Having been exposed almost exclusively to KT88 and EL34 designs this past year, I jumped at the opportunity of reviewing something different—a 300B-based push-pull amplifier. With the Ref 3.1 you get the full enchilada—an integrated amplifier complete with a phonostage, gorgeous cosmetics, and excellent fit and finish. In particular, take note of the attention paid to important detail such as the floating 300B tube sockets and chassis isolation feet. Much to my surprise, especially in view of its modest asking price, the Ref 3.1 turned out to be a major find, a virtual sonic jackpot in not one but several system contexts. (Jonathan Valin came to a similar conclusion about the Audio Space Reference 2 preamp in Issue 173.)
What is the attraction of the venerable 300B power triode in push-pull application? Certainly not power output. A pair of KT88s is capable of 40 or even 60Wpc depending on the operating point. The Ref 3.1 is rated at 21 Class A watts per channel. That’s a 3dB sacrifice in power output relative to power tetrode or pentode designs, but not all watts are created equal. For starters, there’s the issue of bass damping. In practice, obtaining decent bass damping from a beam power or power pentode output stage has proven to be a tough proposition. In the case of a triode, its low plate resistance and the fact that the optimal output transformer’s primary impedance is several times this plate resistance results in a source impedance that may be as low as a third of the load impedance—even without any global feedback. Recall that the damping factor is defined as the ratio of the load impedance to the amp’s source impedance, which argues for low source impedance. However, it is not clear that a damping factor greater than about five makes an audible difference. The situation is far less favorable in the case of a tetrode or pentode stage. Here a high plate resistance is coupled to a primary winding whose impedance may be smaller in value in order to reduce distortion products. The end result is a source impedance that may even exceed the load impedance, giving poor bass damping. Hence, there’s a clear need to incorporate lots of global feedback to counteract confused pentode bass.
The age-old debate about triodes versus pentodes is not just about bass damping factor. There’s also the nature of the distortion spectrum. In general, it’s fair to say that triode sound is smoother and more liquid relative to that of pentode designs, whose sound is often dominated by odd-order distortion products. I am of the opinion that a designer should be allowed artistic license to voice a product to either his liking or to a particular standard. He should have control over the tonal balance, bass character, and distortion spectrum. In the case of the Ref 3.1, designer Peter Lau made the right call to offer the design in two flavors: KT88 and 300B. I can’t say that I’ve auditioned the KT88 version, but to be honest, the 300B version is so musically compelling, its siren song so irresistible, that I have no interest in its beam power cousin.
Audio Space is headquartered in Hong Kong while its manufacturing base is located in mainland China. Part musician, audiophile, and electronics engineer, Peter Lau has over 25 years of design experience and numerous patent awards. It was not surprising therefore that the Ref 3.1’s technical design made a solid impression. Solid-state rectification is used throughout. All filament voltages are DC. The B+ supply features a Pi filter and a generous capacitor reservoir. The phonostage uses a pair of 12AX7 dual triodes (one per channel) and is suitable for high-output moving-magnet or moving-iron cartridges with nominal output in the range of 3 to 5mV. If you would like to use the 3.1 as a basic power amplifier in conjunction with your favorite external preamp, you can switch over to the direct input which bypasses the input-selector switch and volume control. Speaking of the volume control, it is a high-quality Alps potentiometer. Unfortunately, this pot is manual in operation. I was told there was simply no spare room for a motorized pot.
The first stage consists of a 12AX7 configured as a paraphase phase-splitter. The driver stage uses one 6SN7 per channel, my favorite driver tube. All of the stock tubes are Chinese new production sourced from the Shuguang factory. As I understand it, quality control issues have forced Shuguang to suspend 6SN7 tube production, but Audio Space managed to stockpile this tube while the going was still good. However, Peter Lau freely admits that the Ref 3.1 can undergo an instant upgrade by rolling in a pair of vintage 6SN7s. I’m not a fan of modern 6SN7 tube brands, having developed a strong dislike for both Russian and Chinese versions. And as you will shortly discover, that wasn’t a problem for someone like me with an extensive collection of NOS and used vintage 6SN7s.
The output stage idles comfortably at a plate current of about 50mA and plate voltage of around 375V DC. Bias is fixed, meaning a negative bias voltage is applied directly to the grid of each 300B. This increases efficiency and keeps the signal path as simple as possible. The other option is cathode bias, also known as self-bias, which requires a resistor in the signal path as well as a bypass cap. The downside to fixed bias is that idle current may drift as the tube ages, which mandates monitoring of tube current on a periodic basis. To that end and to facilitate output-tube changes, a bias meter and individual bias pots are provided. My recommendation is to check tube bias on a weekly to monthly basis depending on usage. During the evaluation period I did not observe any significant bias drift. Global feedback is selectable on the front panel and may be set to either “Low” or “High.” A quick listening test was sufficient to convince me of the sonic superiority of the low-feedback setting, and that’s how I conducted all subsequent testing. As far as warm-up is concerned, you’ll need to be patient with this amp for about 15 to 30 minutes, after which it starts to sing.
On rare occasions, after only a few musical bars flow by, a new component’s presentation immediately sinks in. That is exactly what happened with the Ref 3.1—a case of love at first listen! The overriding first impression was of extreme soundstage transparency coupled with crystalline-like clarity. This was high definition sound (HD) on a par with the best that solid-sate designs can offer, but with a spatiality solid-state rarely approaches. The Ref 3.1 floated a wide and deep soundstage. Image specificity was excellent, though not quite as palpable in its 3-D realism as that conjured by the much more expensive Esoteric A-100. HD sound is a far cry from the essence of vintage tube sound, which to my mind, is best characterized by the metaphor of a warm bath, replete with harmonic textures that are thick, rich, and overly liquid. That’s a fair description of the amplifier that started it all for me, the Dynaco ST-70—tube virtues without the incisiveness of the real thing. By contrast the Ref 3.1 elucidated musical textures with a delicacy only a handful of other (and much more expensive) amps can replicate. The mids sang sweetly without a trace of tube brightness. Tone colors were vivid, and, in general, timbres were reproduced with exceptional fidelity. When partnered with the medium-sensitivity Esoteric MG-20 loudspeaker, the amp’s distortion spectrum remained benign even when driven hard. In fact, it sounded far more powerful than its nominal rating of 21Wpc would suggest. Of course, it was even more comfortable with a high-sensitivity speaker. Microdynamic and rhythmic nuances were given full scope, allowing musical lines to boogie along with turbocharged kinetic energy.
Another big surprise had to do with bass definition. Well, maybe I should not have been as surprised considering the bass damping factor advantage of triodes over tetrodes and pentodes. But bass lines were reproduced tightly and with exceptional pitch definition. For example, resolving a Fender bass and upright acoustic bass playing in unison was a piece of cake. In these respects, the Ref 3.1 blew away the recently reviewed Esoteric A-100 and the Air Tight ATM-1S. Its low internal source impedance was also a key in minimizing load interactions. It made no attempt to dominate the load’s tonal balance, allowing each speaker’s tonal character to shine through.
It should be noted that my listening impressions to this point were based on using digital front ends or feeding an external phonostage into the “Direct” input. I also spent some time auditioning the built-in moving-magnet stage using a Grado Reference moving-iron cartridge. Its performance was commendable in a number of respects including a low-noise floor, very good detail retrieval, timbral accuracy, and excellent image focus. However, it exhibited dynamic reticence in scaling the range from loud to very loud. Transient speed was also a tad blunted relative to the rest of the circuit.
The only significant sonic quibble noted with the stock tube complement was a slightly grainy character through the presence and treble regions. Not being a fan of any sort of textural abrasiveness I started rolling in vintage 6SN7 types. I knew I was on the right track when my first pick, a Sylvania chrome-top, brought about a smoothing of the upper midrange and treble. As a bonus, the lower midrange gained harmonic richness, fleshing out that big tone a vintage 6SN7 is known for. I also tried GE and Raytheon types, but my favorite in this application turned out to be a CBS-Hytron brown-base 5692, the most spacious and transparent of the bunch. However, that was not the end of the story. Having recently purchased several NOS Sylvania 7N7 Loktal types (with triangular black plates), I decided to give them a tryout as well. The 7N7 is internally electrically identical to a 6SN7 but with a different pin-out. Thus, with a 7N7 to 6SN7 adapter, it may be substituted for any 6SN7. The Loktal (for Lock-In), was a trademark of Philco-Sylvania. Other manufacturers (e.g., Tung Sol and Raytheon) referred to them as loctals. This type is distinguished by having the lead-out wires thickened to form the connecting pins and the use of a metal base shell instead of the typical Bakelite base. The 7N7 has specifically developed a reputation as an inexpensive yet excellent alternative to vintage 6SN7 types. And I totally concur. I found it to possess excellent detail and spatiality and could live with its sonics happily ever after.
It should be obvious by now that I’m wildly enthusiastic about this integrated amp. The Audio Space Reference 3.1 (300B) offers a path toward enlightenment—of being one with the music. Outfitted with a vintage pair of 6SN7 or even inexpensive 7N7 triodes, its musical pleasures would be captivating at any price point. Considering its asking price, it earns my nomination for a Product of the Year award.
Audio Space Reference 3.1 (300B) Integrated Amplifier
Input impedance: >56k ohm (RCA/Direct In)
Impedance taps: 4, 8, 16 Ohm
Output power: 21Wpc (Class-A push-pull)
T.H.D: < 1%
Input sensitivity: 200mV, 3–5mV (MM phono)
S/N ratio: > 80dB (hum noise < 3mV)
Tube complement: Four 300B, two 6SN7, four 12AX7 (ECC83)
Dimensions: 18.5" x 16" x 8"
Weight: 65 lbs.
Audio Space Acoustic Laboratory Ltd.
10/F., Wah Yuen Factory Bldg., 16 Elm Street,
Tai Kok Tsui, Kowloon,
+852 2851 0722
3431 Pomona Blvd., Unit G
Pomona, CA 91768
Final Sound 1000i electrostatics, Esoteric MG-20, Venture Audio Excellence III Signature, and Basszilla Platinum Edition Mk2 DIY speakers; Kuzma Stabi Reference turntable outfitted with Kuzma 313-VTA and Graham Engineering model 2.5 tone arms; Grado Reference, Dynavector XV-1s, and Shelter Harmony phono cartridges; Live! MC-10 and Shelter 411 Type 2 step-up transformers; Air Tight ATE-2 phono preamplifier; PrimaLuna Eight CD player, Weiss Engineering Jason Transport and Medea DAC, Concert Fidelity CF-040 DAC, Altmann Micro Machines Attraction DAC; Concert Fidelity CF-080 line preamplifier, Spread Spectrum Technologies Ambrosia preamplifier, First Watt B1 buffer preamplifier; Esoteric A-100, LAMM Industries M1.2 Reference, and Pass Labs XA30.5 amplifiers; Bybee Speaker Bullets; FMS Nexus-2, Acrotec 6N and 8N copper, Kimber Select KS-1030, Kimber KCAG interconnects; FMS Nexus speaker cable.