Note: Measurements performed by BHK Labs can be found through this link.

Calyx FemtiLess is quickly becoming more. Due to the ever-accelerating pace of technological progress, amplifiers that are smaller, lighter, and use much less energy are increasingly approaching performance levels previously attained by only the biggest, baddest, most power-guzzling behemoths, which have seemingly forever ruled the audiophile landscape. Those who still believe that “digital” amplifiers, in whatever form, can’t possibly compete with sheer horsepower and cubic inches, are in danger of ignoring some seriously impressive components. I’ve heard some of the top speakers from the likes of Vivid Audio, Raidho, and Joseph Audio demoed at shows with digital-switching amplifiers, and with extremely impressive results. So if digital amps are good enough for those folks . . .

Calyx is one of the companies that has joined the camp of smart, environmentally friendly amps. It’s a division of Digital & Analog Co., Ltd., which has been making class-D ICs since 1999 for other companies, and decided it might be a good idea to use their own products to build, well, their own products. Given the extremely positive impression that Calyx’s Femto DAC made on our own Doug Schneider, I was intrigued to hear their entry in the power-amp race: the Femti mono amplifier ($1950 USD each).

Actually, to call the Femti 125 a monoblock is a little misleading and limiting, given the multiple configurations available. Each Femti can be used as a single two-channel amp, in tandem with another Femti in what Calyx calls biwire mode, or as a true monoblock pair -- all functions selectable with the turn of a knob or two. Pretty neat. You could start with one Femti, and add a second as funds permit.

Calyx holds the details of the Femti’s design architecture pretty close to its corporate vest, but I’m told there’s a custom-designed input stage coupled to a class-D output stage based on an unmodified ICEpower module. Calyx’s main design goals are to minimize noise and maximize linearity, to let the music’s natural dynamics and nuances come through. No argument here. During the development phase they discovered that, especially since the Femti is designed to be used in multiple operating modes, noise was being injected into the output stage. Reportedly, a lot of work was done to the input stage to eliminate noise and optimize grounding, to ultimately meet their standards. A substantial aluminum chassis is used to further reduce EMI and vibrations, and to control heat within the unit.

Each Femti weighs a solid 8.6 pounds, given its relatively compact dimensions of 8.7”W x 2.4”H x 9.6”D -- think of two Mac Minis on steroids, painted matte black. The nifty Calyx logo is deeply engraved in the top panel -- pity you don’t see them much once the amps are in their final resting place. If you look closely, the logo is actually an artful G clef impregnated by a bass clef, which I guess covers pretty much everything. The power button is located on the right side panel, and around back you’ll find an IEC connector, four speaker terminals, two RCA inputs, and a knob for selecting among the three operating modes. There are no balanced inputs, and no room for them -- only single-ended interconnects need apply.

Calyx Audio Femti

While it’s nice that the Femti takes up little space and is easy to move about, there’s a price to pay -- that real estate ’round back is a little tight even without the balanced inputs, especially if you’re going the biwire or stereo route. Thankfully, the amps are heavy enough that no matter how much stuff I had hanging out the back, they remained firmly planted. One nuisance was that what I thought were fairly standard spades on my speaker cables wouldn’t fit into the Calyxes’ output terminals. Thankfully, distributor Chris Zainea came to the rescue and forwarded some banana adapters that worked perfectly, but be aware that this could be a problem. Ironically, I couldn’t get the banana adapters to work with any of the comparison amps I had on hand -- the plugs were either too big or too small -- so I had to go back to spades for those amps. It’s enough to drive a guy, um, bananas. Anyway, that little hiccup aside, the Femtis seemed very solidly built, and behaved flawlessly throughout the review period.

If you’re thinking that because these are “digital” amps they must put out some juice, you’d be correct. In fact, these little dudes together, in monoblock mode, produce 250Wpc into 8 ohms or 500Wpc into 4 ohms. In stereo or biwire mode, those numbers drop to 65 or 125Wpc, respectively. They never ran out of gas in any mode in my system, and bass and dynamics remained stellar across the board. So while the little Femtis may fit well in your crowded New York City apartment, they’ll still have the potential to piss off your neighbors with their output capability. And their claimed input impedance of 47k ohms indicates that they’ll play nice even with most tubed preamps. In fact, I’ve learned that Calyx is planning on introducing their own tubed preamp sometime this summer.

The Femti’s distortion is said to be 0.005% at 10W, its frequency response 10Hz-22kHz, its dynamic range a very respectable 117dB in stereo and 121dB in bridged mode. The Calyx uses only 12W at idle, which should make you happy and piss off your electric company while you’re busy pissing off your neighbors.


The digital-switching amps that have passed through my room have greatly surprised me -- both the NuForce Reference 18s and the Rogue Medusa sounded very different from my preconceived notions of what such amps “should” sound like. Perhaps even more surprising was that they sounded more different from each other than have any two of their analog brethren I’ve had here -- night and day, really. Triangulation is a powerful thing. Just think how many times GPS has saved your butt and you get my point.

So I was very interested to hear how the Femtis would compare with the NuForce and the Rogue, not to mention some analog designs. It didn’t take long to hear that these guys hailed from the neutral, straight-shooter school. There was no tonal embellishment going on -- in fact, the Calyxes sounded a tad lighter and tonally less saturated than I prefer, and in that way reminded me quite a bit of the Medusas. Realizing that my system was wired entirely with Acoustic Zen Silver Reference interconnects, I swapped the pair between my preamp and the Femtis for their fuller-sounding and mostly copper siblings: Acoustic Zen’s original Matrix References. Ahh. Tonal harmony achieved, all was again right with the world. We beat on.

One thing was for sure -- from the get-go, these little guys sounded clean as a whistle. By that I mean that, once I’d addressed the interconnect situation, I couldn’t readily identify any area of weakness, or where they might be adding or subtracting anything to or from the musical signal. Every time I cued up a track, I just listened, without being distracted by anything whatsoever. I never thought to myself, “Gee, I could use a little more of this or a little less of that.” What I could tell right off the bat was that the Femtis’ sound resembled neither the relatively large, billowy soundscape of the NuForce amps nor the otherworldly precision and dimensional imaging of the tubed Medusa. Nothing stood out that I could point to as a sonic signature. Just good, clean sound.

Case in point: I recently picked up Puente Celeste’s Nama (CD, M•A Recordings MO84A) -- think Será Una Noche, also from M•A, but recorded in a smaller space. Everything sounded just right. As is characteristic of recordings from this label, sounds came from every direction within its relatively smaller ambience, and the obvious high quality of the recording and the resulting relaxed feel and excellent imaging came through immediately. Again, I wanted for nothing while listening, nor could I point to any obvious flaws, embellishments, or diminishments in the sound. What came through was a quick, snappy, dynamic delivery that served the pace of the music very well. No slugs, these Femtis.

Calyx Femti

Moving on to something a little larger in scale, I tried a disc with an interesting mix: Harold Farberman’s Concerto for Jazz Drummer and Orchestra, with Louis Bellson on drum kit and the composer conducting the Bournemouth Symphony; and Rodion Shchedrin’s Carmen Suite, again with Farberman, this time conducting the Kroumata Percussion Ensemble and the Helsingborg Symphony (CD, BIS 382). The concerto is an acid test for an amplifier -- Bellson’s drums are seemingly recorded at a bit of a distance, but his cymbals were apparently miked relatively closely and very clearly; any hash or distortion in the treble has nowhere to hide. The Femtis acquitted themselves very nicely indeed. They also managed to capture a good dose of the tom-toms’ tonality, which is very important given that they’re set a little farther back and could easily lose their individual sound and character. Front-to-back imaging was also very good -- it was easy to place objects from the front of the stage through the front wall, although in this regard the Calyxes didn’t quite match the natural strength that many tube amps seem to have in this area. The Rogue Medusa, for example, a tubed/solid-state hybrid, did a better job of hanging images in a clear space with more of an overall 3D presence. But the Femtis lacked for nothing when it came to the big bass-drum whomps that occur in the concerto’s first movement. The dynamic punch was notable, as was the tonal integrity of the drum in a larger setting; likewise, the brass had very impressive senses of both tone and bite -- two things that I often find are out of balance in other amplifiers, one way or the other. And the Femtis’ precise imaging and ability to capture minute reverb trails that disappeared nicely into space did a very good job of communicating that I was listening to a large orchestra recorded in a large space.

I did most of my listening with the Femtis in bridged monoblock mode. Switching to stereo mode and using only one amp, I expected to hear a decline in dynamics, and perhaps some sign that the solo unit was working a good bit harder. Wrong again. To my amazement, I detected little to no loss of punch or dynamics, and no sign of stress or distortion that would indicate to me that the little guy was stressing its digits in any way. And I pushed that lone amp hard with some demanding recordings. I was getting excited, about to write that you can get the lion’s share of the monoblocks’ performance for half the price. Then I switched back to monoblock mode.

That somewhat dashed my hopes. Tonally and dynamically, stereo mode sounded pretty much identical to the doubled-up version -- great news, right? But when monoblock mode came back into play, in certain areas the quality of sound jumped up to another league. I mainly noticed that the atmosphere around musicians got noticeably clearer, and that dimensions that had been obscured in stereo mode were now readily audible. The result was that each element within the soundstage now occupied more of its own space, with a greater sense of more air around it. A visual analogy: Think of when you buy a new electronic device (e.g., a cell phone) whose visual display is protected by a clear plastic film. It looks perfectly fine until you peel off that film -- then, everything is a little clearer and more distinct, and the extra bit of contrast makes everything “pop” more. That was monoblock mode. Still, stereo mode retained an impressive proportion of the monoblocks’ positive traits. Buying one Femti, then living for a while with the prospect of adding another if/when funds permit, would be an attractive and exciting proposition.

Noting my complete failure to accurately predict the results of a comparison of the sound of the Femti when used as a stereo amp and when used as one of two monoblocks, I switched to biwire mode with no idea of what to expect. Biwire mode sounded all but identical to bridged monoblock mode; any differences I may have heard could easily have been the results of my using a different run of my reference cables to the upper binding posts. I think the benefits of biwire mode might vary greatly depending on the design of a given speaker, so take my observations at arm’s length here. But add the fact that biwire mode requires two pairs of speaker cables and the benefits would have to be pretty clear to tempt you to go that more expensive route. But any way you choose, it’s nice that Calyx has seen fit to offer a way for the Femti to follow.


For these comparisons, my buddy Rich (not Buddy Rich) generously loaned me his Bryston 3B SST2 stereo power amp ($3850). Its price is in the range of the Femtis, and it’s a very well-regarded analog amplifier owned by many audiophiles.

The Bryston and the Calyxes sounded much more alike than different. The major differences were that the Bryston sounded a little fuller and richer in tonality, while the Femti countered with just a slight advantage in revealing dimensional images within and around the stage. The differences were such that if you were happy with one, I’d think you could live pretty happily with the other.

What neither amp could match was the open, holographic dimensionality exhibited by many tube amps and some solid-state designs. The Rogue Medusa ($3995) significantly bettered both in this regard, but with its stock tubes it also sounded tonally lighter and thinner, with less weight and oomph. I’d roll tubes to try to restore some of the guts of the music while retaining the Medusa’s dimensional magic.

The NuForce Reference 18s ($7600/pair) were another story entirely. From what I remember, they had an overall softer way in the upper octaves and put forth more substantial and rounder images within a wall-to-wall soundstage that was bigger but less clearly defined.

The amp I found to sound most similar to the Femtis was my longtime reference, McCormack Audio’s DNA-0.5 Rev.A. From the midrange up, I found it hard to hear any difference between it and the Femtis, and the imaging and soundstaging were likewise similar. The major difference was from the upper bass down: the McCormack sounded fuller and a bit slower (hence, probably, my preference for using it with the silver interconnects), while the Calyx sounded less full, but tighter and better defined. In short, while these amplifiers sounded far more similar than different, I’d say the Calyx was the more linear.

I usually spend much more time in a review focusing on how a component sounds with various recordings, and relatively less on comparisons. But in this case I found the Femtis to be so neutral in overall character that I felt the need to flip that method around, in the hope that comparisons with other known quantities could tell you a bit more about how the Calyxes “sounded,” given my inability to pinpoint areas in which it made any sort of statement of its own, whether positive or negative. Suffice it to say, if you’re looking for a blank canvas of power on which your upstream components can paint their sonic pictures, the Femtis are among the flattest and cleanest I can think of.


If you want a straight-shooter kind of amp -- one that tells the truth without overtelling it -- and that also exhibits iron-fisted control over bass while still conveying the music’s tonal purity, the Calyx Femti should be high on your list. I’m especially thinking that people whose systems need a bit of tightening up from the upper bass down might very much benefit from the tight, linear sound of the Calyxes. Also, I often hear of people looking to put their beloved tube preamplifier on a slight diet while adding just a touch more definition up top -- I think the Femtis could excel in that capacity as well.

I found the Calyx Audio Femtis very easy to listen to and live with. Their strengths were notable, their weaknesses nearly nonexistent. Tonal balance, while perhaps just a very slight bit on the leaner side, was overall very good, and very linear from top to bottom. And, as expected, bass and dynamics were excellent. That you can buy one Femti now and achieve a great proportion of the performance of a pair of them is an extremely attractive proposition, but I recommend eventually getting two, given the significant boost in performance I heard. The Calyx Femti does a whole lot right and very little wrong, and does it in a neat, tidy, power- and space-efficient package at a price within reach of many audiophiles. In the world of power amplifiers, less indeed seems to be becoming more.

. . . Tim Shea

Associated Equipment

  • Speakers -- Soliloquy 6.2
  • Amplifier -- McCormack DNA 0.5 Rev.A, Bryston 3B SST2
  • Preamplifier -- Bryston BP6 C-Series
  • Digital sources -- Oppo DV-970HD universal A/V player (used as transport), Electronic Visionary Systems Millennium DAC 1 D/A converter
  • Interconnects -- Acoustic Zen Silver Reference II and Matrix Reference
  • Speaker cables -- Acoustic Zen Satori shotgun biwire
  • Digital cable -- Apogee Wyde-Eye

Calyx Audio Femti Stereo/Mono Amplifier
Price: $1950 USD each.
Warranty: Three years parts and labor.

Calyx Audio
Kumkang Venturetel. 414,
Bisan-Dong 1108,
Dongan-Gu, Anyang-Si,
Gyeonggi-Do, Korea 431-050


US distributor:
On Song Audio Distribution
Phone: (269) 569-5540


Canadian distributor:
Charisma Audio
Suite 86, Unit A14
4261 Highway 7
Markham, OntarioL3R 9W6
Phone: (905) 470-0825
Fax: (905) 470-7966