output with the input shorted, was very good, at 81dB. This ratio improved to a superb 99.9dB, right, and 98.1dB, left, when the measurement bandwidth was restricted to the audioband and to 103.2dB and 100.9dB, respectively, when I switched an A-weighting filter into the circuit. Repeating the S/N measurement from the unbalanced outputs reduced the wideband, unweighted ratios by 15dB, and even with an A-weighted measurement, the unbalanced ratios were 10dB lower than the balanced ratios.

This can be seen in fig.2, which shows the spectra of the REF 6SE's balanced (blue and red traces) and unbalanced (cyan, magenta) noise floors while it reproduced a 1kHz tone at 1V into 100k ohms. Some very low-level, odd-order harmonics of the AC supply frequency can be seen in both channels, perhaps due to magnetic interference from the power transformers being picked up by the tubes' steel pins. However, even with the unbalanced outputs, all the spuriae lie below –100dB (0.001%) and will be inconsequential.

1020ARC6SEfig2

Fig.2 Audio Research Reference 6SE, spectrum of 1kHz sinewave, DC–1kHz, at 1V into 100k ohms with balanced outputs (left channel blue, right red) and unbalanced outputs (left cyan, right magenta) (linear frequency scale).

While I was measuring the Audio Research preamplifier, I had some anomalous but unrepeatable test results, which led me to wonder whether, as the REF 6SE's top panel is acrylic rather than metal, it doesn't shield the circuitry from RF interference. I investigated this by measuring the wideband, unweighted signal/noise ratio at the unbalanced output before and after turning on a compact fluorescent light bulb. (CFL bulbs may be efficient, but they emit RF interference.) While bringing the CF bulb close to the Audio Research's top panel reduced the wideband ratio by 8dB in the left channel and by 20dB in the right, the degradation was just 1dB in both channels with the CF bulb a couple of feet away. Even with its acrylic top panel, the REF 6SE appears to be sufficiently shielded from RF interference.

Fig.3 plots the THD+noise percentage against the left channel's balanced output voltage into 100k ohms. Below 1V output, the distortion is buried beneath the noise floor, indicated by the upward slope of the trace with decreasing voltage. The distortion slowly rises above 1V but is still just a low 0.1% at 60V RMS, which was when the Audio Precision generator was outputting its maximum signal of 15V. With clipping defined as when the THD+N reaches 1%, the single-ended output clipped at 10V into 100k ohms, which is still well above any level needed to drive a power amplifier into overload, though the distortion at lower levels was higher than it is with the balanced output. The balanced output clipped at 2.5V into 600 ohms; the REF 6SE should not be used with power amplifiers having an input impedance of less than 10k ohms.

1020ARC6SEfig3

Fig.3 Audio Research Reference 6SE, balanced distortion (%) vs 1kHz output voltage into 100k ohms.

I examined how the THD+N percentage changed with frequency at a balanced output level of 2V into 100k ohms, which is sufficiently high to ensure that I was measuring THD rather than noise. The right channel's distortion (fig.4, red trace) is slightly higher than the left's (blue), but both are still extremely low. Fig.5 indicates that the only distortion harmonic present in the balanced output was the second, which would be subjectively innocuous even at higher levels. The second harmonic lay at –100dB, 0.001%, in the left channel (blue trace) and –93dB, 0.0022%, in the right channel (red). The second harmonic rose to –70dB (0.03%) at the same output voltage from the unbalanced outputs. Intermodulation distortion was also extremely low (fig.6). The second-order difference product at 1kHz with an equal mix of 19kHz and 20kHz tones lay at just –93dB (0.0022%) in the left channel and –97dB (0.0014%) in the right. Higher-order intermodulation products were even lower in level.

1020ARC6SEfig4

Fig.4 Audio Research Reference 6SE, balanced THD+N (%) vs frequency at 2V into 100k ohms (left channel blue, right red) (linear frequency scale).

1020ARC6SEfig5

Fig.5 Audio Research Reference 6SE, balanced spectrum of 50Hz sinewave, DC–1kHz, at 1V into 100k ohms (left channel blue, right red) (linear frequency scale).

1020ARC6SEfig6

Fig.6 Audio Research Reference 6SE, balanced HF intermodulation spectrum, DC–30kHz, 19+20kHz at 2V into 100k ohms (left channel blue, right red) (linear frequency scale).

Like the original REF 6, Audio Research's Reference 6SE offers measured performance from its balanced outputs that is above reproach. However, as its unbalanced outputs offer higher distortion and noise, the REF 6SE's balanced outputs are to be preferred, as long as the preamplifier is partnered with power amplifiers featuring high input impedances. As JVS's Dan D'Agostino amplifier has an input impedance of 100k ohms at 20Hz and 1kHz and still 70k ohms at 20kHz, it was a good match for the Audio Research preamplifier.—John Atkinson

COMPANY INFO
Audio Research Corp
6655 Wedgwood Rd. N, Suite 115
Maple Grove, MN 55311
(763) 577-9700
ARTICLE CONTENTS

COMMENTS
CG's picture

This beggars the obvious question - Which provides the magic? The setup or the gear?

Certainly it's a combination, but is setup undervalued? It reads to me like the system setup for Jason was roughly as significant in defining the sound quality and appeal as the new preamp. Perhaps I'm reading a lot into this, but what's going on? Is this like cooking, where the preparation and the seasoning often matters as much or more than the raw foodstuffs?

I've often marveled at how enthusiasts are willing to buy preamp after preamp, as one example, but will not even consider adding modest acoustic treatment of their listening quarters. Is that really sensible?

One famous owner of an equally famous cable company spoke many years ago in Stereophile how the selling prices of his products were based on the magnitude of sound improvement brought by using those cables over others. The actual cost of the product for him and the rest in the supply chain was irrelevant. If a certain cable made as much improvement as jumping to a three times as expensive amplifier, shouldn't the pricing for that cable be the same as for the amplifier? Whether you agree with that pricing philosophy or not, and it certainly has fanned some of the flames associated with the cable argument, there must be some basis in the results since people continue to buy more expensive cables. Herd mentality?

I'm not offering an opinion here. Merely asking.

Allen Fant's picture

Bravo! JVS
I really enjoyed the introduction leading into this gear review.
A Reference System + Equally Good Music = Happiness! Simple.

latinaudio's picture

... your article dismantles all comments directed to overvalue the relative low cost and high quality of the Benchmark LA4. It's a good device, but it's "light years" behind the best despite its flawless specs.
I'm tired of reading from objectivists that Benchmark products are "supreme" just because JA achieves excellent results when measuring them.
Audio Research, Dagostino, Ayre, DCS, etc. are out of my reach, but from what you and other reviewers have expressed they undoubtedly sound "in another league". So no: the Benchmark products are not what its fans say they are: cheaper yes, but better never...

Kursun's picture

Yes, the more money you pay sweeter it gets, for some!
As a matter of fact, overpricing is a well known marketing strategy as a bait for those people.

latinaudio's picture

If it sounds better it's simply better, what else is it about when we evaluate equipment? If it´s more expensive isn´t necessarily better (PMC fact.8 signature comes to mind).
Until the day you know what to measure exactly (and how to do it!)you will have to trust your ears absolutely. Like the ones from the stereophile reviewers, which is why I assume everyone reads this magazine, or not?

Glotz's picture

... Does not dismantle the other reviews that this and other magazines provided.

You rush to negative conclusion in the same way others rush to praise it.

What about the amp's input and output impedance mismatches with the LA4? JA specifically addresses the good match for the ARC Ref 6SE with the D'agostino, but does that imply it's also a good match for the LA4. Dubious.

While I do agree the objectivists are largely full of it, and on this site they have too overhyped various components in the line, but does it mean that JVS was 'correct' in his dismissal of the preamp to the others?

What about the additive nature of the Ref 6SE? The treble and bass in particular have been addressed over time and tailored. Is that less accurate as a result? Yeah, probably. Does it matter to great sound? No. It sure don't!

I have heard this preamp for about an hour with 160M's with equal gear in a well-set up dealer's system, with great a great LP playback in tow.
I love it!

But that doesn't necessarily mean that the LA4 is worse or less accurate, just different, and different in JVS' system. He, rather, decided to make over-arching claims that the king has no clothes and referencing JGH and others.

I prefer if on the other more 'subjectivist' end of the spectrum we would ease up on the pronouncements that so well polarize audiophiles, or at least refrain from pure 'finality', when the job is to report on what one hears...

tonykaz's picture

It's a nonsense question, isn't it?

A person must discover the totally unique creature that they are.

It can take decades to purge cultural norm guidance.

Once someone stops being the person someone else demanded, life begins it's wonderful and exciting journey.

No-one is a Black Sheep, the people saying Black Sheep are the problem.

Be the unique person you are and celebrate life, everyone will stand with you. ( even if you're a trouble-maker like me )

I've always been pulling for you.

Two thumbs up

Tony in Venice

doug beechwood's picture

So the unexplored question here is... why is there a 20A IEC power connector on a preamp that draws 130 watts? And what connector is on the other end of the supplied power cord; a 15A or a 20A?

The specs shows the unit suitable for operation at 240VAC. Does this mean it has a switching power supply?

I'm awaiting delivery on 290 lbs. of an air conditioner & furnace. Installed by others, with a city building permit, and a 3rd party HERS test. Painfully expensive, but still $7000 cheaper than this item.

stereosnarf's picture

I've owned several pieces of ARC gear and they are great, but...

It really is all about the sum total of all the components and it's tough to know in advance what plays nicely with what. A mismatch of just one component can leave you feeling cheated.

An example; I own a Parasound JC5. I also auditioned the A21+. They are great amps... until you hook them up to a Magnapan speaker. That pairing is awful. The bass shrinks dramatically and the highs bloom a bit too much. Switch over to some cone-driver speakers (Ryans) and suddenly all is well. The difference is stunning. I can drive the Maggies with ARC or Manley gear and the Maggies and Ryans sound much more similar.(and great)

In that light, it's even easier to understand why manufacturers want you to use their components throughout your system. Theoretically, all mismatch has been eliminated.

tonykaz's picture

ARC was the most desirable Audiophile Brand.

I owned a few pieces and thought it was over rated but it had a nice industrial feel for owners.

Conrad Johnson, that I represented, was also over rated ( in my opinion ).

Today I wouldn't own either brand at eBay prices, much less new prices. ( I will buy at Yard Sale prices for resale ).

A person gets Bragging rights with ARC and pricy replacement tubes.

Still, ARC stands by all their stuff, no matter how old. Gotta love 'em for that.

and

I miss my local ARC Dealer, Harry Francis of Audio Dimensions in Birmingham, Mi. Hello Harry if you are reading this, I wish you were here in Venice-- I'll buy you a nice Sea Food Dinner at Sharky's when you're in town !

Tony in Venice

JHL's picture

...tinged toward negative personal anecdote from the past that dings the integrity of manufacturers or the choices of listeners isn't as pertinent as commentary on the real sound and owner feel of contemporary equipment.

It's free press. Commenters should remember the topic and show some appreciation for the effort to produce it.

tonykaz's picture

Accurate phrasing for sure.

I buy and sell this type of gear on the regular. ( eBay and elsewhere ).

My opinions are based on personal experience ( decades of personal experience ).

My 'Value for Money' equation is the main ingredient colouring my opinions that I'm able to share, they are just as valid as your opinion of my writings.

I share my Price Tranche philosophy with Steve Gutenburg's comfort level in pricing.

Super pricy gear deserves plenty of extra scrutiny and disapproval when justified.

ARC's integrity is outstanding, their dealers are typically outstanding.

Owner Feel is real for ARC, I agree.

ARC's Value for Money is supported by the outfits continuing Service ( for a hefty price ).

ARC has a kind of Aston Martin luxury positioning in the Marketplace.

I'm always happy to buy another ARC piece, I've just never had one that stood out as a keeper and I'm a little suspicious of reviews touting an ARC piece in the Concluding comments. ( which I suspect the reviewer MUST write )

Tony in Venice

ps. if you own an ARC piece, share your performance evaluation experience.

ps 2). the high MSRP Retail Price of this piece is what drives support for the higher used prices of ARC gear.

JHL's picture

...how to string that all together to reflect or expand on the article but you're right that opinions happen. Some hew closer to pertinent reality than others too, which if we're on about sound and hardware, helps.

tonykaz's picture

I've never owned any piece of Audio Gear that performed as well as a Magazine Reviewer reported. I attribute it to Manufacturers carefully selecting pieces to send to magazines. Typically, any two ARC SP 6b's will sound different. Certainly, the Conrad-Johnson Premere 1 Amps, I sold, did not sound like the reporting that Tony Cordisman did on the original review sample ( a rave review ), the new ones I sold ( as a full line dealer ) were not wonderful in any way. ( the MV-45a was the Pavaratti of the Conrad-Johnson range ).

Tony in Venice

JHL's picture

...that about the only way I can, which is as an opinionated negative anecdote from the past with a side of projected manufacturer incompetence.

How it reflects on JVS and the 6SE is unknown.

tonykaz's picture

Jason just wrote his best piece yet, he's a unique person with something to say.

ARC is ( as far as I can tell ) a wheezing Dinosaur, attempting to recapture the illusive Bill Z's tremendous success.

Magnapan, on the other hand, remains the Outstanding Audiophile Contributor from the frozen north of Minnesota.

My opinions, only!

Tony in Venice

JHL's picture

...are a lot of great engineers and a lot of entirely competent manufacturers.

Given that neither original company or chief engineer remain, and given that you're projecting intent against type, how does criticism follow?

tonykaz's picture

Audio Gear priced at Automobile price levels leaves the gate wide open for generous criticism.

The gear should sound outstanding but it typically doesn't ( my experience ).

You can critique me for being critical, if you like.

Being an Audiophile is all about discrimination and critical evaluations.

Plus, there is plenty of room for differences of opinion.

I like ARC for it's ability to attract buyers not for it's sonic performance.

so, I buy ARC at the right price and sell.

Tony in Venice

JHL's picture

...more followup about ARC or its engineers or purported values then. An improvement over imagining them.

I imagine a world in which comments threads trailing free editorial content apply to better sound and camaraderie, not asserting a corporate ethic or quizzing the guy taking the gear home or the socio-economic failings of whomever. John Lennonists will appreciate that stereophile's utopia.

tonykaz's picture

I imagine a world of wonderful music not based on pricy.

All of us are peaceable comrades with unique and valid opinions.

We have a passion for our little niche hobby, mine has been lifelong.

I also have a certain love for Stereophile and JA1 since his HFN&RR days, I don't subscribe or read the other Audiophile Glossys. Stereophile's Editorial Content Percentage is the largest of any Journal I'm aware of. ( I do follow the Audiophiliac on YouTube, of course, and miss Mr.Dudley & Tyll )

Stereophile attracts the intellectual vein of Readership and commenters with it's outstanding Editorial Guidance and mostly verbose & poetic reviewers.

Stereophile is "Home Sweet Home" for Audiophiles Globally.

Tony in Venice

MatthewT's picture

Ever get boring and repetative to write? It does reading it.

"I'm always happy to buy another ARC piece, I've just never had one that stood out as a keeper and I'm a little suspicious of reviews touting an ARC piece in the Concluding comments. ( which I suspect the reviewer MUST write )"

teched58's picture

Seeing your (tonykaz) sig, all this time I thought you were in Venice Venice (i.e., Italy). Who'd have thunk it's actually Florida!

tonykaz's picture

Florida is Quality of Life!

Venice Italy is history !

Venice Florida is an incubator for old people, swimming in the pool, year round sun shine, rainbows weaving thru shades of gray rain clouds, fresh seafood at the Amish Market and very little culture.

Italy is thick with the Cultural Civilization of my ancestors.

I have a Colnago Master lite steel bicycle hanging in my office, it's as close as I'll probably get to living in Italy. ( from the Eddy Merckx era )

Tony in Venice

FredisDead's picture

JVS is no doubt a very nice guy. But IMHO his talents lie in show coverage. His reviews are often too much a personal pursuit and a reflection of himself rather than the subject of his review. It as though where you, me, and all of us would look at the subject of this review and see an ARC Ref 6SE, he is looking at a mirror. The new editor made a mistake. This review should have been assigned to Mike so that the rest of us could read something meaningful, particularly if we want to know the qualitative difference between the Ref 6 and Ref 6SE.
This review is absolutely worthless. In fact, it is outright painful to read.

tonykaz's picture

I've been overly harsh too.

I need to soften up.

I'm blaming my anger on what the donkeys did to my Bernie.

Oh well, moving on.

Sorry isn't strong enough.

Tony in Venice

gtaphile's picture

I wish the review samples of these hybrids would include NOS replacement tubes as part of the kit. In my opinion having played with all aspects that one should play with (power conditioning, PC's, IC', equipment stand, footers, room treatments etc.) to optimize the preamp, the NOS changeover is the largest improvement. Clearly the opaqueness is lessoned as well image focus is improved.

ejlif's picture

I don't see the point of reviewing a preamp when you are digital only and running a DCS system known to be just as good if not better than running through any preamp at any price. If you have multiple sources and need switching then you are stuck with having to use a preamp but digital only, no preamp is a no brainer unless you just want to spend more money. The cables alone used are more than the cost of the preamp. 30K + for a marginal improvement and possibly worse sound is not much of a sales pitch.

Jason Victor Serinus's picture

that your statement is not true. dCS never claims its volume control is the end-all, be-all. If it did, it would not display with preamps from other companies at shows. Regardless, even if the dCS volume control were superior to every possible preamp design, it would still make a useful base point for comparison.

ejlif's picture

it is made clear by your review that a spending an additional 30K may get you nothing and if not worse sound maybe. It seems that most reviewers of DCS say ditch the preamp. I have a Rossini and clock but never had preamp with it. I'd like to add vinyl but just to much to try and find a preamp that will not actually degrade the sound of the Rossini. I think I'd go for an AD converter and phono pre before going to a regular analog preamp.

Jason Victor Serinus's picture

Glad to learn that you're asking based on personal experience. While the AR preamp definitely costs less than $30,00 - it's under $20,000 - the preamp I now use with my Rossini DAC/Clock combo is the D'Agostino Momentum HD preamp. It's more than $30,000 - $40,000 to be exact. Please see my review. I never cease to be amazed by this baby. It heightens color contrasts, but not in an exaggerated way; enlarges images in natural proportion; and is a monster when it comes to bass. It's also as smooth as can be. I had heard it with the Vivaldi stack at at least one store demo and perhaps a show before it came to me for review.

Ortofan's picture

... "heightens color contrasts", "enlarges images in natural proportion" and "is a monster when it comes to bass", is this device somehow extracting additional information from the input signal or is it creating these artifacts?

Is it simply a preamp - as in providing gain, volume adjustment and input switching functions - or is it additionally performing some sort of processing function to subjectively enhance the audio signals passing through it?

Jason Victor Serinus's picture

Your comment presupposes that there are absolute colors and image sizes on a recording, and that any preamp that, in contrast to other preamps, enlarges images, heightens contrasting colors, and transmits more bass information creates artifacts that are the results of "processing" and "enhancement." I would argue instead, based on what I hear, that the D'Agostino Momentum HD preamplifier moves reproduction one step closer to the real thing.

Take, for example, Seattle Symphony's recording of Richard Strauss' Also Sprach Zarathustra. I attended one of the two performances from which the live recording was assembled, and sat 8-10 rows back. There is no way that my Alexia 2s and the rest of my components, in my 16 x 20 x 9 room, can reproduce the sheer volume of the instruments that created the huge, glorious sounds I heard. My system can't move as much air, and if I turn the volume higher in an attempt to do so, I overload the room and potentially damage my hearing. The D'Agostino Momentum HD preamp moves me one step closer to what I heard compared to any other preamp I've ever used in my system, or what I hear when I rely on the dCS Rossini DAC's volume control. It takes me one step closer to the truth of my actual experience.

Having used the now departed from Serinusland dCS Vivaldi DAC in my system, I know that if I had it in my system instead of the Rossini, images and bass would be larger and weightier, colors would be even more saturated, and instrumental textures and the contrasts between them would be more palpable. Is the Vivaldi, in turn, adding information, bits, artifacts, or whatever you might wish to call them? Is it enhancing the signal? I would suggest instead that it enables me to hear more of what's on the recording. To my ears, the D'Agostino Momentum HD preamp does the same thing.

Ortofan's picture

... wasn't the Audio Research Reference 6SE the subject of the discussion?

Nevertheless, the question remains as to what aspects of the transfer function of a preamp (D'Agostino Momentum HD, or otherwise) might possibly account for its purported ability to allow a listener to subjectively "hear more of what's on the recording" than when it is not in the system?

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