International Audio Review, Dec. 2000

Single Ended Tube Power Amps

deHavilland Aries

By J. Peter Moncrieff

Most other single ended tube power amps can only dream of performance like this. The deHavilland Aries actually delivers. And it delivers at a price far less than other SET amps with equivalent performance. The Aries sells for $5,995 per monoblock pair. The only other SET power amps we judge to have comparable sonics are the recent Cary 1610-SE at $39,995 (7 times the price) and the Venture Reference One at $24,000 (4 times the price).

Freedom to Choose the Best Speakers

The deHavillandís most astonishing feature (and its most useful) is its power, 40 watts per monoblock channel. Not 3 watts, or 5 watts, or 8 watts, or 12 watts like most other SET amplifiers. That 40 watts is a pretty amazing achievement for a single ended class A triode amp. It wasnít all that many years ago that the most powerful amps you could buy at any price put only 40 watts per channel (e.g. the Marantz 2, 5, and 8B), and those amps achieved their power using the far more power efficient topologies of push-pull instead of single ended, class AB instead of class A, and pentode instead of triode output tube.

That 40 watts of power puts the deHavilland in a whole other league than most other SET amps, since thatís 10 times the power of most. That dramatic margin of power is vitally important for better sound. Note that we said important for better sound, not merely important for louder sound. Why? The usefulness, the significance of this enormous extra power is not merely additional loudness. It is freedom. Freedom, and neutrality, and accuracy, and bandwidth. Freedom to choose a better loudspeaker. Freedom to choose a loudspeaker that is more neutral and more accurate and with wider bandwidth.

Most other SET amps, with only the usual 3 to 8 watts, restrict your choice of speakers to only the most efficient. This is a very restricted, narrow choice, since the vast majority of speakers have only moderate to low efficiency, and they are unusable if your amp puts out only 3 to 8 watts. Since the speaker is the least perfect link in your chain, it is the most important link for you to make as good as you can. So naturally youíd want to be able to choose from as wide a selection as possible, not just a limited selection. Moreover, the laws of physics pretty much dictate that very high efficiency speakers are less neutral with worse tonal colorations, and have less flat frequency response within their bandwidth, and have a less wide bandwidth, and have worse colorations from time ringing material resonances, and have poorer stereo imaging. For example, speaker designers can reduce undesirable diaphragm resonances by making a diaphragm thicker and heavier, or by adding damping material that also makes it heavier. But adding weight reduces speaker efficiency, so the most efficient speakers need to keep their diaphragms light, and thus they tend to have worse diaphragm resonances. Also for example, high overall speaker efficiency requires high efficiency in the bass, which requires a large cabinet, which usually means a large front panel area, which degrades stereo imaging (unless the system uses modular enclosures).

If your power ampís output is so miniscule that it forces you to select an inferior choice for the weakest link in your chain, the speaker, then the degradations you suffer from having to use an inferior speaker will more than offset any gains from using a slightly better sounding amp -- especially given the fact that amps are one of the stronger links in the chain. The amount to be gained from using a better amp is likely to be much less significant than the amount to be lost from using an inferior speaker, so SET amps with miniscule power output compromise your overall system, regardless of how good the amp itself might intrinsically sound.

The deHavillandís 40 watts free you to choose almost any speaker you want, including the finest, most accurate, most neutral, best sounding ones -- and still have that special SET sound to combine with the speaker of your choice.

Facing SET Challenges:

-- Distortion

Besides limited power output, there are several other challenges that SET power amps must deal with, and here too the de Havilland surpasses most.

Consider the challenge of distortion. The deHavilland sounds much cleaner and purer than most other SET power amps, so that we can enjoy and focus on the music, without the annoying distraction of distortion artifacts. Most other SET power amps exhibit easily audible distortion, often sounding like a dirty grundge through the midranges, a fuzzy smear in the trebles, and boomy sounding harmonics in the bass. SET power amp circuits do not have the distortion canceling benefits of balanced or push-pull topologies (which inherently cancel even order distortions), and various other factors (e.g. having to push large currents through transformers) conspire to worsen other various distortion mechanisms. We have openly criticized many SET power amps for having objectionably high levels of readily audible distortion (e.g. the older Cary models, and most SET amps using 300B tubes). We hear their distortion front and center stage, which in our view negates whatever tonal charms these amps may bestow upon music with their romantic midrange bloom. The de Havilland deserves great praise for conquering this SET distortion bugaboo, where so many others have failed.

Two factors deserve special mention, for helping the de Havilland to achieve audibly lower distortion than most other SET power amps.

The first factor is, again, power. The deHavillandís dramatic margin of 10 times more power has a byproduct of achieving lower distortion at ordinary listening levels, from a triode output stage.

Why is that? Letís follow a chain of reasoning over a few paragraphs. Firstly, even though triodes are less powerful than other tubes as output tubes, triodes are desirable because their transfer (amplifying) characteristic is inherently linear, so they can be used with little or no negative feedback, whereas other types of output tubes (tetrodes and pentodes) are inherently less linear, so they require more negative feedback.

But there are complications. A triodeís transfer characteristic is truly linear (straight) only for a portion of its operating range, and beyond that it gently curves in a nonlinear (non straight) fashion. Tetrode and pentode output tubes, are made linear by feedback, up to the point of clipping, at which point the sound usually turns so ugly that every listener is instantly warned to back off on the volume control, the result being that tetrode and pentode output tubes are usually not listened to in their nonlinear regions, and are heard only in their linear regions (as created by feedback); since tetrode and pentode tubes can put out plenty of power, there is usually no rationale for pushing them into their ugly sounding clipping region. In stark contrast, triodes are often listened to in their nonlinear regions. Because the triodeís transfer characteristic curves gently outside its linear region, there is no sudden onset of ugly sounding clipping. Instead, the sound simply starts becoming slightly dirty, grundgy, and fuzzy -- imperceptibly so at first, then gradually more and more noticeably as the music gets louder.

Many listeners (especially those accustomed to vinylís distortions on louder music passages) donít key in on this distortion that occurs during louder music peaks or passages, so they typically run their SET power amps well into the curved region of the triodeís transfer characteristic, so these listeners are actually listening to their SET power amps producing a lot of distortion.

Now we get to the key point in this chain of reasoning. These listeners donít really have much choice anyway. Given the miniscule 3 to 8 watt power output of most SET amps, most listeners have to run their amps into the curved nonlinear region just to get a reasonable normal listening level out of most any speaker (save a fully front loaded horn system). On the other hand, an amp such as the deHavilland, with 10 times the power, affords the listener the unique luxury of being able to achieve normal listening levels while still keeping the triode output tube within the ideal linear (straight) region of its transfer characteristic. In sum, the deHavilland has the headroom to allow you to hear a triode in its ideal linear region, with lower distortion; most other SET amps do not.

The second factor is the engineering by the deHavillandís designer, Kara Chaffee. Kara has developed some unique circuitry to give this amp extra headroom and lower distortion (and wider bandwidth as well). And Kara has also provided generous margins of headroom in the circuit parameters and in the parts chosen for the circuit. For example, the driver tube, that is merely used to drive the output tube actually supplying the power, is a KT88/6550. Note that this tube is so robust and powerful that it is usually used as an output tube for 60 watt amplifiers. And here in the deHavilland this tube is being used merely to drive another output tube putting out merely 40 watts. By the way, the triode output tube itself in the deHavilland is an SV572.

-- Bandwidth

Another challenge faced by SET power amps is bandwidth. The deHavilland reaches to both frequency extremes much better than most other SET power amps.

SET power amps are famous for their rich, musical midranges, but most are notoriously weak at capably extending their reach up into the trebles and down into the bass. SET power amp topology faces difficult challenges here. For example, output transformers like being fed in a balanced or push-pull mode, but they dislike being fed in a single ended mode, and they present some special limitations when the SET power amp designer forces them to operate in this single ended mode. These limitations imposed by the output transformer can act to worsen distortion and to limit bandwidth, perhaps at both ends of the musical spectrum. Most SET power amps have bass thatís weak and/or poorly controlled, and trebles that are dull, rounded, and soft (as well as distorted).

The deHavilland employs expensive custom Electraprint transformers, and these, combined with the deHavillandís unique circuitry, give the deHavilland its ability to reach toward the frequency extremes of the audio bandwidth better than other SET amps.

-- Tonal Neutrality

Another challenge faced by SET power amps is tonal neutrality. The deHavilland sounds more tonally neutral and accurate than most other SET power amps, which are tonally colored. Indeed, for many listeners the rich, warm midrange bloom that SET power amps bestow upon music is part of the charm that makes them attractive.

However, there is more to music than midrange bloom, and these other SET power amps usually shortchange the rest of the music in various ways. Common problems among them are that musicís trebles are to tonally recessed and not articulate enough, and that musicís bass is too warm, prominent, and flabby in the upper bass while being too weak in the lower bass.

The deHavilland has the midrange bloom that SET lovers crave, but it then balances this with a much more neutral rendition of musicís treble and bass spectral regions than most other SET power amps can manage. Thus, the overall musical portrait from the deHavilland is much more balanced, accurate, and neutral.

Construction and Controls

The de Havillandís construction features a number of costly aspects, which are all oriented toward producing better sound. Parts are expensive and oversize. Point to point 3D wiring is used throughout, to avoid the sonic limitations of PC boards. These are big and heavy (55 pounds) amplifiers, and youíre clearly getting your moneyís worth in parts alone. Itís also worth noting that your money is not wasted on cosmetic window dressing. You wonít find chrome plated potting cans on the transformers, or a fancy milled chassis box, etc. The front control panel is a simple handsome thick plate of brushed aluminum, and thatís it.

The deHavilland has some interesting and useful controls. An optional stepped attenuator (not simply a cheap pot) allows you to control the input volume level. We found this very useful for being able to run our DAC directly into the power amp, for optimum fidelity. It was also useful to be able to turn down the input volume to zero for safety while changing system connections upstream.

This power amp is easy to sum up. The deHavilland Aries gives you more than any other SET power amp at up to 4 times to 7 times its $5995 price. The deHavilland Aries gives you two crucially important things that these other SET power amps donít: better intrinsic sound (lower distortion, wider bandwidth, better neutrality); plus the invaluable ability to pick and choose the best sounding speakers to mate with the amp.

More  by J. Peter Moncrieff

International Audio Review, Issue #80 , 2002 -- deHavilland 845 SET

     "The sonics of the 845 deHavilland  excel over the sonics of their earlier 572 model in nearly all aspects. It sounds slightly more liquid, and so is more musically beguiling. But this liquidity does not bring with it a dark or excessively syrupy sound as in many other SET power amps. Instead the 845 deHavilland sounds remarkably clear, vibrant, and alive, even more so than the 572.

     " The 845 penetrates deeper into the musical texture, and is even better than the 572 at revealing music's subtle inner details, especially throughout the midranges (a typical strong region of the spectrum for SET amps).

     " Further helping this amp's musical revelation is the fact that the 845 deHavilland sounds even cleaner and purer than the 572, which in turn sounded cleaner than most other SET power amps of its day when it was introduced. The 845 allows you to enjoy the music without a haze of grundgy or juicy distortion coming along for the ride, as it does with most of the lesser SET power amps. You simply hear more of the music when you are less distracted by hearing distortion.

     " The clean pure sound that this 845 amp achieves, even without any overall feedback to lower distortion, is also a tribute to Kara Chaffee's design work and to the inherent linearity for the whole circuit that Kara has achieved.

     " When an amp excels at revealing music's subtle inner details, as the 845 does, then it naturally also excels at stereo imaging, since the subtle imaging cues that make an image truly believable are themselves inner musical details imbedded in a good recording. The 845 amp excels at projecting dynamic music in a tactile, coherent way that makes you believe you can reach out and touch the musicians. 

     " The 845 continues the deHavilland tradition of handling the frequency extremes much better than most other SET power amps. In the trebles most other SET power amps turn fuzzy soft, veiled and defocused, with grundgy distortion making treble definition even worse. The deHavilland 845 has none of these problems, and handles trebles with very good cleanliness and focus.

     " In the bass, most other SET power amps turn flabby, loose, boomy, and muddy. The deHavilland 845 has much better bass definition than most other SET power amps, without any loose flab, or ill defined boomy heaviness. The low bass might not have the sheer impact that the best push-pull can achieve, but the 845's tight bass definition is a notable performance for a SET power amp.

     We heard the 845's outstanding clarity and dymanics being brought to the fore by a slight tonal emphasis in the upper midrange, but on your speaker system this might not happen. We can assure you, though, that the 845 is not a shy, polite, retiring amp that's all warm and mellow, dark and syrupy. This amp has moxie, and brings you the music full of life."

 

Complete Report at http://www.iar-80.com/

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