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Equipment Review

December 2001

Naked Truth Audio Calla Mono Amplifiers

by John Potis


Review Summary
Sound "A completely new perspective" -- "present the music in a less up-front manner that seems to invite a more relaxed yet seemingly greater understanding of the musical event"; "they image spectacularly" and their "tonal balance, most of the time, seemed perfect; but "it is in the midrange where the Callas are their most enchanting."
Features OTL design that uses 6AS7 and 6SN7 tubes; available factory-direct.
Use "Require a speaker with a nominal impedance of something on the order of 8 ohms or greater and one that doesn’t dip too far below that at any point"; produce a fair amount of heat.
Value "It is the new perspective on the music that should put the Naked Truth Audio Calla OTL monoblocks on your audition list."

Consider the ice cream lover who craves only butter pecan. Though it is his flavor of choice whenever available, one day the ice cream parlor runs out and he’s forced to try something new. After careful consideration, he settles on double chocolate fudge. Upon first taste, he’s not convinced that it's worthy of his affection, but he continues on. A few tastes later, he begins to observe that the new flavor has some characteristics that may just deserve exploration, and he starts to develop an interest. By the time this new discovery is fully consumed, he’s come to understand what it is that lovers of chocolate ice cream taste in their preferred flavor, and his respect has turned into genuine admiration.

Audio equipment is a lot like ice cream. It comes in a mind-blowing array of flavors, and once you move past the mass-market brands, most of it is very good. And just when you think you know what it is you desire, something may come along to make you question your own faith -- something just a little different.

The Naked Truth Audio Calla monoblocks, $4995 USD per pair factory-direct, are output-transformerless (OTL) tube power amplifiers and a very new flavor to me. OTLs lack the usual output transformers common to other types of tube amplification. Normally the output transformer has two basic functions. First, it acts as impedance-matching device in that rather than looking at a speaker’s rather low and varying input impedance, the tube amplifier sees the higher and more stable impedance of its own transformer. Secondly, it works as a protection device that prevents driver-destroying DC from reaching the speakers. But transformers do have their downside as well. Think for a moment about all the reviews you may have read in which the manufacturer claims that the secret to his amplifier is the unique (and usually very expensive) way in which his output transformers are made. By most accounts, output-transformer design is still very much a black art.

OTL amplifiers skirt these issues, but compromises are made. On one hand, OTL amplifiers enjoy a reputation for having wonderful imaging and soundstaging qualities along with spectacular timbral accuracy. On the other hand, gone with the impedance-matching device is the ability to operate into low-impedance loads, and even dealing with the rest of the speaker population can be somewhat unpredictable. And without the DC protection offered by transformers, the danger of nothing standing between the amplifier and speaker in the event of malfunction has to be overcome. Naked Truth has answered the protection problem by choosing a tube with an internal fuse in order to protect the speaker in the event of a problem. As for speaker compatibility, well, that’s always an issue and part of the challenge of building a great-sounding audio system.

Some assembly required

As I unboxed the Naked Truth Calla monoblocks, I realized that these were hardly plug-and-play pieces. Sure, with 16 6AS7 triodes, six more 6SN7s and two IEC power cords, there was going to be lots of plugging in, but before I could get to playing, I had to go through an important, yet fairly simple regimen for tube biasing. Making a long story short, biasing was a fairly simple task that didn’t require any tools or meters. The Callas have LCD displays on their rear apron that tell you everything you need to know, and the straightforward directions make biasing a very simple operation. Naked Truth recommends 10-20 hours for the tubes to fully break in. Inputs are via either RCA or XLR connections and outputs are via nice brass WBT binding posts. Large rear-mounted rocker switches apply power while the amps remain muted. The filament voltage is applied for 30 seconds before the DC voltage power is applied.

A matter of perspective

With a rating of 60 watts each, the Naked Truth Calla monoblocks have enough power to drive the majority of speakers out there; however, as previously mentioned, they do require a speaker with a nominal impedance of something on the order of 8 ohms or greater and one that doesn’t dip too far below that at any point. Fitting the bill and in the house were Silverline Sonata IIs and the Soliloquy 6.5s.

With a total compliment of 22 tubes, these amplifiers generate some heat. In my small 150-square-foot listening room, the amplifiers were capable of raising the temperature substantially -- and quickly. Compounded by the acerbic smell that the amplifiers emitted (for the first 10 to 15 hours), my first impression of the amplifiers was not all that positive. I didn’t really get these amplifiers. They were certainly smooth and soundstaged very well, but they didn’t seem to stand out. And given their fairly lofty price, I had a hard time finding much value.

Associated Equipment

Loudspeakers – ACI Sapphire III, Silverline Sonata II, Soliloquy 6.5.

Amplifiers – Audiopax Model 3 integrated amp, Wyetech Labs Onyx mono amps.

Preamplifier – Herron Audio VTSP-1A.

Digital – Bel Canto DAC1.1, Music Hall CD32 CD player, Pioneer DV-525 DVD player.

Interconnects – DH Labs Silver Sonic BL-1, Silverline Audio Conductor.

Speaker cables – DH Labs Silver Sonic T-14, Silverline Audio Conductor.

Accessories – Vibrapod isolation pods.

But I kept listening, and I began to understand. And once I did get it, I started to like it. And the more I listened, the more I liked what I heard. I had just finished reviewing the Wytech Onyx SET monoblocks, and I had been doing a lot of listening to the Audiopax Model 3 SET integrated too. Both of these amplifiers exude intimacy and immediacy, and I was hearing less of these from the Callas. At first I felt a little alienated from the music -- in comparison to the SET amps I was using. But the more I listened, the more I realized that the vast majority of what I was listening for in the music was still present. I wasn’t missing out on much of the detail at all. And while I can’t say I sensed the intimacy that I had been experiencing, neither did I sense anything really between the music and me. I then realized that I was hearing most everything there was to hear but from a different perspective, one I grew to appreciate and enjoy.

I’ve commented before on the audiophile’s quest for detail. And without question, the amount of detail rendered from even the most modest of high-end systems greatly exceeds that heard in the concert hall. Live audiences are not nearly as privy to the precise imaging that most of us hear at home, and neither are they exposed to the intricate details that we now take for granted from our home listening experience. But at the same time, none of us are fooled by our systems into thinking that we are listening to live music, and some say that there is evidence that we are actually enjoying the music less than ever. Some complain that today’s systems are too hi-fi and unnatural-sounding.

Well, the Naked Truth Audio Callas are the answer to many of these criticisms. This is not to say that the amps don’t image well or that they don’t do the detail thing. They do. In fact, they image spectacularly. What’s more, they soundstage even better, and here is where you get a completely new perspective. Without losing detail or focus, the Callas move your concert-hall listening seat several rows back, giving you a slightly more distant perspective. I’m careful about not using the term "laid-back" because these days it seems to suggest a deviation from a neutral tonal balance, which the Callas do not exhibit.

But these amps do present the music in a less up-front manner that seems to invite a more relaxed yet seemingly greater understanding of the musical event. Soundstaging is expansive and cavernous, and with most of the recordings I used I had the feeling that I was looking down at a stage of musicians. Not that the image was portrayed lower than it had ever been before; it was actually completely unchanged in this regard. But when I closed my eyes, I had the distinct impression that I was sitting further from the stage, which means that I was now sitting above the orchestra and the mental image of height was adjusted for in my head. If you are thinking that it takes some startlingly natural sound to create such a mental image, you are right.

I was listening to the recent release of the Beach Boys Pet Sounds [Capitol 72435-21241-2-1], which features digital remastering in stereo as well as the original mono mixes, when I realized that there was more to the Callas than cavernous soundstaging prowess. They also excelled at image layering. Although the recording has its origins in mono, the stereo version contains an amazing amount of dimensionality. Though somewhat truncated in terms of ultimate depth, there is nonetheless a great amount of detail relating to the front-to-back layering to be found. Images are mainly contained by the locations of the speakers but given the somewhat distant perspective it all makes perfect sense.

Tonal balance, most of the time, seemed perfect. Linearity seemed equally flawless; the Callas sounded as smooth and natural as any amp I’ve heard, and live music. Truth of timbre was, at times, absolutely astounding, and I was made aware of a real feeling of harmonic accuracy. That’s not to say that everything was perfect, though. No, I did have the distinct awareness that I was listening to tubes via the Callas. Nobody will ever mistake this sound for solid state. In the classic tube sense, deep bass was a little bit loose and it lacked the last iota or two of detail. Even in comparison to the Wyetech and Audiopax SETs, the Callas produced bass that was just a hair on the woolly side. But don’t interpret that as being bloated or slow; it was not even close. The bass was just a little more full-bodied and had a little more bloom. It was not an entirely unpleasant effect and one that may be welcomed in systems suffering from slightly over-damped bass. In my case, it was nothing obtrusive at all, just something noticed via direct comparison. Particularly over the Soliloquy 6.5s, the bass was enormously powerful and rhythmic.

The highs also could sound classically tubey, though to a lesser degree. They were extended and as smooth as glass -- without the glare. They exuded utter refinement, and I eventually found that they could go a long way toward taming the treble response of speakers selling at a fraction of the Callas' price. For example, the inexpensive ACI Sapphire III is an excellent example of a sub-kilobuck monitor but, particularly when pushed, it can get a little ragged on top as compared to its more expensive competition. But the Naked Truth Callas helped the Sapphire IIIs absolutely transcend their class with more treble elegance than I’ve ever known them to possess. Once in a great while I had the sneaking suspicion that treble response was just a touch rolled off, particularly as compared to solid state. Most of the time listening provided no clue to this, but triangles and chimes seemed to indicate just a smidgen less treble energy. I seriously doubt that we are talking about much more than a dB or so, and I would suppose that if any listener found this significant he could abolish the discrepancy through a change of toe-in at the speaker. We are talking about a minor deviation here.

Which brings me to the midrange. It is in the midrange where the Callas are their most enchanting and where they sound like no other amplifier I’ve heard -- ever. It is the midrange that will either make or break the Callas for you, but it is the midrange that you need to experience. It’s not what I would term hyper-detailed, but there’s a bounty of detail to be discerned. It’s just not of the in-your-face variety. It’s exceedingly transparent and just a little distant -- liquid but without quite the sense of intimacy that SETs make their goal. Images absolutely dissolved the wall behind my speakers. I was reminded of the old Ohm Acoustics magazine advertisement showing the living room with the speakers placed at one end but with no wall behind them; it had been replaced by a concert hall. That’s just about what I heard. Midway through the review period, I took delivery of an Audio Magic Stealth Power Purifier and plugged all analog equipment into it. I found it did miraculous things for the Callas in the way of midrange clarity and purity.

Neil Young’s Unplugged [Reprise 45310-2] and "The Old Laughing Lady," in particular, showcased the soundstaging abilities that I’ve talked about. The stage was deep and cavernous, and Young was placed well behind the speakers. All images were very well focused, but the rear of the stage seemed to go back forever. The harmonica from "Pocahontas" had a strong image at center stage, but it also seemed to bloom as it occupied a still larger sense of space. Images throughout had a rightness of scale too. I never had the feeling that I was listening through a microscope as minutia of detail was never emphasized, yet I was privy to all the important events such as the popping of the microphones, every misplaced foot stomp on the stage, and lots of amplifier hum -- it was all naturally laid out before me. In other words, it was all there, just never thrust at me. There was total musical awareness without forced hyper-detail.

As for bass weight and control, the Callas did a very good job controlling the woofers of the Soliloquy 6.5s on James Horner’s soundtrack from Casper [MCA MCAD 11240]. The plucked double basses 5:30 into "No Sign of Ghosts" had every bit of the detail and heft that one experiences in a live venue, if not more, even if they did suffer somewhat to brawny solid-state amplifiers in comparison. Bass-drum resonance filled the illusory hall on "Strangers in the House," illuminating a huge window on the performance. "First Haunting/The Swordfight" showcases the bass like few other pieces in my music collection, as double basses, bass drums and even some synthetic bass lines all set my room in sympathetic motion. Violins from "The Lighthouse - Casper and Kat" were just intoxicating in their authenticity. "Casper’s Lullaby" features an oboe that rises from the music with an emotional component that it never had before, partly due to their contrast with its tonal opposite: the piano accompaniment. Where the distant miking of the piano creates a large and blooming -- if somewhat soft and muted -- presence, the ever-so-delicate oboe is heard with fine precision.


I'm sure by now the point regarding the Callas' somewhat distant nature, which lets one stand back and breath in the big picture, versus a SET's up-close-and-personal demeanor has been made. In direct comparison to both the Wyetech Labs' Onyx SET monoblocks as well as the Audiopax Model 3 SET integrated, there were other differences as well. James Taylor’s Hourglass [Columbia CK67912] was interesting yet completely enjoyable over the Callas. I listened for the complete hear-through intimacy of the SET amps, but the Callas didn’t quite give it to me. But as noted earlier, this has nothing to do with a perceived layer of haze or grunge that some electronics can heap onto Taylor's voice. I heard none of that. Consistent with everything else I’ve heard from the Callas, the music is placed at arm’s length in an authentic replication of space, but somehow Taylor’s voice still manages an uncanny authenticity. There is no perceived loss of detail, and JT's voice is cleanly reproduced with no colorations of any kind.

In terms of bass, the Callas traded a little of both the SETs' detail for a little more heft. In this regard, the Callas reminded me of a good push-pull tube amp: not too loose down low, but just enough there to lend some added life to the bass lines on what may be, perhaps, overly damped speakers. Difference in perspective aside, all these amps have crystalline clarity through the midrange. In the uppermost octave, the Calla was just a touch polite -- not so much as to create a dulling, but rather a slight softening of the treble.


Very literally, just when I thought I knew a lot of the answers, Naked Truth Audio changed the questions and raised new issues with its Calla OTL monoblock amplifiers. Clearly these amps are not for everybody. They represent a very different philosophy on reproducing music that may not appeal to everybody, and they are not compatible with every speaker out there. Potential buyers will want to think about how to deal with the considerable heat generated too.

What the Callas have not changed is my mantra that this is all about system building, and once all the proper pieces are in place, I have no doubt that many of people are going to love what the Callas do. The basic building blocks are all there. It’s the fine full-bodied bass, which is joined to clean, luxurious treble by one of the most seductive midranges I’ve ever heard that will appeal to most listeners. But it is the new perspective on the music that should put the Naked Truth Audio Calla OTL monoblocks on your audition list -- as well as ensure that the Callas are not just the new flavor of the month.

...John Potis

Naked Truth Audio Calla Mono Amplifiers
$4995 USD per pair factory-direct.
Warranty: Return in 30 days in the condition received for a full refund; 90 days for repair or replacement of all parts not damaged by abuse; five years for repair or replacement of anything not damaged by abuse except tubes; lifetime repair of any manufacturing defects.

Naked Truth Audio
P.O. Box 1353
Ojai CA 93023
Phone: (805) 646 8383
Fax: (805) 646 9683

E-mail: info@nakedtruthaudio.com
Website: www.nakedtruthaudio.com

Naked Truth Audio responds:

In reference to the smell the Callas gave off, this is not normal to the amps. I had a swarm of bees move into the storage shed where the amps I sent were stored. I suspect they left something in the amplifier that smelled when it got hot. For some reason they liked the box my VPI came in and made their hive in it.

You are right that transformers have always been a difficulty. I am no expert in transformers, but I will try to shed a bit of light on the subject. A transformer is the electronic equivalent of a lever. A transformer is basically two windings physically close to each other but insulated from each other. There is an iron core that helps magnetically couple the two windings together. The primary sets up a magnetic field that is picked up by the secondary, thereby transferring the signal. To get good coupling across the audio range is a difficult task. Plus, you have the problem of saturating the core and of the transformer's tendency to act as a filter. In SETs, this problem is compounded by DC current flowing in the primary winding of the transformer. In a push-pull amplifier, you can effectively cancel this.

I must disagree with you that transformers help with speakers that show large swings in impedance. The transformer will transfer those swings back to the tube just as moving one side of a lever affects the other.

Vacuum tubes are generally happier playing into a higher impedance than a speaker represents. This is the purpose of a transformer. The typical output tube wants a few thousand ohms when connecting to the plate of the tube. The 6AS7 wants a few hundred. Driving off the cathode and using 16 tubes per amp, two per envelope, brings the impedance down to where the tubes are happy playing into even 4 ohms. Unfortunately the damping factor and the power start to fade at lower impedances. This is why tube amps get mushy in the bottom with low impedance, poorly physically damped speakers. A little bit of this adds warmth; a lot of it and the sound turns into mud. Years ago 16- and even 32-ohm speakers were much more common. These suit tubes better. I suspect they were phased out because transistors put out more power into lower impedances, much more drastically than tubes do into higher impedances. Halve the load impedance on a transistor amp and, if the power supply can do it, it will double the power. This also means that with speakers that vary a great deal in impedance across the audio spectrum you are better off with tubes. This is the opposite of what most people think.

Most good audio transformers are good to about 50kHz. I have found that transients represent a much higher frequency than this. I have found that good transients make a huge difference. They give you a better representation of sounds like drum strikes or the plucking of a string. They also give you better imaging and a better illusion of space. Perhaps this is why you feel that you are sitting further from the musicians.

I have been modifying tube amplifiers for years with my friend Tré. We had tried dozens of output transformers and had settled on those from a Citation II. We had been kicking around the idea of an OTL. About five years ago, he built an OTL and it knocked my socks off. The amount of detail and the imaging blew me away. It had always bothered me that drums just didn’t sound real to me on any stereo. Listening to Tré's OTL was the closest by far they had come to real. We knew that speakers could be a bit of a problem. Our dynamic drivers seemed to do fine. We have found that most good-quality dynamic drivers do well with OTLs. If a driver varied a great deal in impedance across its frequency range, we really didn’t want it anyway. That is one of the determining factors we use in choosing the drivers we use for our home systems.

There are two approaches to building a stereo system. The first is to find components that cancel each other's deficits. Like an amplifier with a poor damping factor and overdamped speakers. The second is to try and get components that are in themselves as close to right as possible. This is my philosophy. Unfortunately this is often a more expensive route, though I am appalled at how bad some really expensive equipment is.

OTLs do have their downside. With the exception of the Futterman, which has other problems, they do consume quite a bit of electricity for the power they put out. This equates to lots of heat. They require many tubes to bring the output impedance down. They do not like many electrostatics as they have a strong rise in impedance at lower frequencies. This means they get more power just where they cannot really handle it. Quads tend to freak out if played loud.

On the other hand, triodes are inherently very linear devices. They will pass DC to several mHz very accurately. OTLs take advantage of this. The only limit in most OTL amplifiers is the coupling capacitors. We have been thinking of a way to build an OTL that is completely direct-coupled, thereby giving us an amplifier that is flat into the mHz range.

Along with building an OTL to get rid of transformers, we have built nuvistor-based (a subminiature tube) head amplifiers to replace our step-up transformers. It had the same effect of opening up the soundstage -- one more veil lifted. This may be the next Naked Truth Audio product.

For more information on the Calla, please visit my website.

Bill Sibner
Naked Truth Audio

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