Atma-Sphere “UltraViolet” UV-1 Preamplifier

Wonderful Tone Color

Equipment report
Tubed preamplifiers
Atma-Sphere Music Systems UV-1
Atma-Sphere “UltraViolet” UV-1 Preamplifier

Designer Ralph Karsten recounts that the UV-1 preamp is based on a circuit that has been percolating at Atma-Sphere for many years but has never before been commercialized. Bowing to feedback from friends and associates who felt it was something special, Ralph finally put the UV-1 into production in 2014. Admittedly, it represents a significant departure from Atma-Sphere’s main product line and is intended as a cost-effective entry-level offering. In its basic form as a line preamp, a single 6SN7 dual triode is used per channel. Both mm and mc phono options are available. The mm option deploys a pair of 12AT7 triodes in a circuit topology that is in some ways similar to that of the vintage Dynaco PAS-3, but with more accurate RIAA eq and a few other refinements. For the low-output mc option, Jensen step-up transformers are added at the phono inputs to further boost overall gain. When Ralph asked me which version I wanted to review, I opted for the mm phono version. I have several excellent mm cartridges on hand, and having the chance to spin vinyl is always a good thing. The UV-1 can also be configured as a headphone amplifier, and the Aux output can be specified as a second set of outputs or as tape-out jacks.

The 6SN7GT tube was originally released on the eve of World War II and saw extensive use during the war years, primarily in radar installations, as its high power dissipation and construction made it both reliable and rugged. After the war, it was widely used in early TVs, audio amplifiers, and various industrial and military electronics. Even the ENIAC, the first programmable electronic computer, used thousands of 6SN7s. Karsten considers the 6SN7 sonically superior to nine-pin miniature types for line-drive applications, and I have to confess that I’ve been partial to it as well for many years. In fact, the 6SN7 has gained a following as a linear, low-distortion triode, which is capable of a musical tone. Morgan Jones in his book Valve Amplifiers summarizes his test results of 529 dual triodes and concludes that the reputation of the 6SN7 family (including 12.6V and loctal equivalents) is well justified when it comes to low distortion. By the way, the popular 12AU7 nine-pin miniature finished last in these tests. It’s likely that designers are barking up the wrong tree when they sidestep the 6SN7. In the case of the UV-1, Karsten is definitely barking up the right tree. There are several ways to connect a dual triode gain stage. The mu-follower has been a popular choice for high-fidelity audio applications. Another totem-pole topology that has seen much use is the series-regulated push-pull (SRPP) circuit. Instead, Karsten opted for a simpler, classical approach, a voltage gain stage that is direct-coupled to a cathode follower output stage. The one obvious limitation to using a medium-mu triode in this configuration is voltage gain, which in this case is only about a factor of six. The power supply is said to be pretty traditional but uses HEXFRED rectifier diodes for low diode switching noise. Note that the UV-1 inverts signal polarity because it has an odd number of gain stages.

There aren’t many functional features; forget about a tape loop or a remote control. But at least a shunt-type balance pot is provided in addition to the obligatory volume control. I find a balance control to be a useful feature and one that is lacking on many modern line preamps. Many years ago, I recall J. Gordon Holt raising a preamp to chest level and asking me what was wrong with it. When I shook my head in puzzlement, he pointed out that it lacked a balance control and that he could not recommend it on that basis alone. And he had a point. Left-right channel balance is rarely perfect. A phono cartridge’s intrinsic channel balance is typically no better than 1dB, and matters can be made worse due to cartridge set-up misalignment or the application of improper anti-skating force. Speaker drivers are usually not matched to better than 1dB. And I haven’t even mentioned power amplifier-induced channel imbalance, or the effects of speaker positioning and room acoustics. Some purists dismiss a balance control on the grounds of “less is better,” but I find it essential for centering the soundstage in my listening room.

Starting with the linestage section, it quickly became apparent that the UV-1 delivers on the promise of the 6SN7 with a big-tone, authoritative midrange. Even with the stock Chinese 6SN7s, the sense of space was abundant and vocals were portrayed with notable palpability. Here was a linestage that could propel the music forward with an excellent feel for its emotion and drama. Once the initial surprise wore off, it was time to take stock of the unit’s shortcomings. The major fly in the ointment turned out to be the treble range. The lower treble was a bit grainy, an issue I’ve noticed with other Chinese 6SN7s. And when pushed hard, the sound became slightly glassy. Detail resolution also needed some help, especially transient decays, which tended to get lost in complex passages. My perception was of a broad-brush spatial portrayal, which made it rather difficult to peer deeply into the inner recesses of the soundstage. The UV-1 benefitted significantly from being plugged into Monarchy Audio’s AC-Regenerator operating at 117V/120Hz. I noted smoother upper octaves and crisper bass lines. In general, the AC-Regenerator built on the UV-1’s sonic strengths.

Of course, I don’t expect Atma-Sphere to ship the UV-1 with anything but new-production 6SN7s; unfortunately, current Russian and Chinese tubes are sonically inferior to U.S. ones from the 1940s, 50s, and 60s. The serious audiophile should (and likely would) contemplate rolling in vintage alternatives—that is, after all, an integral part of the tube experience. I happen to have a decent stockpile of vintage 6SN7s, but I decided to cut to the chase and roll in my two top picks. The Sylvania 6SN7WGT added a degree of musical finesse that no new-production 6SN7 can match. The treble range was now noticeably smoother. Resolution of transient decay improved, as did the ability to follow distinct bass lines. In all, very welcome enhancements that reduced the fuzz and grain surrounding instrumental outlines. Historically, the immediate predecessor of the 6SN7GT was the 6F8G/VT-99. It was developed by RCA in 1937 but is rather unwieldy to use because of its anode cap. The pinout is different but with an adapter (I bought mine from the Lowther club of Hong Kong), the 6F8G can be substituted safely for any 6SN7GT. It’s a tall tube, made even taller with the adapter in place. However, the additional overall height isn’t an issue with the UV-1 because it has no chassis cover. Trust me, this tube is worth trying. OK, its sound is slightly euphonic, but it speaks to the heart, and I for one was moved by its siren call. Expect extremely passionate voicing and much sweeter textures. The UV-1 also gained dramatically in terms of soundstage transparency and dynamic headroom.

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