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StereoKnight Reference Silverstone-B&R and Magnetic Enigma-1.0R Preamplifier Review

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StereoKnight Reference PreamplifierRemember the simile/metaphor for an ideal amplifier: the “straight wire with gain”? It was always silly to say that such-and-such product was so close to a “straight wire with gain”, that you wouldn’t be able to tell the difference. Really? Has there ever been more obscene marketing hyperbole? Okay, well, “perfect sound forever” must rank first, but “straight wire with gain” is right behind it.

But there is something that is very close to a “straight wire with gain”: a wire wrapped around a magnetic core. Even though that means it wouldn’t be straight anymore, it’s a transformer, and whether it’s a traditional design, or autoformer, it can amplify a voltage without resistors, capacitors or active devices. James at StereoKnight sent the Silverstone-Balanced, a balanced passive line stage that worked wonderfully with his M-75 monoblock tube amps. The two were designed together, so it follows that they should work well together. When the diminutive Silverstone-Balanced was paired with the M75s, it acted as a phase splitter when amplifying a single-ended input. A single-ended input would drive the balanced outputs of the Silverstone-Balanced, and bypass the phase splitting input transformer on the M-75. The results were great: transparent, ultra clean, tonally accurate, and noise-free.

James subsequently sent his two upscale preamps for review, the Reference Silverstone-B&R and his Magnetic Enigma-1.0R. The Silverstone B&R is a remote-controlled version of the little Silverstone-Balance, but with a few extra taps on the transformers to allow a few more dB of gain. The Magnetic Enigma is the Silverstone B&R with two active tube stages. What I wanted to know was how these work with a diverse selection of equipment, not just the M-75 amps. How would they compare to other line stages, and can a transformer completely replace an active linestage?

Simple or Complex?

The Silverstone B&R looks like an active preamp; it’s heavier than some amps I’ve had, so you’d be excused for thinking it was active. The Silverstone packs a full complement of input and output transformers, with computer-controlled relays to switch taps, adjust balance, select inputs and outputs. The digital stuff is segregated to the front of the chassis where the ICs handle the house keeping chores. There is an aluminum wall separating the two, and StereoKnight goes to lengths to make sure the control circuitry doesn’t inject noise into the signal carrying circuits. Build quality looked first rate, with lots of machined billet aluminum, all joined together with good quality socket-head machine-thread screws. With the cover off, it lost none of its rigidity, something that is a little rare in consumer electronics.

StereoKnight Silvertone B&R preamplifier

Both preamps have a similar exterior appearance, even down to IEC power inlets. It’s not until you open them up that you see how much stuff is added with the active circuitry of the Magnetic Enigma-1.0R. The Enigma packs eight transformers: four input transformers, two balanced output transformers, a power transformer for the tubes’ power supply, and a separate transformer for the digital switching stuff. There are 66 relays (33 each channel) to step up and down the balanced transformer taps. There is active voltage regulation for the audio power supply, with MOSFETs, polypropylene bypass caps and lots of other stuff on a densely packed PCB. Per StereoKnight:

“The unique design of the new tube preamp model “magnetic Engima-1.0R” can be implied from the prefix of the name: “magnetic”, which indicates the direct origin from StereoKnight magnetic passive preamp. Not only does Enigma-1.0R use 4 transformers for volume attenuation, but also another 2 transformers for signal output and two more for power supplies! “

When using either of these preamps, I never lacked for connections or flexibility. The only possible issue might be integration into a full-featured home theater. I don’t do the surround-sound thing, and really don’t watch a lot of films. As the center of a high-end system, both preamps had plenty of connectivity and was quite easy to figure out.

Living with the Silverstone B&R

As I stated previously, what struck me as novel with the passive Silverstone B&R is that you can derive a balanced output from a single-ended input, though this functionality is not unique to StereKnight. At which stage is this accomplished, I’m not sure. I would guess that it happens in the input transformer since the volume control is balanced. When I had the StereoKnight M-75 mono-block tube amps here for review, I was driving them with the balanced output of the Silverstone B&R whether the source was balanced or single-ended. An interesting feature of the M-75 amp is that it is balanced from the input and uses a phase-splitter transformer to drive the input if you are using a single-ended preamp. If you use a single-ended preamp, you flip a switch on the back of the M-75 and it takes the RCA input to a phase-splitting input-transformer. The transformer produces the needed plus/minus signals for the balanced circuitry of the amp. Again, I don’t know how novel this is, but it was my first time to use a balanced amp with a phase-splitting-input-transformer for non-balanced inputs.

When driving the M-75s via the XLR output of the Silverstone B&R, single-ended sources sounded much better than the alternative option of using the RCA output of the Silverstone and the RCA input of the M-75 amps. Is it because the signal is routed through another transformer at the input of the M-75 amps when it is accepting an unbalanced connection? Is it because the balanced connection is inherently better than the unbalanced/RCA connection? Is there some kind of problem with a balanced-transformer-volume-control driving an unbalanced line? I don’t know for sure, but it’s probably a combination of factors. Just the thought of a CD player driving the balanced circuit in the amp because of the magic of magnetics makes me smile. It’s just so elegant!

I didn’t have the luxury of trying out numerous amps that had both single-ended and balanced inputs. When I was using the Sanders ESL Amp, it was mostly paired with the Sanders 10B speaker’s electronic crossover which acted as a one-input balanced-preamp, so I didn’t use a separate preamp. When I got around to using the ESL amp with the Magnepans, I preferred the balanced connection from the Silverstone over the single-ended.

In all instances, I had better dynamics, a more stable image and better low bass when going balanced. I did not hear anything lost or destroyed when going from single-ended to balanced: no smearing, no losses. If anything, the music had less of a mechanical-electrical character when running balanced. Perhaps it’s because balanced cables reject more noise. Also, might not a balanced cable have much less characteristic sound, cable sound that is, because of common mode noise rejection? That might not account for better low bass though. I’m not going to say that balanced is always better, and that unbalanced products are inferior from the get-go. That would ignore the fact that there are unbalanced products that can go head-to-head with the StereoKnight components and sound just as good with unbalanced connections.

So, if I haven’t lost you yet: balanced output driving a balanced amplifier gave better sound with the Silverstone B&R preamp. All my positive observations are based on the balanced connection. When in single-ended mode, it sounded excellent, but lost some bass depth, transparency and dynamics. That brings the StereoKnight back down to earth, but still as good as most gear I’ve heard.

The Silverstone B&R did something that only fine equipment can do: time dilation. It seemed to me that time passed more slowly with the Silverstone. It’s hard to explain, but it’s not unlike being in an accident. People will say “it seemed like forever”. Another example is when your favorite sports team is in the playoffs. A big play, either for or against your team, will feel like it took much longer to transpire than it actually took on the clock. The commonality is that in the case of the accident, the “big play”, and good audio equipment, you are immersed in the event. Extraneous concerns are forgotten. You mind is paying much closer attention than normal, whatever normal is. I think what is going on here is the lack of electronic smearing, of the signal being pushed through nichrome wire, through signal obliterating capacitors, through solder (solder sucks as a conductor) or through anything that amplifies. The clarity of eliminating all that crap helps the brain suspend disbelief (suspension of disbelief). The immersion in the music is easier when you hear more detail and less of the unnatural electronic-ness added by amplification.

It’s hard to qualify the sound of a transformer volume control, or TVC. It’s hard enough to describe the differences between interconnects. So, it comes down to explaining the difference between this TVC and an active preamp. In comparison, the Silverstone B&R has much better delicacy, more exact timing, better microdynamics, virtually perfect tonal balance, no noise (none), and dimensionality on par with anything active. What it seems to lack, and I don’t know if this is an artifact of amplification, is slam or weight. There is the distinct possibility that the active circuit is dumping (like losing control) or that the active circuit is rounding off the attack which makes the shape of the note seem “hefty”.

You can run into instances where the output impedance of a TVC is a mismatch with the input impedance of the following amp. I didn’t run into that issue during the review, and I did audition two transistor amps with the Silverstone. If you didn’t know, a tube design usually has higher input impedance than a transistor design. A passive preamp might have difficulty driving the lower input impedance of a transistor design. That’s a generality.

The Magnetic Enigma-1.0R

The Enigma uses 6922, same as the 7308, E88CC, etc, and the 6H30. I’m going to go out on a limb and publicly state that I don’t care for the “Russian super tube”, the 6H30. In everything I’ve auditioned with this tube, I’ve heard some glare and graininess. It’s an opacity overlaid with electronic edge. Manufacturers started using this tube because of the impressive specs. But just because a tube looks good on paper doesn’t mean it will sound good. If you try to use that justification, you are no better than the automatons who declared “all amps sound the same”, “perfect sound forever”, etc… I’m not alone in this opinion. A number of friends feel the same way. I try to avoid the 6H30, but its ubiquity makes that almost impossible.

Apparently, there is a premium version of the tube, the “DR” tube. According to internet, font of truth that it is, the DR version is way better. The problem is that they are overpriced for what they are—a surplus military tube. I don’t have a problem shelling out hard earned cash for Telefunken, Fivre, MOV, Bendix, Tung-Sol, RCA Special Red, Ediswan, etc.. Those old tubes sound great and usually last several times longer than current production. In the case of the SQ tubes and many industrial types, they’ll last 10,000 hours or more. If the internet is to be believed, not only is the DR version of the 6H30 overpriced, it doesn’t last long enough to justify the cost. By the way, I really like the 6N23EB: it’s the only dependable low-noise 6DJ8 I have found for amplifying low-output moving-coils. I just wanted to point out that I’m not against Russian tubes, I also like the 6C33C, though its propensity to burn up sockets scares me.

I heard some of the glare with the Enigma that I’ve heard with other 6H30-equipped gear. There was plenty to like, though. The Enigma was very quiet, had realistic space, and possessed a powerful sound that makes classic tube preamps sound puny, such as those using popular low-transconductance tubes like the 12AX7. In the end, I wasn’t quite satisfied.

Not wanting to give up on the Enigma, I searched for an alternative and came up with the 6N6P, which is electrically similar to the 5687, a fine sounding tube. Also, a friend suggested the 6GU7 and 6CG7. The 6CG7 is a 9-pin version of the 6SN7. The 6GU7 is similar to the popular 12BH7 (one of my favorites). Rolling through just a handful of these alternatives showed that there is plenty to be gained (or lost?) by experimentation. Not only that, but you can roll quite a few choices of 6DJ8 and the many variants.

I wound up getting very nice sound with a British military E88CC variant, one of the CV valves, and 6GU7 RCAs. It wasn’t a perfect tube substitution: the sound went from slightly hard and cold to slightly too romantic. But, romantic matched well with the Sanders ESL amp and the Magnepan ribbon tweeters.

Getting down to serious listening, I heard a lot of the core sound of the Silverstone B&R: plenty of detail, no noise (with fresh tubes it sounded as quiet as a transistor design), good speed, and more “oomph” than the Silverstone. There was more bloom too, though perhaps it’s an artifact of the tubes. If I were comparing it to a “typical single-ended tube preamp”, the Enigma is faster, quieter, lower in distortion, and bigger sounding. It’s a successful design, and one that you might prefer over the passive Silverstone.

The Enigma made the most of big-band jazz, large orchestral pieces, and anything with big dynamic swings. It doesn’t have the speed or transparency of the Silverstone, yet with the right music its power sounds authentic.

Something I noted on both units is that if you turn off the power, the circuit is “open”. In other words, when the power goes off, the unit’s output isn’t loaded. It’s like when the power amp was on, there was an interconnect connected to the input of the amp, but the other end of the interconnect wasn’t connected to a preamp. It caused a hum when I left the Sanders ESL amp idling, so I turned off the Enigma. I’m not sure there is a real problem with the design, but I wanted to mention it.

Any Number Can Win

That’s the name of a great Jimmy Smith record, itself a reference to a cool French film that came out in 1963. On the cover of the Jimmy Smith record (on Verve) are good-looking girls from all parts of the planet. Indeed—you’d be a winner with any one of them. The same holds true for both these offerings from StereoKnight. The Silverstone B&R injects very little character, it is as close to a chameleon as I have used. My attempt to define a “Silverstone” sound was frustrating, and in the end I have to say it is neutral.

The Magnetic Enigma-1.0R has a more traditional high-end sound, where everything is slightly bigger and more powerful than reality. It reminded me of the NAT Plasma R preamp I auditioned last year, although the Enigma is a little more quiet—probably the transformers at work. As the StereoKnight was shipped to me, I had mixed feelings. It sounded very good, but the character of the 6H30 was intrusive after the purity of the Silverstone. I substituted some alternative tubes and found a combination that was warmer and more organic. After the tube swap, it was a beautiful-sounding preamp: slightly euphonic, and a little golden. The change of tubes lowered resolution slightly but I was very pleased with the results.

Which will sound better to you will come down to your taste. For me, the vanishingly low distortion of a transformer volume control and the effortless clarity of the Silverstone made enough of an impression on me that I bought it. Unlike a makeshift volume control in a plastic box, the Silverstone B&R offers the functionality, looks, and build quality of an active preamp, but with none of the electronic distortion. In other words, I had my cake and was able to eat it. Considering the attractive pricing, it makes it easy for me to recommend either unit. Which you prefer depends on you.

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2 Responses to StereoKnight Reference Silverstone-B&R and Magnetic Enigma-1.0R Preamplifier Review


  1. Ed Fox says:

    Dear Phillip Holmes,

    In your review of the Stereoknight Magnetic Enigma 1.0 preamplifier in August 2010, you discussed substituting vacuum tubes. Your quote: “I wound up getting very nice sound with a British military E88CC variant, one of the CV valves, and 6GU7 RCAs.” Did you mean you replaced only the 6H30 with either of these tubes, or did you also replace the 6922’s with E88CCs? Sorry, I don’t know much about tubes. I am testing this amp and I agree with you about the harshness. I wanted the Silverstone passive you reviewed, but James Zhang did not have any more (he is going out of business). I really wanted the passive amp, but my only option was to try his hybrid. Any help with exactly what you did with the valve swaps would be appreciated.

    Thanks for your help.

    Ed

    • Phillip Holmes says:

      The 6dj8 tubes can be substituted with many different tubes, including the 7308, 6922, 6n23p and other pin-compatible tubes, like the 6cg7, 6gu7, 6fw7, 6kn8, etc..

      I wouldn’t put any of those tubes in place of the 6h30 unless you know the circuit. You can try the 6N6p Russian tube, which is much more musical than the 6h30.

      I hope that helps.

      Thanks,
      Phillip

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