January 2004

Art Audio Diavolo Stereo Power Amplifier
Single-Ended With Balls
Review By George Papadimitriou (Pappas)
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  Single-ended tube amplifiers. Have you ever noticed how controversial this amplifier design is? You've probably heard opinions from both sides of the camp. There are those who focus on the strengths of single-ended designs which include phrases like: finesse, purity, wholeness, a more relaxed and easy presentation, a sense of organic realness to the music. Then of course, there are those people who are quick to point out the weaknesses which tend to be statements like ethereal, anemic, light, no grunt or balls and can't drive the loudspeakers that I want to use.

Having heard a number of low power single-ended amplifier designs, I can honestly say that there is validity to both arguments. In general, most single ended tube designs are a combination of the above strengths and weaknesses. There seems to be a very easy, relaxed quality to these amplifiers and at the same time these amplifiers just cannot drive most loudspeakers available in the marketplace. Their lower power ratings, usually between 3.5 to 8 watts or so, for the most part, just cannot provide the iron-fisted control required to "properly" drive most conventional loudspeakers. This really puts these amplifiers at a great disadvantage because loudspeaker matching becomes paramount. Not only paramount, but a major problem.

In designing the Diavolo power amplifier, Art Audio wanted to combine a single-ended amplifier that would combine the strengths that this class of amplifiers is noted for: purity, finesse, truth of timbre and an easy listening experience; while minimizing the weaknesses by creating more power to drive most loudspeakers, have better control of the drivers of a loudspeaker, as well as producing deep satisfying bass. I believe that they have succeeded with the Art Audio Diavolo power amplifier.

A useful option on this amplifier is the availability of a volume control that allows this amplifier to be used without a conventional pre-amp, as a single source amplifying device.

Design And Construction

This design uses zero feedback and also has high peak current delivery. There is an automatic biasing circuit that eliminates setting the bias. The construction is dual mono on a single chassis. The Diavolo employs a tube (CV-378/GZ-37) choke regulated power supply preceded by pi filter. The output transformers are a novel split core type not normally associated with single-ended design and custom designed by Art Audio to give a wider band width than most generally available transformers. The tube sockets are ceramic with silver plated pins. The entire amplifier is finished in polished (non magnetic) stainless steel chassis with gold plated transformer caps and name plate. The total power output is 13 watts per channel of single-ended power. The output impedance of this amplifier is 6 ohms however special custom impedances from 2 to 8 ohms are available at no extra charge. The tube compliment is a pair of each: 6DJ8, 12BH7, CV378, and EAT power tubes. This amplifier weighs a very heavy 60 pounds and is very well finished.

Removing the bottom cover from the Diavolo showed that this amplifier is extremely well built. There are 4 Hovland Musicaps (2 per channel) on the PC board. I was happy to see that all the wiring was directly soldered using no clip-on connectors. The volume control was a high quality Alps unit (this volume control is optional on this amplifier) that is inserted between the RCA input connectors and the PC board. Good quality, thick circuit boards are utilized with a star grounding configuration terminated at the middle of the amplifier. The body of the amplifier is made from thick chrome stainless steel that is very strong and stiff, showing very little deflection to finger pressure.

The output transformers use thick solid core wire, as opposed to other designs that can be quite thin, and these transformer wires are soldered directly to the loudspeaker output jacks with no flexible end leads. This eliminates another piece of wire as well as another solder joint. This amplifier uses excellent ceramic tube sockets with the best grip of the tube pins I've ever encountered. The tube socket pins that hold the power tubes have extra thick metal to provide a better grip on the pins of the EAT power tubes. On the rear panel, there are two transistor heat fins that are most likely for cooling of the power tube heater voltage regulators.

How It Sounds

Art Audio wanted to produce a single-ended amplifier with some power, grunt and drive; an amplifier that would drive most real world loudspeakers. They have succeeded. Let me first say that, this amplifier sounds fantastic. It has mass, drive and oomph. It certainly does not sound like a wimpy single-ended triode amplifier at all. Our esteemed editor, Steven R. Rochlin, said of this amplifier in his review several years ago in Ultimate Audio magazine that the Diavolo sounds like a 50 watt amplifier. This is so true. This amplifier has incredible balls for a single-ended triode amplifier and in this regard, it smokes my Audionote Meishu 300B integrated amplifier. The Diavolo strikes an almost perfect balance between the light, airy and ethereal type of sound and the warm, dense more massive solid type. This is an extremely difficult trick to accomplish. Traditionally, most of the single-ended amplifiers that I have heard have great strengths in subtlety, finesse and purity. Their sound is very open and relaxed, but they do not have the energy drive to really move the music into the room. It's almost like the music is massless and lacks the solidity that I hear in live music. I'm not talking here about just loudness but a sense of texture and physicality to the music.

The soundstage is very large with good width and depth, that in my small room is a great accomplishment. This large and open soundstage is accomplished without a thinning out of the sound. Many amplifiers that I have heard, both transistor as well as tubes, create a soundstage that loses its mass and density as the width and depth of the sound stage increases creating a sound that is too light and airy. 

The Diavolo is very detailed, but unlike other amplifiers, does not throw it in your face. The subtleties, the voice inflections are all there, but they are integrated and are part of the music. This is exactly how real music sounds. This detail is expertly combined with great body and warmth. You hear the subtleties in the music and the richness of texture as well. I believe this is the hardest thing to achieve in amplifier design; getting an amplifier to sound detailed with great definition and at the same time produce an amplifier that produces a sense of body, richness and musicality. One can hear this in natural instruments like brass and woodwinds. With these instruments the amplifier has to reproduce the quick attack of a trumpet or trombone for example, and still produce the resonant, throaty sound of those instruments.

Too often amplifiers either produce the leading edge of the transients, the "bite", or they reproduce the roundness, warmth and body of that instrument. Very, very rarely is there an amplifier that can produce both. The Diavolo is such an amplifier. You can hear an example of this when you listen to the 24 carat gold compact disc of Frank Sinatra "Duets" from DCC. The last song on this CD is called "All the Way/One For My Baby (and One More For the Road)". On this track you can hear the attack as well as the phrasing and tempo changes of the piano, and at the same time hear the woody, resonant quality as well. You can also hear on this track a large amount of detail and subtlety. You can hear the warmth of the strings, the brushes striking the snare drum, the deep resonant stand-up base as well as the power and raspiness in the voice that is distinctively Frank Sinatra. Here is a great example that, with the Diavolo, you get the detail as well as the texture of the music. 

As I stated in the title, this single-ended amplifier has "balls". It has the ability to drive a wide range of loudspeakers. It drove my Green Mountain Audio Continuum 2's with ease. These are a twelve inch three-way with 90dB sensitivity. The Diavolo was able to produce very good bass, extraordinary for a single-ended design, that gave a richness, a depth and a body to the music. Very nice. Another benefit of the Diavolo is that the centre image was extremely stable and did not wander. The high frequencies had good attack, good definition, yet remained sweet and tender. They were never edgy or sharp. 

This amplifier came equipped with an optional volume control that enables the Diavolo to be used directly with a source like a CD player, tuner or phono stage. Using it in this way, the sound was more articulate, more open and airy, but less robust, thinner, with less oomph to the music. While the sound directly with the source was clearer, it was also less dynamic. 

You can also fine tune the sound of this wonderful amplifier by substituting a different power tube in place of the EAT tubes. By substituting the KR 32 tubes, one gets a warmer, richer presentation but with a slight loss at the leading edge of the transients as well as a slight loss in microdynamics. But the warmth, the solidity are exceptional. Very easy to listen to. The KR 32 tubes might be a good match if a system is a little on the thin side or the loudspeakers have tipped up response in the upper mid-range and treble like so many modern loudspeakers. It is nice to have that option to fine tune the sound of this great amplifier to suit your particular system and musical taste. 

A couple of observations to note is that the power transformer ran quite hot, that I understand is normal in single-ended designs. My Audio Note Meishu exhibits the same characteristics. Another thing to know is that the loudspeaker binding posts are tapered and can shear off bare wire loudspeaker cables. In addition, these posts are knurled and need to be hand tightened; you cannot use the six-sided Postman or Audioquest hand wrenches. Two of these posts also became loose and would revolve as I tightened the loudspeaker cable on the post. These are very easily tightened up however. Perhaps a better solution here would be to use the high quality Cardas binding posts that sound excellent.

Conclusion

The bottom line is that this is one fantastic amplifier. If you have reasonably efficient loudspeakers and like the finesse and texture that single-ended amplification provides, but hate the lack of drive and energy that most single-ended amplifiers produce, you're in luck. This could very well be your amplifier. The Diavolo combines the strengths of single-ended with the ability to drive most loudspeakers as well as the ability to give you the drive and "energy" of the music. It gives you, in one amplifier, both the detail and resolution as well as the warmth, texture, and emotion of the music. In short, it combines the resolution with musicality. Well done Art Audio.

Specifications

Type: Transformer-coupled, zero feedback pure "Class A" triode stereo amplifier

Output Power: 13 watts per channel, two channels

Input Sensitivity: 350mV

Input Impedance: 380 Kohm

Output Impedance: 6 ohm (special custom impedances from 2 to 8 ohm available at no charge)

Frequency Response: 9Hz to 60kHz

Tube Complement Per Channel: 6DJ8, 12BH7, CV 378, EAT 32 or KR VV32B power tubes

Dimensions 18.25 x 13.75 x 10 (WxDxH in inches)

Weight 60 lbs.

Price: $6,600

Company Information

Art Audio
34 Briarwood Road,
Cranston, Rhode Island 02920

Voice: (401) 826-8286
Website: www.artaudio.com