Blue Circle Audio does things a little differently. They hand-build each component, to order, at their facility in Innerkip, in Ontario, Canada. This means that their products are easily customized, with wide varieties of finishes, faceplates, and numbers of inputs, outputs, and switches, to meet a particular customer’s needs.

The man who runs Blue Circle, and its driving force, is the inscrutable Gilbert Yeung. I’ve known Yeung for many years, and believe that, in designing his products, just about everything he does is in the interest of making them sound better. Sometimes, such things as a Blue Circle product’s appearance take a back seat to his ultimate goal of sound quality. You need only look at his Thingee products -- phono stages, PLCs, DACs, and headphone amplifiers stuffed into pieces of acrylonitrile-butadiene-styrene (ABS) pipe -- to get an idea of how much Yeung values being able to provide the maximum quality of internal parts and sound possible at a specific price point. With his preamplifiers, remote volume control is an option that uses a completely separate circuit so as not to compromise the sound quality of the front-panel control. We’re talking old-school here.

Blue Circle Audio BC2K4sh

So I was surprised when I heard that Yeung was replacing his highly regarded and long-running BC200 series of hybrid tubed/solid-state power amplifiers with a new series of amplifiers, the BC2Ks, that have class-D output stages. Not only that, but Blue Circle’s website states that these new amps are “less expensive, and most importantly, more musical than the components they are replacing.” I wanted to review one.

Coming full Circle

Like most Blue Circle products, the new BC2K power amplifiers come in a variety of configurations. The four models available have power-output ratings ranging from 150 to 2000Wpc into 8 ohms, and twice those specified outputs into 4 ohms. Some models are bridgeable, and the most powerful are monoblocks. Each model is also available in what Gilbert Yeung calls a selectable hybrid (“sh”) configuration, which adds a 6SN7 tube to the input stage. Yeung and I decided that the 300Wpc stereo BC2K4sh would be the most sensible choice for my system. The BC2K4sh costs $5595 USD; the all-solid-state BC2K4, which lacks the tubed input stage, costs $4295.

Blue Circle Audio BC2K4sh

The class-D output stage of the BC2K amps features switch-mode power supplies with power-factor correction and slow-turn-on circuitry. Yeung says that the class-A solid-state input stage is based on that of Blue Circle Audio’s top power amp, the NSL, with a “differential-balanced parallel topology.” The tubed input stage of the ’sh models can be switched in and out of circuit with a toggle switch on the faceplate. To prolong tube life, the tubed input stage can also be powered off, using a rocker switch on the front panel. According to Yeung, turning the tubed input stage on or off or switching it in or out of circuit while the BC2K4sh is powered up won’t damage the amp. About the only thing that could go wrong is if the user doesn’t turn on the power for the tubed input stage, switches it in circuit, hears (of course) no sound, increases the volume on the preamp to compensate, then powers up or switches out of circuit the tubed stage. This could result in high volumes when the sound does come on.

Blue Circle Audio BC2K4sh

Also on the front panel is a rocker switch for the main power, and a Blue Circle logo that illuminates at power-up. My review sample was fitted with the standard stainless-steel front panel, which looks like something I might have built in high school metal shop. In fact, the BC2K4sh looks pretty rudimentary. Granted, it’s more attractive than the components that Yeung makes out of ABS piping (not saying much), and optional faceplates of wood or Plexiglas are available, but I’d expect something that costs $5595 to look a little more polished.

At 35 pounds, the BC2K4sh is relatively heavy for a class-D amplifier. (The all-solid-state version weighs 23 pounds.) On the rear panel are two pairs of very-high-quality, gold-plated binding posts, XLR and RCA inputs, an IEC power inlet, and a ground-lift switch. Available as options are 5V or 12V triggers, and an automatic, variable-speed ventilation fan.

Blue Circle Audio BC2K4sh

Blue Circle’s previous hybrid amp, the BC204, was an 85-pound beast specced to output 150Wpc into 8 ohms and store 363 joules. The BC2K4sh is specified as being able to store 522 joules and output 300Wpc into 8 ohms or 600Wpc into 4 ohms. (Blue Circle doesn’t specify its output when bridged.) Details of the BC2K’s exact design are scarce on Blue Circle’s website, but Yeung makes a point of how important he thinks the power supply is to the “musicality” of an amplifier’s sound. According to him, this musicality requires a massive power supply that can store large amounts of energy, which is dependent on its capacitance and supply voltage.


I used the BC2K4sh primarily with my Oppo BDP-105 universal BD player used as both DAC and preamp. Nordost’s Quattro Fil balanced interconnects and Super Flatline Mk.II speaker cable connected the Blue Circle to Definitive Technology Mythos ST-L or KEF R900 speakers. I also used my reference Anthem Statement D2 surround-sound processor as a DAC and preamp, though I find myself relying on it less and less as a reference component -- I’m more than satisfied with the sound of the Oppo BDP-105 through its dedicated balanced stereo outputs.

Blue Circle Audio BC2K4sh

Gilbert Yeung suggested that, during the listening period, I leave the BC2K4sh’s tubed input stage powered on at all times, so that it would always be fully warmed up when I switched it in circuit to compare its sound with the amp’s pure-solid-state sound. However, in the interest of prolonging tube life, I don’t recommend that you do this.

Kind of Blue Circle

When I first powered up the BC2K4sh, I wasn’t sure what to expect -- mating a solid-state output stage to a tubed input stage isn’t new for Blue Circle, but the BC2K line’s class-D output stage is indeed a first for them. But to establish a baseline, I first listened to the BC2K4sh in pure-solid-state mode.

Recently, I’ve had some great-sounding class-D amps in my system: the Anthem Statement M1, Bel Canto Design REF600M, Cyrus Stereo 200, NAD Masters Series M27, and NuPrime Audio IDA-8. The BC2K4sh was right up there with the best of them. Even working exclusively in solid state, the BC2Ksh had a neutral, fleshed-out sound that just sounded right. The recording by the King’s Singers, an a cappella men’s chorus, of Don McLean’s “American Pie,” from the group’s Good Vibrations (16-bit/44.1kHz FLAC, RCA Victor), sounds good through most systems but seldom great -- the recording is a bit dry, especially in comparison to the lusher sound of this album’s title track. Through the Blue Circle, each singer sounded exactly as I imagined he would sound in a recording studio, with fewer of the spatial cues heard in recordings made in larger, livelier venues. While “American Pie” can sound somewhat flat through some systems, the BC2K4sh conveyed a touch of smoothness in the voices that resulted in a beautifully organic sound. The more lyrical “Good Vibrations,” with its copious harmonies, sounded stunning through the BC2K4sh, the wonderfully pure voice of the countertenor floating ever so delicately on a soundstage defined by the other voices, equally as well composed and with precisely positioned voices. And this was in pure solid state mode. In fact, the BC2K4sh sounded so darn good with voices that I couldn’t resist switching its tubed input stage in circuit to hear if it could sound any better.

Blue Circle Audio BC2K4sh

And it did. Eva Cassidy had never sounded more real and alive through my system than with the BC2K4sh in hybrid mode, playing a Japanese edition of her Songbird (16/44.1 FLAC, Blix Street/JVC XRCD24). Cassidy’s voice was expressive and fleshed out in a way I hadn’t heard with any of the class-D amps mentioned above. This was evident with every cut on this album, but especially with her cover of Sting’s “Fields of Gold,” which can sound a bit flat through extremely neutral, unforgiving amps. With the BC2K4sh in hybrid mode, the gently strummed guitar sounded steely and realistic yet slightly warm, perfectly framing her moving singing. The simple arrangement of “People Get Ready” was sublime, with exceptional clarity and precise placement of cymbals and hi-hat.

A Tale of Two Cities

Adele’s 25 (16/44.1 FLAC, XL Recordings) is “mixed pretty hot,” as Hans Wetzel puts it, especially in the album’s first single, “Hello” -- something I’ve noticed on some of her other recordings. However, in the Blue Circle BC2K4sh’s hybrid mode, Adele’s voice sounded magnificent, with a slight sibilance that gave it a lively quality, each word delivered with the breathy but towering character that is her trademark. The piano sounded just as spectacular, with an ultradeep, pounding rhythm. In pure solid-state mode, Adele’s voice was still very enjoyable if not quite as smooth, but the piano really rocked my listening room, with more definition and a brawny punchiness in the powerful chorus. With very well-recorded albums, such as Enya’s Dark Sky Island (16/44.1 FLAC, Warner Bros.), the differences between the BC2K4sh’s two modes were less audible. Enya’s super-silky voice might have sounded even a bit silkier in hybrid mode, and the deep, whompin’ bass slightly more controlled in pure solid state, but these differences were fairly subtle. Overall, I preferred the hybrid mode for its slightly smoother midrange: It made voices pop a little more, with only a slight loss of definition in the bass.

Blue Circle Audio BC2K4sh

Even more choices

As good as Blue Circle’s BC2K4sh is, it faces stiff competition at its price point. Especially in pure-solid-state mode, it reminded me in many ways of Bel Canto Design’s excellent e.One REF600M monoblock ($4990/pair). Both are very neutral, and share a smooth, clear midrange combined with a tight, powerful grip on the bass. If I had to choose between the two, I’d pick the Blue Circle for its slightly more liquid midrange in hybrid mode, which gave voices astonishingly realistic presence. The REF600M checks all the audiophile boxes, but through the BC2K4sh, Holly Cole’s voice in “The Briar and the Rose,” from her Temptation (24/88.2 FLAC, Blue Note/Analogue Productions), was more palpable, with an unforced naturalness that captivated me -- it was easier for me to suspend my disbelief that she was standing there between my speakers. In pure solid-state mode, it was a bit of a wash between the BC2K4sh and the REF600Ms: the Blue Circle’s astounding midrange presence was no longer as prominent.

Blue Circle Audio BC2K4sh

As I noted in my review of the Bel Canto REF600M, Anthem Statement’s M1 monoblock ($7000/pair) has astonishing bass clarity but sounds a bit dry, though its control over a speaker is nearly absolute. This clarity and smoothness extends throughout the entire audioband -- the M1 is as uncolored as any other amplifier I’ve heard at its price point. Its retrieval of detail is also amazing -- I could more easily pick out individual horns in the Canadian Brass’s intro to “The Briar and the Rose,” and compared to the BC2K4sh, Holly Cole’s voice was more emotive and pronounced, if not as silky, without sounding forward. The M1’s near-total transparency is quite thrilling, and invaluable as a reviewing tool, but I couldn’t help but be smitten by the BC2K4sh’s more easygoing and instantly involving character in hybrid mode -- it made everything sound especially right. Sure, its bass wasn’t as controlled as the M1’s -- or even as controlled as its own bass performance in pure solid-state mode -- and there was a bit less detail. Other than that, the BC2K4sh’s sound quality was hard to fault.

Decisions, decisions

Blue Circle Audio’s BC2K4sh may not be much to look at, but with it, Gilbert Yeung has created a thoroughly modern hybrid power amplifier that should appeal to anyone looking for a high-performance, high-powered amp at a reasonable price. In 2005, in his review of the BC2K4sh’s predecessor, the BC204, Marc Mickelson said that it had “a natural presentation that’s never harsh or merciless in the way it conveys detail.” This perfectly describes the sound of the BC2K4sh. Although it’s based on a class-D design, its hybrid sound is totally natural in a way that sounds so right it’s difficult not to listen to it for hours on end and fall in love with it.

. . . Roger Kanno

Associated Equipment

  • Speakers -- Definitive Technology Mythos ST-L, KEF R900
  • Amplifiers (monoblocks) -- Anthem Statement M1, Bel Canto Design e.One REF600M
  • Sources -- Oppo Digital BDP-105 universal BD player, Anthem Statement D2 A/V processor, Asus VivoBook X200MA computer running Windows 8.1 and foobar2000, Bel Canto Design mLink USB converter
  • Cables -- AudioQuest Carbon USB, Analysis Plus Digital Crystal digital, Analysis Plus Solo Crystal Oval and Nordost Quattro Fil interconnects, Analysis Plus Black Oval 9 and Nordost Super Flatline Mk.II speaker cables
  • Power cords -- Essential Sound Products MusicCord-Pro ES
  • Power conditioners -- Blue Circle Audio Peed Al Sea Thingee, Zero Surge 1MOD15WI

Blue Circle Audio BC2K4sh Stereo Amplifier
Price: $5595 USD.
Warranty: Three years parts and labor (with warranty registration).

Blue Circle Audio
Innerkip, Ontario N0J 1M0
Phone: (519) 469-3215
Fax: (519) 469-3782