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The Amplifier/Speaker Interface
Tom believes that today's HiFi design culture has been looking at things "the wrong way 'round. First, HiFi requires a source such as vinyl, CD, radio or tape. Then we need a whole heap of voltage & current gain to drive a loudspeaker or two. After the signal leaves the source, the major factor determining a HiFi's ultimate sound quality is the amplifier & speaker combination being used. Some might say, 'Aha! But don't all amplifiers sound the same? It's the speaker that sounds different. You know: The boom & tizz, the sibilance & bass, man!'..."


Tom stresses that it's the interaction between amplifier & speaker that determines the sound as a whole. You have to engineer with that in mind to extend the edge of the art in amplifier design. "There are more permutations of amplifier & speaker combinations than you can count. To get an optimum match between most amplifiers and speakers, you have less chance of success than winning the lottery. One of our design goals had to be that the Linear A would work equally well in all those interaction situations to ensure consistently good sound for all of our customers. Keep in mind that the laws of physics will apply to anything electrical and mechanical. We're faced with designing for an infinite variable of customers with different room sizes, aesthetic tastes, budget, differing equipment and the often-overlooked fact of the frailty of human hearing slowly fading from constant use. The end result is that the same system in different rooms and with different ears will sound different for better or worse."


Tom uses the following analogy to illuminate loudspeaker/amplifier interactions: "The problem that most speakers pose for an amplifier is that from an electrical standpoint, they are moving targets with respect to impedance versus frequency. Imagine you're sitting at a piano. The furthest key to your left plays a 27Hz note, the furthest key to your right a 7000Hz note. Loudspeakers try to mimic the sound of those two notes from a recording yet each note presents a different electrical load to the amplifier due to the way loudspeakers are constructed. In the same way, a 40-watt light bulb presents a different load to the mains than would a 100-watt bulb. Just as one light bulb emits more light, one note is easier to drive from an amplifier's point of view."

"It's a widely held belief in the audio community that a high damping factor is all that's required to drive any speaker impedance curve. Actually, the truth is that the damping factor merely stops the speaker from ringing or bouncing like a spring once the music stops. As has been shown by Gilbert Briggs, founder of Wharfedale, the amplifier output impedance only needs to be half the speaker's impedance to have this effect. I can hear you laughing and thinking we've lost our marbles - but how come all amplifier manufacturers quote different power outputs into 4/8/16 ohms when the magic low output impedance means it allegedly produces the same power irrespective of load? Worse even, transformer-coupled tube amplifiers use multiple impedance taps. Look at any loudspeaker. Very few are ruler flat over their frequency range and most look like the side view of a roller coaster fairground ride. A 32-ohm peak at bass resonance and crossover point requires double the voltage of an 8-ohm load as does a 16-ohm peak against a nominal 4-ohm load. Remember that amplifiers clip when supplying higher voltages and that all amplifiers have a falling power output into rising impedance. As a result, the driver has a frequency response rolloff as the impedance rises.


The rising impedance with frequency, as seen by the amplifier, produces a benign inductive load. However, the corresponding fall with frequency is capacitive and will cause the amplifier output devices to overheat. Because the Linear A uses an output transformer and operates in Class A, it is immune to any of those ills usually associated with amplifiers. Since the valves don't see the loudspeaker (only the transformer), and because they are on all the time, they don't draw more current at higher power. However, the major advantage of the Linear A is that we have matched the transformer and composite valve to produce over 20 watts between 5-10 ohms. The speaker output remains the same when its impedance curve fluctuates anywhere between 5 and 10 ohms. The fall in power above 10 ohms is offset by the improved output transformer efficiency. Since transformer losses are "current squared x winding resistance", the current requirement falls off with rising impedance. In sonic terms, it means your loudspeaker presents a less peaky sound output if you are using a Linear A."


The Chassis
The appearance of the Linear A's chassis has been somewhat controversial among the visitors I've had to my listening room. As pal Pete put it: "It would probably be considered more attractive had not the toaster oven been invented first." I was teasing Ed Sheftel about the toaster oven appearance after I had uncrated the amp. Ed quipped back that after I got a chance to listen to it, its appearance would transform and I would begin to think of it as beautiful. It's true; I finally did get used to the appearance and didn't think about it any further.

Tom explained that he tried to make the Linear A as unobtrusive as visually possible. While a Class A amplifier is going to be sizeable, the Linear A matches the industry standard shelf width. Tom believes most Linear As will spend their lives tucked invisibly out of the way on a bottom shelf so appearance shouldn't be a big issue. For aesthetic sissies, the Linear A is offered in a Special Edition (upgraded cosmetics, identical circuit - see left).
However, to focus on appearance is to miss the point completely as I now sheepishly acknowledge. Tom designed the chassis very deliberately to perform important functions: to keep signal paths short and help isolate the circuitry from vibration. The transformers are mounted directly below the circuitry to provide the shortest possible signal path. The circuit boards are raised for decoupling. The chassis is designed to isolate active components by floating them in space on a centered plane that is mechanically decoupled from the rest of the chassis. "One of my main design goals concerning the chassis was to reduce the unwanted electromagnetic fields generated by putting a large mains transformer in a metal box. This can effect the movement of electrons through circuits creating a negative audible effect."

Listening Impressions
Before we cover the results of my listening sessions, let me say a word about cables: The Linear A must be used with shielded interconnects from the preamplifier. When I tried unshielded interconnects, the Linear A went completely berserk, screeching like a banshee at the top of its lungs and scaring the living daylights out of me. I found Nirvana S-L shielded interconnects to work well between the critical preamplifier and amplifier interface. I used unshielded 47 Labs Cable Kit interconnects for all other duties to excellent effect. Speaker cables were either Keith Aschenbrenner's Auditorium 23 cables or the 47 Labs Cable Kit cables.


Given that the amplifier is rated at 25 watts per channel, it's the most powerful amplifier I've had on my 103dB sensitive Avantgarde Duos in a good long time. On the Duos, it sounds like a muscle amp. It brings them alive in ways nothing else has yet. I suspect that the Linear A's extra power would also work great with excellent less-efficient speakers such as Wilson Watt-Puppies or Quads, for example.


I listened to vinyl through my idler-wheel drive Garrard 301 Project turntable equipped with a Cain & Cain plinth, the new $1100 47 Laboratory 4723 MC Bee phono cartridge and the Origin Live Silver tonearm. Phono amplification during the Linear A listening sessions was either the Tom Evans Groove Plus or the combination of Fi Yph phono stage (I bought it after the review so yeah, I really like it) and a stunning hand-made moving coil step-up transformer using vintage UTC microphone transformers by NASA engineer Tom Miccolis. For digital listening, I used my customary Meridian 501.20 & Audio Logic 2400 valve DAC combination.
Overall Impressions of the Linear A
After I powered up the Linear A, I noticed how quiet it was on my highly sensitive Avantgarde Duos. Even with my ear inches away from the drivers, I could only make out the faintest amount of amplifier noise. I listened to a lot of music over the course of the review, both digital and vinyl but mostly vinyl. It's hard to resist the superior resolution and musicality of vinyl and becomes almost mandatory as the primary medium when evaluating a cutting-edge amplifier like the Linear A. I've been on a buying spree lately of the excellent Classic Records remasters and enjoy Neil Young's Greatest Hits; Graham Nash's Songs for Beginners; Crosby, Stills, Nash
& Young's Déjà Vu; Carole King's Tapestry; the Mercury Living Presence remaster of Balalaika Favorites; the Living Stereo remasters of Clair de Lune, Rachmaninoff Rhapsody on a Theme of Paganini, and the Royal Ballet Gala Performances; Lou Donaldson's Lou Takes Off; the 45rpm singles cuts of "Autumn Leaves" and Alison's "Uncle" off Cannonball Adderley's Something Else; and the 20th Anniversary Commemorative 12" single of Bill Henderson's "Live at the Times".
I also spun a few of my favorite Acoustic Sounds test pressings like Count Basie's Farmers Market Barbeque, Curtis Counce's You Get More Bounce With Curtis Counce, Doc & Merle Watson's Pickin' the Blues, Cookin' with the Miles Davis Quintet, Bill Evans Trio Waltz for Debby, Gene Ammons' Soulful Moods of Gene Ammons, Chet Baker's Chet and the Tony Bennett & Bill Evans album.


I knew that Tom Evans was a big Rock & Roll fan so one of the first albums was Englishman Graham Nash's 1971 Songs for Beginners remastered by Classic Records on 200-gram Quiex vinyl. This is a terrific-sounding rock album of great music by great musicians: Graham Nash on vocals & acoustic guitar, Rita Coolidge on background vocals, Jerry Garcia on piano and steel guitar, David Crosby on electric guitar - to name a few.


On the opener "Military Madness', the first thing I noticed was how well the pulsing of the beat resonated through the music, making me want to tap my foot to keep time. In the well-established British tradition, the Linear A gets the beat right, vitally important for bringing Rock & Roll to life. The dynamic rise and fall of strong and weak beats of the drums gives the rhythm life and coupled with the bass drive, evokes a strong sense of exhilaration while listening to the music. The percussion has plenty of startle factor due to the excellent dynamic abilities of the Linear A. Kick drum gets a big lively feel with a tacit sense of air pulsing around the drum head. Speaking of air, you can hear the interaction of vocals and instruments with the air volume of the recording venue in a more tangible manner than I've heard before. The vocals are natural and the harmonies grand. I couldn't help but reflect on how appropriate the song's social commentary was for our own time with its message of tolerance, nonviolence, the value of relationships and helping the poor. You really get a sense of the emotive impact from the music through the Linear A. It makes the music sonically beautiful, emotionally hard-hitting and philosophically thought provoking.
In the song "On Better Days", there is an impressive and startling crack of the drumsticks with a rim shot off the snare drum. You get a definite sense of the air swooshing out of the hi-hat, something I've never noticed before on this album prior to the Linear A. On "Wounded
Bird", the acoustic guitar sounds great. You not only get a sense of the individual strings but also of the body of the guitar and the airspace within it, making the acoustic guitar sound very natural. Even with solo acoustic guitar, you can really feel the sense of rhythmic flow the Linear A provides.


What did I learn while listening to Songs for Beginners? First off, this Tom Evans amp is a great rocker due to the way it handles the beat to bring the rhythm alive. You can tell that Tom's been enjoying listening to Rock. There's something else rockers will appreciate: the Linear A will play louder without your ears shutting down than any amplifier I am familiar with. At levels where I'd normally be reaching for the volume control, I could listen along with no sense of stress or discomfort. How the Linear A accomplishes this little feat I have no idea (perhaps ultra-low distortion and absence of compression?), but it's very apparent and pleasurable. The Linear A is also very detailed and articulate so you get a real sense of hearing all the detail on a recording. It does not sound etched or analytical at all, making detail recovery and unraveling of fine nuances even more amazing. Acoustic instruments sound completely natural and organic as do vocals.


The Linear A's fine resolution of micro dynamics combined with its exceptional ability to reveal detail give a very real sense of texture and feel to the music. Articulation at lower frequencies is phenomenal. I've never heard any other amplifier in my listening room come close. The Linear A also gives an enormous sense of recorded space that infuses the listening room with the air of the recording venue. There is a strong display of SET-like magic with well-defined and illuminated images. The soundstage offers a good separation of images left to right & back to front. Image sizes are life size and have a lot of presence. It's easy to hear all the different instruments contribute to the musical whole and it's easy to pick out a particular instrument and follow it through the mix. The way the Linear A handles percussion is phenomenal.


Okay, so the Linear A rocks to beat of the band. But can it play something more complex and less beat-oriented like classical music? To find out, I pulled out Classic Records' remaster of the Mercury Living Presence Balalaika Favorites [SR90310] and cued it up on the big Garrard. Right off the bat, I was impressed with the amp's ability to both sound slightly warm & dark while being able to retrieve more information off a record than any amplifier in my experience – an intoxicating combination. The level of transparency and detail recovery coupled with the natural tonality of the Linear A made for a captivating listening session. I could hear every little thing. The individual strings of domras, balalaikas and goosli were resolved with great vividness and articulation. I also got a realistic sense of the sharpness of attack on the individual strings and their decay. That I could hear this contribution both for individual strings as well as the strings of an instrument or orchestra as a whole was really quite remarkable.

The perspective of the Linear A puts you up close to the musicians. With Balalaika, I felt like I was in the first row of the hall, so immediate and live was the sound. Soundstaging too was superb. In width, the soundstage extended beyond the outer edges of my speakers while depth wise, I could hear very very deep into the stage. The Linear A even produced the height dimension of the soundstage very convincingly to render life-sized instruments in a big domed space.


Where the Linear A really excels beyond its peers is in its ability to recreate the sound space around the players and how that relates to the sense of the hall ambiance itself. You can hear reverberation and decay of the instruments to an extraordinary degree. Dynamic contrasts are another strength. I could easily track all the dynamic envelopes of individual instruments and enjoy the dynamics of the softest and loudest sounds simultaneously with nothing lost. None of the fine details were smeared, covered up or crowded out. Imaging as well was extraordinary. It wasn't simply in the relative nearfield of the soundstage that instruments were easy to pinpoint. I could readily identify the exact location of an instrument deep in the soundstage with rather remarkable palpability and focus.


So it's clear that the Linear A is a remarkable sounding amplifier. However, what makes it such a great amplifier is that it not only sounds better than any amplifier in my experience but also plays music more convincingly. The Linear A reveals the full drama of every cut on Balalaika by portraying the atmosphere, attitude and feeling of the musical performance. As good as the sound was, I never once forgot that my focus was on listening to great music. No amplifier in my experience recreates the feeling of a live performance like the Linear A does. It never failed to draw me into the music.


That's pretty much how it went with every album I played. Whether rock, classical, jazz or bluegrass, the Linear A proved superior in every respect and by a rather large margin to any other amplifier I have experienced in my system. It excels at audiophile sonic trickery while never letting it get in the way of the music. It opens a clearer & more detailed window onto the music than anything in my experience, making everything else sound a little veiled and opaque by comparison. The wonder of all this is that it can do that while still sounding warm and natural.
Summing up the Linear A
I have to congratulate Tom Evans on what is truly a remarkable achievement in amplifier design. This chap is one of the true innovators of modern audio and marches to the beat of his own drum. His Linear A sounds better and plays music better than any SET amplifier in my experience - by a substantial margin. Coming from an SET devotee like me, I hope you realize the high level of praise that represents.
The design of his Linear A is much more extraordinary than its plain appearance would suggest, hiding as it does the composite 10 design with eight EL84s and op-amp drivers.
EL84s are cheap and easy to come by, quite unlike some of the exotic direct-heated triodes. And unlike many single-tube triode amps, the Linear A is powerful enough to drive medium-efficient real-world loudspeakers.


How about downsides? The looks here aren't likely to incite passion in too many folks and shielded interconnects become a must. Really, the only major downside of this amplifier is that at $8500, I can't afford one. I thought about loans, selling one of my guitars, living without any creature comforts, munching on oatmeal and other crazy ideas. I finally came to my senses but it was a struggle. If you can afford a Linear A and have reasonably sympathetic speakers to match its power, I cannot imagine you being anything other than deliriously happy.


The Linear A is simply the finest amplifier I have encountered in all my years in audio but that's just the start of this story. There's a lot more to talk about when it comes to Tom Evans' innovative products. Watch for upcoming reviews of the Vibe preamplifier with the optional Pulse power supply and the Groove Plus phono stage.
Manufacturer's reply
Tom and I would like to overwhelmingly thank Jeff Day for his deeply researched, thoughtfully constructed and highly insightful review of the Tom Evans Linear A Stereo Amplifier. Jeff articulately captures Tom's creative goals regarding the development of the Linear A. This review is a "work of art."

Thanks.
Ed Sheftel, on behalf of Tom Evans Audio Design

PS. Of course, none of this could have been conceived without the gracious approval of Srajan for allowing us to submit our Linear A, Vibe, Pulse and Groove+ for review. Thanks for your kind consideration on this matter. It is greatly appreciated.
Manufacturer's website
US distributor's website