February 1999Art Audio Diavolo Amplifier
by Ian White
What is it about single-ended amplification that causes audiophiles to drool? I have always been somewhat baffled by all of the hoopla surrounding the "single-ended" issue, and it was with great interest that I took on the review of the Art Audio Diavolo. Other than some time spent listening to the Wavelength Cardinals at Jack Goodmans, I mean Steve Rochlins home in New Hampshire, and a two-hour session with an Audio Note Kit 1 and a pair of Lowther speakers about two years ago, my experience with single-ended designs has been fairly limited. While I prefer the sound of tubes to solid-state devices, I refuse to throw myself into the ludicrous "push-pull/single-ended" debate. I see no point to it. Does it really matter to you the reader if the amp that makes the most beautiful music in your home uses tubes or transistors? I believe that this exercise is about finding the amplifier that sounds the best to your ears in the context of your system. Some of you will choose solid state and others will choose tubes. Its the end result that matters.
On with the show
Art Audio (UK) has been manufacturing tube amplifiers for close to a decade, and designer Tom Willis has introduced a real thoroughbred with the 13Wpc Diavolo (which means devil in Italian). The Diavolo is a dual-mono, pure class-A transformer-coupled, single-ended, zero-feedback design whose overall level of finish should be a point of reference for many in the audio industry. This 60-pound monster has a polished, non-magnetic, stainless-steel chassis and gold-plated transformer caps. It is a most strikingly beautiful piece of industrial design. You can order the Diavolo with an integral volume control too, which adds $200 to the cost.
The Diavolo, 18 1/2"W x 14"D x 10"H, is a lot heavier than it looks, and I must remind you to bend those knees before lifting it. Power output is rated at 13Wpc (at 1% distortion). The auto-biasing circuitry (one per channel) is set to 110ma. All eight of the tube sockets are ceramic and have silver-plated pins. Art Audio uses low ESR/ESL Aerovox capacitors in the power supply, along with smaller-value Rubycon and Phillips capacitors. German-made Wima metalized polypropylene capacitors are used mainly in the signal path. Art Audio also uses mil-spec, thick-layered, two-ounce copper-trace circuit boards, and all resistors are 1% low-noise metal oxide. The output transformers are a modified split-core design. According to Art Audio, these make the Diavolo a truly full-range design, with deep bass, extended treble, and realistic dynamics. Gold-plated RCA jacks, five-way nylon-capped binding posts, and very high-quality heat sinks round out the Diavolos parts list.
The Diavolos 470k input impedance and 350mV sensitivity make it a fairly easy amplifier to drive. My Copland CTA-301 Mk II preamplifier reached rather loud volume levels using only the beginning third of its volume control. The Diavolo will not run out of steam unless you push it to really dangerous listening levels. The output impedance is 6 ohms (which drops to 4 and extends to 8 ohms). The quoted frequency response of the amp (full rated output) is 20Hz20 kHz, -.5dB.
It's the tubes, stupid!
To understand the sonic signature of the Art Audio Diavolo, one really has to pay some attention to the tubes used with it. Art Audio designed the Diavolo around the KR VV32B triode because of its unusually high amperage capacity (3 amps). The VV32B's sound is audibly quite different than that of the Western Electric 300B, KR 300BXLS, and other 300B variants that I have listened to. The VV32B is not an overly warm-sounding tube in the midrange, and it does not sound super sweet in the treble either. This tube sounds very clean (without being too analytical) in the midrange, but it does suffer from a certain degree of brightness in the treble, which would most certainly explain the mildly aggressive nature of the amp. The brightness in the treble was more apparent as I changed from one brand of cable to another (Cardas, van den Hul, Nirvana). Do not use unshielded cable with this amplifier. I started my listening sessions with van den Hul cable, and the unshielded First interconnect was a sonic disaster with this amplifier (way too hot on top). The Cardas and Nirvana were dramatically smoother-sounding.
If the VV32B's aggressive treble was somewhat of a detraction from the overall sound of the tube, its bass performance was absolutely KILLER! The VV32B stomps all over the 300B tubes when it comes to the midbass and below. Its definition, articulation, and slam factor are in a totally different class. The 300Bs that I have listened to lack authority. The KR VV32B is a really good-sounding triode, but you need to pay close attention when selecting your cables.
While I was able to forgive the VV32B's mildly aggressive treble due to otherwise stupendous sound, I have a problem of a different nature to report. This tube rings like crazy, even at moderate volume levels. While I certainly enjoyed my time spent with the amplifier, I was really annoyed by the ringing of the output tubes. At first, I wondered if I was driving the amplifier too hard, but when I listened to music at really quiet levels, I could still hear the ringing. I placed a call to Joe Fratus (Art Audio USA) and detailed my problem with the tube. KR manufactures three different versions of the VV32B tube (cylinder blue glass tube, blue glass tube, glass tube that looks like a 300B), and my review sample of the Diavolo came with the cylinder blue glass tube. Joe sent me a replacement set of 300B-style 32Bs, and I sent the other set of tubes packing. The new tubes were dramatically quieter, but I did still hear some ringing at loud volume levels. I suspected that if I better isolated the amplifier, I might alleviate some of the ringing. BINGO! I placed three Black Diamond Racing Cones underneath the amp and also stacked three #4 Vibrapods under each of the tubes on the underside of the chassis. The ringing didn't completely vanish, but the difference was quite audible. Another contributing factor to the ringing has to be the fact that the tubes are naked. Art Audio does offer a specially made tube cover for an additional $300, but I consider this unacceptable. The KR tubes are not inexpensive (not to mention the amplifiers cost), and I can't help but wonder if this great amplifier would sound almost unbeatable if the tubes were quieter. Happily, Art Audio has just begun supplying the Diavolo with a new iteration of the VV32B tube that they claim alleviates the problem with ringing.
In addition to the power tubes, Mullard CV-378s are used for rectification, NOS Amperex 6922s for gain, and EI Yugo 12BH7s for additional gain and cathode follower. These tubes were dead silent throughout the review process.
The White House equipment room, pre-impeachment
I listened to the Art Audio Diavolo in systems using the Copland CTA-301 Mk II preamplifier, EAR 834P phono stage, and Copland CTA-501 integrated amplifier. For comparison purposes, I used the KR Enterprise 18 BM single-ended power amp, OCM 200 power amp, Copland CTA-505 power amp, and Primare 30.1 integrated amplifier. Digital sources included the Copland CDA-288 CD player, Rega Planet CD player, and Panasonic DVD-A310 DVD/CD player. My analog front-end was the always-reliable Audiomeca Romance/Wilson Benesch ACT .5 tonearm/Benz-Micro Glider cartridge combination. The Diavolo drove the Meadowlark Kestrel and Hot Rod Shearwater loudspeakers. My Martin-Logan Aerius sat silent in a corner during the review. Cables included Cardas Golden Cross and Neutral Reference; van den Hul The First, Second, Revolution; and Nirvana S-L interconnect and speaker cables. Everything except for the power amps was plugged into the Chang 6400ISO and 3200ISO line filters. Each power amp was connected to a Foundation Research LC1 power-line conditioner. Black Diamond Racing Cones and Vibrapods supported all of the equipment on my Design Progression and DIY equipment racks. Homemade acoustic treatment performed miracles in my listening rooms. Hallelujah!
The devil made me do it
Andrew Manzes recording of Tartini The Devils Sonata [Harmonia Mundi 907213] seemed like the appropriate recording to start off with, and I was most interested in hearing how this wonderfully odd, baroque piece would sound. Manze is an extremely aggressive violinist, and I loved how the Diavolo communicated this. The Diavolo does not hold back when it comes to dynamic contrasts in the music. I found myself deeply engrossed in this recording because of the Diavolos transparency and detail in the midrange. The Diavolos treble balance errs somewhat on the bright side of perfect, which enhances immediacy, but not enough to skew the sound of this recording.
The forward nature of the treble did, however, affect the soundstage depth of the recording. There was a reduction in soundstage depth on this recording and a few other classical recordings that I listened to. Is this a major shortcoming of the amplifier? I dont think so. The reduction of soundstage depth was more than made up for by the detailed, engrossing presentation, and only an anal-retentive soundstage freak would really be bothered by this.
Los Lobos The Neighborhood [Slash/Warner CD 26131] is in my opinion the real sleeper from this East L.A. band, and the Diavolo was wonderful at reproducing David Hidalgos voice. I have only heard one other power amp make his voice sound this good (for me to know and you to lose sleep over), and it costs a lot more than $6000. The Diavolo and the Meadowlark speakers are a terrific combination, and I found the transparency of the sound and slam factor far superior to that of my Martin-Logan Aerius speakers. The Diavolo has a lot of low-end authority for a 13-watt design, and I was really impressed by its ability to maintain a stable soundstage during really dynamic passages. The sound maintained its focus at very loud levels.
The Diavolo gets top marks for being adept with different types of music, and I think that important factor separates it from the other single-ended designs that I have listened to and the push-pull tube designs that I had at home with me. The KR 18 BM is a very good power amp, but the Diavolos low-end performance on recordings like Green Days nimrod [Reprise CDW 46794] and Metallicas And Justice For All [Elektra CD 60812] was downright spooky. Bass drum had real knock-me-down visceral impact, and I was listening to the smaller Meadowlark Kestrels at the time. The push-pull Copland power amps, while having lovely and warm midrange performance, sounded wimpy, mushy, and lightweight in comparison. The opening to the song "One" from And Justice For All was a wonderful demonstration of how a great power amp handles the attack/decay of musical notes and tremendous dynamic shifts that exist in music. When an amplifier makes you put your CD player on repeat just so that you can hear the same song over and over again, its doing something extraordinarily well.
As good as the Diavolo sounds with CD players like the Rega Planet and Copland CDA-288 (it liked the gentle-sounding Rega more), it is even better with vinyl. I cued up my copy of Bruce Springsteens The Ghost of Tom Joad [Columbia C67484] and loved how natural his voice sounded. The opening track begins with a really sharp-sounding harmonica and on really bright-sounding equipment, your ears could melt. I was worried about how the Diavolo would cope, but it came through better than any of the solid-state amps (OCM 200, Primare 30.1). The KR 18 BM sounded much smoother than the Diavolo on this recording, but once again it lacked the Diavolos ability to handle dramatic dynamic shifts in the music at loud levels.
In comparison, the KR Enterprise 18 BM (with its KR 300BXLS output tubes) sounded warmer in the midrange, smoother in the treble, but lacked the emotional impact of the Diavolo. I really liked the KR 18 BM with many classical recordings, but it did not reproduce music with the same degree of scale as the Diavolo. The difference between the two really wasnt that dramatic in relative terms, but it was audible. Both single-ended designs were MILES ahead of the OCM, Copland, and Primare power amps in this regard.
Bottom of the ninth, two out, runner on first, down by a run, ace reliever with nasty splitter coming in to pitch...
Drink Smalls Electric Blues Doctor Live! [Mapleshade 01832] is a major workout for any system. It is a killer performance by the Carolina bluesman and the best live recording of an electric guitar that I have ever heard. The Diavolo hit one out of the park on this recording, and I mean way up there, over the Citgo sign at Fenway Park. The Diavolos speed and midrange clarity really showed up the competition on "Stormy Monday Blues," and it was really hair-on-the-back-of-your-neck-raising sound. The KR 18 reproduced a similar experience, but the other power amps were dramatically inferior. The Diavolo just brings a greater degree of immediacy and emotion to the music, not to mention a better sense of pace and rhythm.
Running with the devil
The Art Audio Diavolo is a very different kind of single-ended animal. Its bass is powerful, defined, dynamic and does not take a back seat to that of any of the push-pull power amps that I compared it to. The Diavolo leans towards accuracy and does sound somewhat forward because of some brightness in the treble, but it is still a wonderfully engaging-sounding power amp that drove the 88dB Meadowlark Hot Rod Shearwaters to ungodly levels without breaking a sweat. It is a single-ended design for real world speakers.
However, the Diavolo is not perfect. The supplied KR VV32B tubes rang like crazy, and I have no doubt in my mind that the ringing affected the sound in a negative manner. The Diavolo needs very high-quality shielded cables (Cardas and Nirvana get a major thumbs-up) and a good degree of isolation to alleviate the tube noise, but nothing in my experience will eradicate it completely.
The Diavolo is built like a tank and looks like a million bucks. At $5995, it is a very expensive piece of equipment, but the Diavolo sounded great the first time I powered it up, and even better the last time, before I begrudgingly packed it up and sent it back to Rhode Island. Its a unique product and well worth listening to.
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