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Equipment Review

May 2001

Audiopax Model 3 Integrated Amplifier and CX303 Loudspeakers

by John Potis


Review Summary
Sound "Richly textured, clean, natural, and as pure as live music" with the speakers and integrated amp paired; also, "works near-miracles with vocals"; "score this system very high for a remarkable degree of detail and presence without ever becoming fatiguing."
Features Single-ended design; speakers and integrated amp are designed to work synergistically, "including reciprocal distortion characteristics [with the speakers] that will reduce significantly the whole system's distortion.
Use Model 3 puts out only 7.5Wpc, so speaker load and drivability are important considerations, unless you use the CX303s; dual volume pots are not easy to set for perfect channel balance.
Value John found the CX303 speakers "competitive" with others in their price range, but thought that the Model 3 "may outperform preamp/amp combos far in excess of its price."

In the April 2000 installment of his "Y-Files" column, Srajan Ebaen wrote of his encounter at CES 2000 with the products of Eduardo de Lima, whom he termed an "unconventional thinker." I’ll thank Srajan right now for his extensive introduction to this review and invite you to read his column right now -- or at least after you've finished here. Srajan goes into much more technical detail than I would feel qualified to offer, even if space permitted.

Suffice it to say right here that Mr. de Lima is not only an unconventional thinker, you may want to call him a backward thinker. In an effort to eliminate the ill effects of relatively high levels of second-order distortion common to most single-ended-triode amplifiers, de Lima elected to take a path less traveled. Contrary to my own system-building advice of picking out your desired speakers and then matching them with a complimentary amplifier, de Lima designed his Audiopax brand of amplifiers first, then went and backward-engineered his speakers to be complimentary to his amps in every way, including reciprocal distortion characteristics that will reduce significantly the whole system's distortion.

Thus the Audiopax CX303 speakers are said to be "high-performance, tube friendly loudspeakers." Intended for stand mounting, they are constructed of 1" MDF with a 2"-thick front baffle intended to minimize resonance. An 8" woofer was developed in house and is said to possess many characteristics that optimize it for use with non-feedback tube amplifiers. Among these are a coated paper cone coupled to a Kapton voice-coil structure, a solid brass phase plug, and a rarely seen treated-cloth accordion-style suspension. Audiopax claims a very simple crossover using high-quality capacitors and inductors but no resistors -- claiming this as key to tonal accuracy and quality. A 1" titanium inverted-dome tweeter, which looks to be sourced from Focal, handles upper frequencies. Frequency response is given as 50Hz to 20kHz (+/- 3dB) and sensitivity is a claimed 92dB with one watt input at one meter. Nominal input impedance is 8 ohms with a minimum of 6.5 ohms, and the recommended amplifier power is 5 to 40 watts per channel. The CX303 stands 19"H by 11"W by 15"D deep and weighs in at 35 pounds. Their retail price is $1580 USD per pair.

The other Audiopax component under review here is the $1980 Model 3 integrated amplifier, which is not what Srajan heard at CES 2000. The Model 3 develops an output of 7.5Wpc using two 6L6 output tubes and two 12AT7 input tubes. Audiopax claims a typical frequency response of 15Hz to 55kHz (-3dB) and an input requirement of 300mV to obtain the full 7.5-watt output. Input and output impedances are stated as 22k ohms and 2.7 ohms respectively. The Model 3's dimensions are a very petite 10"W x 7"H x 13"D, and it weighs 11 pounds. Accommodated are two line-level inputs and a tape loop. Nice-quality gold-plated five-way binding posts are found at the amplifier’s rear, as are the gold-plated RCA inputs and an IEC power-cord receptacle. A true dual-mono design, the Model 3 has volume attenuated via a matched pair of pots on the front panel, right next to the power switch.

A brief clarification

Before I go on, I’d like to explain one issue that astute tube aficionados may have already picked up on. The output tube in use here is the 6L6, a pentode tube, not a triode. I guess that technically the Model 3 is a single-ended-pentode design then. I contacted Eduardo de Lima and asked him to clarify things for me. His reply is as follows:

The Audiopax model 3 can be called a single-ended-pentode amplifier only because its output tube is a 6L6. To be totally accurate, the 6L6 is a beam power tetrode, which is a slightly different kind of pentode. But this same 6L6 when used in our own output stage configuration behaves much closer to a single-ended-triode amplifier. It uses a technique we call LM3 (low Mu triode with higher raw efficiency emulator). By using a screen tap in the primary of the output transformer, a tertiary winding for partial cathode coupling, and the right amount of unbypassed resistance in this cathode circuit, the pentode will behave very differently from a normal pentode.

It is important to note that the right amount of screen and cathode coupling and cathode degeneration are critical to obtain the targeted behavior. It can only be achieved using a very different output transformer. The transformers in the Model 3 are the result of several years making these kinds of transformers. Therefore, we would rather consider the Audiopax Model 3 and our other single-ended amplifiers as "single-ended LM3 amplifiers." A more detailed explanation of this LM3 output stage is available at our website.

Set up and use

Installation was simple and straightforward, but some may quibble with the orientation of the integrated amplifier's binding posts. Due to the limited amount of real estate on the back panel of the small Model 3, the two sets of posts are in close proximity and the pairings are above and below each other, making the use of spade lugs somewhat difficult. Another quibble was with the dual volume pots, which are a bit of a pain. If they at least had detents so you could count the "clicks" in an effort to maintain channel balance, I would be happy. That said, by the end of the review period, I did get pretty good at standing between the speakers, where the amp was located, and adjusting each channel up or down while judging balance by ear. After that, the only problem I had was when I was swapping speakers in or out of the system. Listening for a solid center image in order to judge speaker positioning is a little iffy when you don’t know for sure that the amplifier is perfectly balanced. But that’s more a reviewer’s problem.

The speakers were set on 24" stands, and per the suggestion by Audiopax, they were adjusted so that the speakers pointed upwards just a few degrees (Audiopax recommends up to five degrees). I did settle on a fair amount of toe-in, even though Audiopax suggests little or none. I found that the system gelled well with the speakers and my listening seat forming an equilateral triangle.

System performance

The system sent to me was not exactly factory fresh, so it required no break-in. After a brief warm-up period, I was ready for some listening. First up just happened to be Sarah McLachlan’s Fumbling Towards Ecstasy [Arista 07822-18725-2]. Just as Srajan Ebaen experienced, it only took a few notes to be completely taken unaware. At the very moment when McLachlan started singing "Possession," I knew I was in the presence of something special. It hit me like the proverbial ton of feathers, and I was immediately struck by what Srajan must have been referring to with what he termed "the fabulous immediacy and purity of single-ended triodes." As simply as I can put it, it was as if an aged and yellowing coat of varnish had been stripped away from the music. What was left was richly textured, clean, natural, and as pure as live music. Removed was some kind of previously unnoticed haze that was obscuring the natural beauty of McLachlan's voice, and produced now was presence and harmonic accuracy that invited -- no, demanded -- a likening to the real thing. What I experienced can best be described as an epiphany.

Associated Equipment

Loudspeakers –  Silverline Sonata II.

Amplifiers –  Herron Audio M150 monoblocks.

Preamplifier – Herron Audio VTSP-1A.

Digital – Music Hall CD32, Pioneer DV-525 DVD player.

Interconnects – DH Labs Silver Sonic BL-1.

Speaker cables – DH Labs Silver Sonic T-14.

An epiphany? Surely, I exaggerate. Allow me to explain. The Audiopax Model 3 is the first single-ended amp I’ve ever had extended exposure to, but I’ve read a lot about them over the years. Other writers have referred to them as unpredictable tone controls. This is because the high output impedance of the amplifiers when combined with a widely varying speaker impedance can and will produce frequency-response anomalies that may or may not be to one’s liking. But choosing a speaker with a sufficiently steady (non-varying) impedance goes a long way toward a predictable result. Add to that the fact that not only were the CX303 loudspeakers designed with a stable impedance, they were backward engineered to compliment the amplifier. I suppose one could still claim the tone-control effect, but at least in this case the result was predicted. Predicted? No -- more like conceptualized, calculated and executed.

My other preconception was that SETs were extremely euphonic amplifiers. I expected overly sweet and syrupy sound, but what I found was intensely natural and unadorned raw beauty -- not raw as in the harsh sense of the word, but raw as in unprocessed. I heard intimacy and immediacy I’ve never before experienced.

Often writers speak of a blackened canvas from which the music emerges with unique clarity. I’ve experienced this phenomenon, and it does go a long way toward highlighting microdynamic detail. But I’m not sure that a black canvas is the optimum. In nature, we never experience sound from a complete void, so it’s not entirely natural. At CES 2000, I stopped into the Herron Audio room and heard a presentation that was quite to the contrary of the black canvas. What I heard was music emerging from an illuminated soundstage, and the space between the notes appeared much as one would see sunlight through the slats in a picket fence while speeding by in a car. Rather than emerging from a black hole in space, the music materialized from a real acoustic. This is very much what I heard with the Audiopax system. Music arose from an illuminated soundstage with superbly conveyed essence and microdynamic detail.

CX303 loudspeakers

Taken on their own, the CX303 loudspeakers are very fine examples of what you would expect in their price class. Audiopax specs the speakers down to 50Hz, but in my room they sounded as though they could dig deeper than that. Where bass is concerned, they are on par with speakers such as the Tyler Acoustics Taylo Reference Monitor or the Reimer McCullough. In terms of treble smoothness, they compete favorably with other speakers of the same cost, and while not the ultimate in perceived detail, their midrange was always clean and relatively free of amusical artifacts. Somewhat paradoxically given the overall immediacy of the system, the CX303s were not quite as good at picking up on minute detail as some more expensive monitors. These are not the kind of speakers that are going to amplify problems upstream in either the recording or electronics, and perhaps this is why the system can provide its overall level of resolution without ever making mediocre recordings sound less acceptable than they are. The CX303s are not speakers that will render the bulk of your pop-music collection unlistenable.

In order to get a handle on what the CX303 speakers sounded like outside of the Audiopax system, I borrowed a JoLida 300B-based prototype integrated amplifier (model designation not yet determined). This utilizes four Chinese 300Bs to render 18Wpc -- more than double the power of the Audiopax Model 3. This integrated amp duplicated much of the midrange magic of the Model 3, and overall the JoLida integrated along with the CX303s produced a formidable combination. But when I substituted in the Model 3, it became evident that indeed the CX303s were designed specifically with it in mind. The speaker and amp had rare synergy. Where one zigged, the other zagged. While the JoLida actually had a more weighty bass presentation, the Audiopax Model 3 was abundantly more detailed and more musical down there. The Model 3 also seemed to track the speakers through the upper bass and lower midrange, where the combination produced a spotlessly clean presentation that was completely free of any undue warmth, which would obscure upper harmonic detail. But don’t confuse this aspect of cleanliness with sterility; the combo was not so lean as to be antiseptic, not by any means.

Model 3 integrated amp

During the review period, I used the Model 3 with a multitude of speakers, including the 90.5dB efficient JMlab Mini Utopia. Although not as efficient as other speakers in the house, and one with an impedance that would seem to make it a less-than-perfect match with the Model 3, the Mini Utopia nevertheless faired very well with the Audiopax’s 7.5Wpc. The Model 3 delivered enough power to the Minis so that as long as I was careful with ultimate SPLs, I could drive the speakers remarkably well in my larger family room. This proved enough for all but the largest-scale symphonic pieces and the hardest-driving rock. But very late in the review period, I took delivery of a pair of Silverline Sonata IIs. This full-range speaker with a claimed 95dB efficiency and smooth 8-ohm impedance proved invaluable in sizing up the merits of the Model 3 integrated amplifier, which proved itself to be an outstanding performer -- with or without the CX303 speakers and one that should merit your attention.

What came as the biggest surprise was the Model 3’s bass performance. Where the JoLida provided greater heft and impact over the limited-bandwidth CX303s, with the Sonata IIs and their claimed 25Hz bass extension, the opposite was true. The Model 3 produced greater impact, weight and detail than the JoLida integrated. By any measure, and as compared to the rest of the amps at my disposal, this was very good performance in absolute terms. Only the Herron Audio M150 monoblocks (with Herron preamp), at over three times the Model 3’s price, clearly outperformed it in this area. But where midrange transparency is concerned, the Model 3 was again the winner, bettering the Herrons by an easily perceptible margin. The JoLida integrated did come much closer to matching the Model 3 in this regard, with its triode-like immediacy and transparency. Treble performance between all these amps was pretty much a wash -- all sounded exceptionally smooth and extended.

I must stress here that the Silverline Sonata IIs are very efficient speakers and a very easy load on an amplifier. Substitute in any number of other speakers and the results of these comparisons would most likely be vastly different. But that’s the point! SETs in general, and the Audiopax Model 3 in particular, are all about system building in the strictest sense of the term. They require careful matching of components, not only in order that each can operate to the best of its ability, but also so that they all can go further to produce a truly synergistic combination.

Finally some music

James Taylor’s Hourglass [Columbia CK67912] has become a staple in my evaluation stack of CDs. While not an audiophile recording per se, it tells a lot about a system’s ability to perform the basics. The cleanly recorded vocals, crisply portrayed acoustic guitar, and a very well-recorded drum kit go a long way toward illuminating a system's ability to reproduce acoustic music (and the music ain’t bad either!). The Audiopax Model3/CX303 system navigated the disc with ease. Taylor’s voice was ultra clean and natural without sounding insipid or uninspired. While not isolating the guitar and projecting it in the mix as well as some other highly detailed systems can, the Audiopax system nevertheless sounded authentic and believable. Treble performance was spectacular, as evidenced by clean-yet-unaccentuated high-hat strikes, shimmering cymbals and a commendable lack of sibilant emphasis. Score this system very high for a remarkable degree of detail and presence without ever becoming fatiguing. The bass drum on "Gaia" sounds deceptively big and satisfyingly weighty. Imaging, while not quite as razor sharp as with some systems, was very good, and soundstaging was still better. The speakers completely disappeared into a wall of sound, which not only extended beyond the speakers in the lateral plane, but also surpassed them well into the vertical one.

"Little Dog’s Day" from the Live Recordings at Red Rose Music, Volume 1 sampler [Red Rose Music RRM01] sounded startlingly natural and so real it was eerie. Credit both the recording and the Audiopax system. Looking back, I’m not sure if I’ve made it sufficiently clear that this system works near-miracles with vocals. The midrange is so transparent, so clean and so electrostatic-like that vocals such as Kim Cattrall’s emerge with striking presence and realism. The rendering of the accompanying acoustic bass suffers a little in both weight and detail compared to that with speakers like the ACI Jaguar 2000 and the JMLab Mini Utopia, but given the price differentials involved and the fact that this system is about finesse, not brute force, I can easily forgive this small transgression. At 3:29 into "Hard Times" from the same CD, you can hear -- deep in the background -- a telephone ringing. I'm not sure whether or not this was intentionally mixed into the recording, but given the very diminished volume level of the phone’s ringer, I suspect it wasn't. On some systems, this ringing may go unnoticed, but for better or worse, the Audiopax system’s ultra-clean midband illuminated this would-be sonic intruder to a high degree.


On their own, the CX303 loudspeakers represent good value. As clean and very musical performers, they are competitive in a very competitive market. But when coupled with the Model 3 integrated amplifier, they truly do become greater than the sum of the parts. As designed and touted, the speakers and integrated amp form a synergistic combo that produces striking musicality accented by immediacy and intimacy heretofore unheard, by me at least, at their price point.

As part of an all-Audiopax system or on its own, the Model 3 is marvelous, albeit limited by its power output. Within the context of a system comprising high-sensitivity and easy-to-drive speakers, the Model 3 may outperform preamp/amp combos far in excess of its price. Combining surprising drive capabilities with extremely high levels of refinement and transparency, it sets the pace for systems where musicality, sophistication and elegance are the goal. During the rather extended review period, it proved a very stable and predictable performer, and one whose relatively small size and moderate heat output (for a tubed integrated amplifier) made it easy to live with. If you think you can do with a limited amount of very high-quality watts, I recommend you give the Audiopax Model 3 an opportunity to impress you as much as it did me.

...John Potis

Audiopax Model 3 Integrated Amplifier and CX303 Loudspeakers
Model 3 integrated amp, $1980 USD; CX303 loudspeakers, $1580 per pair.
Two years parts and labor, 90 days on tubes.

22525 Charlene Way
Castro Valley, CA 94546
Phone: (510) 888 9193

E-mail: audiopax@rionet.com.br
Website: www.audiopax.com

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