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The Audio Research Reference 6SE Preamplifier Review

Audio Research Corporation is one of the storied high-end brands in audio. When we started The Absolute Sound back in 1973, Audio Research had been around for a few years, and early on in the cycle, we covered their very famous preamp, the SP-3a. Personally, hearing that preamp when it was released changed my view of the significance of front-end electronics. It was just dramatically better than the preamps I knew. Since then, Audio Research has been known as a top producer of preamplifiers and amplifiers, particularly those built around tube circuit topologies.

It may be hard to imagine this for those of you who are relatively new to the audio scene, but there was a time in the early ’70s and late ’60s when the assumption was that transistor electronics were going to rule the earth. The thinking was that tubes were “obviously” the technology of yesteryear. Simplistic technological determinism rarely works, however. There were a few people like William Z. Johnson, the founder of Audio Research, who said, “Wait a second, tubes have got some life in them because there are some characteristics that tube devices have that transistors do not, and those are desirable characteristics”. Johnson set out to build a whole line of products around tube amplification devices (no technological determinist, he also built transistor-based products later on).  Anyway, that is a very brief history of Audio Research, the point of which is that Audio Research has been around for more than 50 years, making great products that have been highly reviewed by publications like ours. I thought we should catch up with one of their more recent offerings and see how it worked.

What Is a Line Stage?

Before I go into sound quality, which is the main thing we’re here to talk about, I do want to briefly talk about what a preamplifier is. Most people would call this a line stage rather than a preamp because it does not contain a phono preamp with attendant RIAA equalization and the gain stages needed for moving coil or moving magnet cartridges. So in modern parlance the Ref 6 SE is a line stage.

In the current technically diverse world, there are all sorts of different architectures for ultimately driving speakers. It may help some of you to go back to basics and talk about what a line stage is. Simply put, a line stage can be talked about in many ways, but I think of it as having three functions. Function number one, and one that’s often overlooked yet not something to be scoffed at, is that a line stage is a switch. The Ref 6 SE has eight inputs: four balanced XLR-type inputs and four single-ended RCA-type inputs, so you can have eight different devices connected to a line stage like this for lots of flexibility in choosing sources (which can be done via the included remote or via a front panel knob). That’s great if you have multiple sources that you want to use and you want them to be easy to use. For example, in my current reference system, I have two different turntables, three different arms and three different phono preamps. So, I’ve got essentially three cartridge-phono preamp combinations connected up to the Ref 6 SE. That’s one example. On top of that, I have a good old-fashioned CD player and two D-to-A converters that I’m running into it. So, right there I’m using six of the eight inputs on the Ref 6 SE. So, it sounds obvious to have this, but the switch has to be done well to not add distortion. And it has to have the right kinds and numbers of inputs for your needs.

Part two is that a line stage is an attenuator (a volume control). You could say it’s an amplifier because line stages have the capability to have more than unity gain, and that’s certainly a necessary thing for some phono preamps or if you’ve got a low sensitivity power amplifier. But in most cases, you’re taking the output of the DAC and attenuating it to drive the power amp to the right output level. The current reference system that I’m using uses the dCS Lina DAC and that’s got a two-volt output, which would drive the power amps I’m using to full output. So, I have to adjust volume via attenuation of that signal because I almost never want full output from the amps. Again, the volume adjustment and the attenuator/amplifier circuitry has to be done in a very low noise, very low distortion way. This takes careful power supply design, refined circuit design and attention to parts selection.

So, a line stage provides a switch, a volume control, and then it is sometimes important to have impedance matching. You want to have your line stage have an output that’s of an appropriate impedance so that it can successfully drive whatever power amplifiers or power amp you are using. Passive volume controls often don’t do this as well as active circuits.

Sound Quality

Back in the day, it was common to say that we wanted our amplification devices to act like a “straight wire with gain”. What we meant was that you wanted your line stage or your phono preamp or your power amplifier to just have gain and do nothing else to the signal — no other form of distortion or alteration of the signal. It seems like a nice idea, but it’s not so easy to do. This is a bit tricky because line stages have been around in some version of the form we’re talking about here for more than 50 years. While we’re talking about a fairly mature technology, we keep having old devices go out of production and new devices appear to lower noise and distortion. As circuits get better, we realize some designs had distortions we didn’t notice. The other part of the problem in evaluating line stages is that you’re sometimes listening to something and not hearing distortion. While that’s what you want, it is hard to describe “nothing”. Loudspeakers all have obvious distortions and colorations, and that tends to be our model of understanding. But that model doesn’t really set the right mental expectations for line stage commentary. Add to that the common observation that subtle differences and minor differences are not the same thing.

With that in mind, let me tell you a little bit about my experience with the Ref 6 SE. It’s been more than six months that I’ve had the Ref 6 SE, and two big things got revealed in that time. Number one was that, as I go through the fairly large number of products that are submitted for review, it was really interesting to me that about four or five months into it, I started realizing that every time I put something new in – a new power amplifier, a new cable, a new speaker, a new D-to-A converter, a new cartridge, a new turntable – I could almost immediately hear that it made a difference. That, perhaps, doesn’t sound too startling to those of you who are familiar with comparing speaker A and speaker B. They almost always sound quite different, and distinguishing between speakers is not that hard. But when you get down to cables, and sometimes when you get down to D-to-A converters or power amps, there are times when you would think it wouldn’t be that easy. Many people hear minimal or even zero difference between some of these components. And yet, every product I’ve put in the reference system, while the Audio Research Ref 6 SE was in the circuit – made differences pretty much immediately obvious. That got me thinking “Hey, this line stage is at least as good as almost all the products that I’m putting into the system”. And that is, I think, a high compliment. To be clear, the idea is that part of why differences between different equipment can be hard to hear is that the pile of distortions of other products mask the smaller differences between components under test.

That the Ref 6 SE doesn’t do this masking with turntables and cartridges and DACs and amps in the $5000 to $50,000 range makes it a reference-suitable element of some rather high-end systems. It also makes it a great piece to build a high-end system around. The Ref 6 SE is $19,500, which may seem expensive. But if you’re dealing with a $100,000 to $200,000 system, or maybe even a $50,000 to $100,000 system, this is a perfectly appropriate line stage from a budgetary standpoint. Because it just keeps revealing difference after difference after difference, it is at least as good as the other kinds of products you might put into such a system. And, importantly, that may be quite useful when you are building toward such a system.

I did compare it to some lower-priced preamps that I had, and every time, the Ref 6 SE was more revealing. I particularly heard specific domains where I thought it was superior. The big one that I want to talk about is that in terms of soundstaging — width, and depth and the ability to do layering and place instruments and voices in the right position, front to back – the Ref 6 SE was superb. This area of soundstaging is something that requires very low noise. I believe that low noise signature is why the Ref 6 SE keeps revealing itself as sounding better as I’ve tried it with better and better equipment. Note that low noise has its benefits not in simple hiss reduction. It sounds better on the dimensions of soundstaging and the details of the hall or club and the spotlighting of performers. I’ve come to think of this domain as the sine qua non of very good audio electronics these days, because flat frequency response and low harmonic distortion simply aren’t that hard to do now.

The second thing that I realized was that the Ref 6 SE didn’t have the treble or transient hotness that you sometimes get with products that seem detailed and revealing and dynamic and transparent. With some products you realize, after the initial drama subsides, that they aren’t quite right and they don’t seem like live music. In contrast, the Ref 6 SE is not faking its sense of transparency and dynamics. I found that this element of its sound made listening to music very enjoyable and it wasn’t distracting due to some kind of subtle artificial distortion. I will say, though it is just one reviewer’s opinion, that 15 years ago there was a lot of work being done in high-end audio that I think erred on the side of faking it a little bit. You often got some transient hotness that wasn’t really right, and it was a little bit annoying. And you felt like you had to suffer a little bit to get the sense of transparency that a lot of us enjoy when we’re listening to live concerts. So, I thought that transient naturalness was another wonderful element of the Ref 6 SE.

A third strength of the Ref 6 SE, one that I can’t really be sure of because I haven’t had the big CH Precision or a big Soulution preamplifier in my system, is that I feel the Ref 6 SE, if it is shaded or colored at all, is shaded to be 1% off — a tiny amount in my lexicon – on the soft side. And I think that’s actually the side to err on given that you’re going to have errors in any product, and given that subtractive distortions are less distracting than additive ones. I can’t even really be sure of this softness, I just want to say it because I think that’s if there’s any character to the Ref 6 SE, it’s that minute bit of softness. Again, when people listen, for example, to cartridges, they almost always choose a cartridge that’s 5% or 8% or 10% soft (or 5% bright or both). So, I just want to put 1% in context. A lot of people in my experience prefer movement in the direction that Audio Research has, I believe, chosen. I thought the Ref 6 SE was beautifully judged in terms of how it went about this particular way of presenting the music.

Now, as you probably can tell, it’s a little bit hard to review products that are this close to the “straight wire with gain” world. As another way of going at this evaluation, just before writing this I downloaded the firmware that takes the dCS Lina DAC and installs a digital volume control. I also was coincidently sent the Lumin T3 DAC which has a different implementation of a digital volume control. I thought to myself, “Hey, this is actually kind of an ideal scenario. I can compare these DACs through the Ref 6 SE and without it. Without the Ref 6 SE all you’re adding is a theoretically harmless math process that makes the digital volume control work. If the Ref 6 SE is very transparent, the differences should be minimal.”

Yes, let the food fight begin on whether digital volume controls add distortion or not. I’ll leave that for my reviews of these two products. But let me say just as a summary: digital volume controls, as implemented by D-to-A converter companies who really know what they’re doing, work quite well.

So, I thought to myself, “Here’s a good opportunity to A/B no preamp with a preamp”. And I’m here to tell you the differences are microscopic. If I had to say, there might be, occasionally, on certain recordings, a slight benefit to the preamp being out of the circuit. But every time I thought I heard that, I then went back and listened again, I realized, “Oh, I didn’t really have the volume matching right.” Or “Oh, when I heard this character of this song the second or the first time, I played it when I played it again with the preamp in the circuit, it sounded very close to identical.”

If you’re deeply bothered by the last 1% of performance, which after all is part of what we’re here to talk about, and if you don’t need the switch function that I talked about before, you might look at these DACs that have digital volume controls. But the big message, I think, is that at the level of line stage we’re talking about with the Audio Research Ref 6 SE, you are giving up very close to nothing except money (on the class of equipment that I’m using) by having the line stage in the circuit. And I think that’s very good news because for those of us who have significant investments in and love of vinyl and turntables, and phono cartridges and phono preamps, you really need that switch functionality. And I like to use other sources as well. And so I need the switch. And in general, the D-to-A converter companies who have built digital volume controls haven’t always put switches in their products. Some DAC products do have switches, and that’s nice, although they’re generally, I think, not as flexible as the Ref 6 SE and I don’t know if the switch and inevitable additional ADC/DAC functions are as transparent as is the DAC alone with digital volume math. Anyway, I’m here to tell you from what I could hear, the differences are so microscopic that I think you’re being distracted by trying to listen for the differences rather than just enjoying the music.

I was really pleased with the results with the Ref 6 SE. Now, I do have to say, if we’re going to talk about budgets, if you don’t need the switch, if you don’t have to have multiple inputs, if you just want to stream, let’s say, you can save $19,500 by not having this preamp, and that money might be invested someplace else in your system. And that seems like a reasonable choice to me. So, from an economic standpoint, having a preamp if you don’t need it is kind of dumb. But other than that, if you do need the switch and you want it to be a really good and superb switch/volume control/impedance matcher, the Audio Research Ref 6 SE is close to a “straight wire with gain” and thus an excellent candidate for you if your surrounding equipment is or will be of sufficient quality.

Appendix: The Reference System

I should mention the reference system I used because you might want to know the level of other equipment that I’ve tried in coming to these minimalist conclusions. There is equipment well above the price range that I’m talking about that may be better and might reveal that the REF6 SE isn’t up to the task of working ideally with that kind of equipment. And it might be that significantly less expensive equipment won’t benefit from the transparency of the Ref 6 SE, at least not in a cost-effective way.

The reference system I have used during eight months or so with Ref 6 SE is straightforward. As a primary source for streaming of high-res, which is my default source, I am using the dCS Lina 1.1 DAC with the Master Clock. Review here. dCS has a special DAC topology that in my experience works extremely well.

I also have two turntables. I’m using Technics SL-1210G, and that has two different Hana moving-coil cartridges, the Umami Blue, and the ML. And those are being run into the Hegel V10 phono stage. I also have a J. Sikora Initial Max turntable that has two tonearms mounted on it: the J Sikora Kevlar 12” arm which has the DS Audio DS003 cartridge running into the DS Audio preamp box (you need a special preamp for DS Audio optical cartridges). I also have a Kuzma 4 Point 9-inch tonearm with a Benz Micro Gullwing SLR cartridge. The Benz is running into a Musical Surroundings Phenomena III phono stage. So that’s my vinyl setup at the moment.

I have an Oppo BCD-105 CD player, which I mostly use to play some legacy CDs that aren’t available on streaming. The Oppo has ESS DACs in it, though if I used it more I would run it into the dCS Lina.

From the Ref 6 SE I go into PS Audio BHK 600 monoblocks – nominal 600 watts per channel amplifiers. And I have those amplifiers driving Magico A5 loudspeakers.

Audioquest signal cables are used almost exclusively. And Shunyata Venom AC cables come out of a Shunyata Venom power director.

In modern high-end terms, it’s a mid-priced system. Although when we got to calling $100,000 audio systems mid-priced, I can’t really tell you. But here we are. I think it’s very good, but the point of the reference system is to be capable of revealing differences, not necessarily the best system possible for some specific budget. I’ve listened to lots of speakers that I’ve had in here with lots of power amplifiers and multiple preamps that I’ve compared to the Ref 6 SE. I’ve used other turntables, and multiple other D-to-A converters. And as I said, the punchline on the Ref 6 SE is that it reveals differences between those components almost instantaneously without seeming to impose any readily identifiable deleterious sound on the operations. It is also as smooth and glitch-free in operation as I’ve experienced.


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