My first experience with a single-ended Class A amplifier was the Cary Audio CAD-300sei, an SET integrated amp fitted with Western Electric 300B output tubes. I had purchased this amp for my bedroom system (primarily used for background listening), and used it with a pair of ProAc Response One SC loudspeakers. I was quite pleased with the sound, and it was emotionally involving at low volume levels.
My main system at that time was in a dedicated sound room in my house in the Santa Cruz mountains. Over the years, it contained top-of-the-line Class A solid state amps from Krell and Classé Audio, both stereo and mono-blocks. During one of my frequent upgrade periods (my career was spent in High-End Audio), I sold my Krell FPB-600, and while waiting for my new Krell mono-blocks to arrive, I decided to hook up the Cary to my Wilson Watt/Puppy 7s. This combination forever changed my audio world. Although the 15 watt Cary couldn't drive the Wilsons to anything close to a realistic level, the sound it produced was unlike anything I had ever heard before: there was a palpable quality to the music that was atmospheric, holographic, and with a sweetness that greatly stirred my emotions. I was transfixed with this sound, and this led me to selling my Wilsons and buying a pair of Avantgarde Duos. These horn speakers had a high sensitivity so I could experiment with other single-ended tube amps in order to listen to my music at the high volume levels that my house in the woods afforded me.
About 15 years ago, I read about Sugden Audio in Stereophile, a British manufacturer that began hand-building a 10 watt/ch solid state Class A integrated amplifier—the world's first such design—back in 1967. This amp, the A21, was the inaugural home audio product for Sugden, and it received lots of praise from the audiophile press as well as creating many happy customers.
Over the years, Sugden continued to iterate and manufacture this amplifier, each time improving upon the original design. Sugden retained the same principle model number, and to this day 52 years later, one can still purchase an A21. I ended up buying the then current A21a for my bedroom system after reading the Stereophile review. By then, I owned the ProAc Tablette Reference 8 Signatures, and this combination reminded me very much of the Cary/ProAc combo I had enjoyed before, a warm tone with a highly musical sound.
This retelling of my past leads me to the subject of this review, the Sugden A21SE Signature ($3,250) which is the latest iteration of this classic model. I recently purchased this solid state integrated amplifier which replaced my Luxman MQ-88 ($8,000) tube amp that drove my Raidho XT-1 loudspeakers ($8,500 w/stands). It was time for a change since I now live in a small apartment, and I was looking for an amplifier that would immerse me in my music at low volume levels.
I'd like to mention Tone Imports and its proprietor, Jonathan Halpern, for assisting me by answering all my questions about the A21SE Signature prior to my purchase.
My source gear for this review is a MacBook Air repurposed as my music server connected via USB to a Chord Electronics Qutest DAC ($1,900). The before mentioned Raidho XT-1s are the loudspeakers, and the cabling is primarily Kimber. The music used were rips from my CD library as well as 300GB of hi res files purchased from HDtracks and the Pono Store, all spanning many eras and genres.
The A21SE Signature came in a thick cardboard box with foam end caps protecting the amplifier in dutiful fashion. Once I removed the amplifier from the box, I inspected the fit and finish carefully since I am somewhat obsessive about build quality. This is a nicely made 30 watt/ch amplifier, the chassis parts lining up well and the potentiometer and switches all operating in a smooth and solid manner. The rear chassis has five line level inputs (a dedicated phono preamp and headphone amp is available) and two sets of outputs—fixed level outs and preamp outs. The binding posts appear to be akin to high quality WBT posts that take both spade lugs and bananas. A nice touch is a cardboard tag that indicates the employees who participated in hand-building and testing the amplifier.
I began my evaluation first by running the A21SE Signature for 400 hours to ensure it would sound its best. This is my normal practice, both when I worked in high end audio retail as well as in my use of gear at home. Although some manufacturers do not feel there's a need for burn-in, many, e.g., Krell, Classé, Levinson, Sugden, do. A Pure Class A solid state amplifier simply means the output stage is fed with a bias (DC) current to ensure the output transistors are fully on, even when no signal is present. The advantage of this topology over the more commonly used Class AB design is the removal of notch distortion, a type of distortion that occurs when an AB amp switches from Class A operation—usually around 5% of its full wattage rating—and Class B operation which yields far greater output but at the expense of linearity and musicality. An additional advantage in Class A operation is the output stage always operates at a thermally optimal level, and any distortion it creates is a more pleasing type of even-order harmonics versus the odd-order harmonics produced in Class AB designs. Pure Class A amplifiers sound very musical, but since the output circuitry is always full on, they run very hot (and make for great space heaters!), so additional cost is required for proper heat sinking, a beefier power supply, and higher grade parts that can withstand the heat over the lifetime of the product. I refer everyone to The Absolute Sound's Robert Harley article, "A Survey Of Amplifier Types," for further explanation.
Now we come to the most important part of the review, the sound. Imagine you live in a house with a big glass window looking out on a beautiful landscape full of color and detail, however, the glass has a film on it from the environment outside. During the day with the sun brightly shining, the view is spectacular, creating a sense of pleasure in your mind and body, the film barely obstructs the view. As the day goes on and the sun begins to dip down towards the horizon, the colors in the landscape are muted from the growing darkness peering in through the finely opaque window, and any tiny detail easily observed earlier in the day is lost in the background. This scene analogizes how the Sugden sounds. Let me explain. ...
Often when we listen to our music system, we desire to be immersed in a full sound so we become more emotionally involved in our listening. It is the emotion wrought that defines the beauty we experience. We turn up the volume in order to better hear all the detail, especially the micro detail that contributes to the transparency of the sound. The A21SE Signature is unique in my experience of high-end solid state amplifiers in that one doesn't have to turn up the volume. It's as if in our glass window analogy that we cleaned the glass, and even as the sun goes down, the colors remain vibrant, and landscape detail is still discernible. And so it is with the Sugden. I don't have to turn up the volume to hear all the detail in the music, it is beautifully portrayed regardless of volume level. This amplifier is dead quiet, its noise floor seems non-existent. So there is nothing to obscure the micro detail—all the music is there.
Because the Sugden has incredible transparency and reveals the tiniest of details, the soundstage this amplifier creates is enormous. Way better than my far more expensive Luxman that preceded it. A three-dimensional recreation of the music performance is readily heard. It's quite spooky, actually. The soundstage width extends laterally way past my speaker placement (I should give the Raidhos some credit here, its ribbon tweeter is the best performing high frequency transducer I've ever heard). The height of images equals the width in performance, but it is the depth of the soundstage that is most stunning. The sound is holographic.
The frequency range is linear, I don't hear any areas where either too much emphasis or not enough is found. The bass on this amplifier is well controlled, yet it extends deeply giving my Raidhos the appearance of lowering their frequency range by another 15Hz or so, and this quality also is quite noticeable at low volume levels. Sugden claims they beefed up the power supply by 50% for the A21SE Signature, and I hear this in the bass performance. The midband is stunningly sweet with excellent retrieval of spacial detail; female vocalists are generally harder to reproduce accurately than their male counterparts due to their greater vocal range, but in listening to some of my favorites such as Kate Bush, Sinéad O'Connor, Tori Amos, Beth Orton, and Natalie Merchant, the full expression of their voices are delicately reproduced. Unlike the A21a I owned 15 years ago, the trebles have a wonderful clarity while still retaining a sense of warmth reminiscent of good tube amps. Yes, the Sugden as can be expected by now from all I've written does mimic the sound of quality tube amps, and I suspect that the even-order harmonics innate in the Sugden design is what contributes to this quality.
I bought this amplifier for low level listening, but I did try playing my system at higher than normal levels. The 30 watts/ch into 8 ohms, 40 watts/ch into 4 ohms seems to me a conservative rating, so I suspect that those like me who have speakers with low sensitivity will be surprised on how loudly their speakers will play without any sense of strain from the Sugden.
My objective when I started my search for a replacement for my Luxman amp was to be able to listen at levels typical of apartment living, yet still be immersed in the sound so as not to feel I was missing any of the emotional content in my music. My memory of the A21a from years ago left such an indelible impression on me, that Sugden rose to the top of the list of solid state amplifiers that offered great performance at a reasonable price, for high-end standards, anyway. I couldn't be more pleased with the A21SE Signature (other than the plastic remote); it allows me to enjoy my greatest passion in life without feeling I'm missing anything. Add to that the lineage of this design which appeals to my current desire of owning things with longevity in mind, the reputation of Sugden when it comes to service in the event I ever need it, the minimalist aesthetic which mirrors my current lifestyle, my view is any music lover reading this who shares my sensibilities should definitely audition this singular amplifier. In basketball parlance, the A21SE Signature is a slam dunk!