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October 2023

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Art Audio Opus 4 And Carissa Vacuum Tube Amplifier Review
Two amps from Art Audio and two pairs of speakers from Quad determine ideal combinations.
Review By Bob Grossman


Art Audio Opus 4 Monoblock And Carissa Stereo Vacuum Tube Amplifier Review


  My musical listening times have been quite active and busy over the past couple of months as I have mixed and matched various combinations of two vintage Quad speaker models along with two new beautiful amplifiers from Art Audio based in Nottingham, U.K. I have been on a year-long listening and learning pilgrimage revisiting memories and prior experiences from hearing Quads decades ago.

I acquired restored Quad 57 and 63 speakers from the renowned specialist Sheldon Stokes of SDS Labs in New Hampshire, USA as the starting point for understanding and hearing both electrostatic models designed by Peter Walker. The focus of this review project is the Art Audio Carissa stereo amplifier and the Art Audio Opus 4 monoblock amplifier pair. Both amplifiers were used in rotation for both pairs of speakers.

This variable quartet of four moving equipment choices has presented musical lines in a complex counterpoint of developments, harmonies, and combinations equal to any Johann Sebastion Bach prelude and fugue. I had other thoughts at times while I listened to Renaissance music by Gesualdo and Monteverdi. One of my favorite composers Monteverdi, composed music through complex counterpoint and adventurous new harmonies that combined sounds very expressively to establish a visionary innovative style.

Here is an excerpt of thoughts written by John Rahn (2000) in Music Inside Out: Going Too Far in Musical Essays. The term "counterpoint" has been used to designate a voice or even an entire composition. Counterpoint focuses on melodic interaction only secondarily on the harmonies produced by that interaction. In the words of John Rahn: "It is hard to write a beautiful song. It is harder to write several individually beautiful songs that, when sung simultaneously, sound like a more beautiful polyphonic whole. The internal structures that create each of the voices separately must contribute to the emergent structure of the polyphony, which in turn must reinforce and comment on the structures of the individual voices. The way that is accomplished in detail is... 'counterpoint'.



Drifting Back in Time
The Renaissance period composers wrote thoughtful and insightful compositions created 500 years ago which offered new innovations that executed complex counterpoint, daring harmonies, and dissonance for that era which are still completely expressive and emotionally moving music to hear even in our modern era way of listening to music. These days I must sometimes pause and imagine how this music would have impacted and been felt by people back in the old days. It is an unfamiliar perspective for our contemporary knowing to be able to hear so many music compositions and styles spanning centuries.

Today, the thoughts and challenges of understanding harmony and having to compose four-part arrangements when I was a music major in college seemed like a good allegory to my review project of mixing and matching two speakers and two currently made amplifiers that reflect historic amplifier design ideas and valves (or tubes) that are now updated and influenced with new parts and materials of today.

Think about how much music has changed and evolved over the centuries. We also now can hear the past 100 years of recorded musical artists and composers leading and performing their works through the convenience of sitting down with our music systems and picking out anything of interest to hear. I have seen many new equipment developments and introductions, yet many people still enjoy well-maintained and refurbished vintage equipment.

This had me thinking about the innovative genius of Peter Walker who designed and introduced two industry breakthroughs with different-looking and sounding speakers upon making the Quad model 57 and 63 loudspeakers decades apart in the 1950s and 1980s. I marveled at the speakers originally made long ago that have the ability to sound so elegant today with performance qualities that have a vital musical expression that does well in comparison to speakers being sold today.

I wanted to experience and hear new contemporary amplifiers that could bring out some of the best sounds from these classic British-made speakers. My thoughts turned to looking at the products made in England by Tom Wilkins, proprietor, and designer of Art Audio. I reached out for assistance to the USA rep and distributor, Scott Bierfeldt of Verdant Audio. With Scott living in New Jersey, it was a convenient drive for us to get together. Scott has been a helpful resource for explanations on the amps and a gracious provider of the Opus 4 monoblock demo. Although I purchased the Carissa stereo amp last year, I wanted to provide review coverage for the updated model.



Quad 57 And Quad 63 Loudspeakers
The Quad 57 speakers were my first purchase and introduction into the system following a seven-year use of Magnepan 20.7 speakers. I have been using the Art Audio Carissa stereo amplifier for the past year with restored Quad 57s speakers. Although a single-ended triode (SET) is not a typically recommended type of amplifier design for the Quad 57s, the Art Audio Carissa amplifier can produce 18 Watts of triode Class A power along with lots of current reserves from a pair of 845 tubes. The design concept is based upon a premise of not pushing the 845 power tubes too hard so their contribution to the sound is a sweet zone along with allowing the tubes to have a longer life span.

Art Audio's Carissa vacuum tube stereo amplifier was a lovely match for the Quad 57 speakers, which require a modest level of power. Art Audio's Carissa is also lustfully gorgeous decked out with a polished chrome chassis, crystal flute tube chimneys, and glowing blue lights. This is about as close to owning a Bentley as I will get since the amp chrome chassis is polished at the same shop the famous car maker entrusts for their metal work.

The next phase of the review began with rotating the Quad 57s out to bring in the Quad 63s. Initially, I continued to use the Carissa stereo amplifier for the 63s before eventually substituting the Art Audio Opus 4 pair of monoblocks. The Opus 4 is beautifully designed and built to a high standard of fit and finish. Their narrow footprint allowed the pair of amps to sit adjacent to each other on the same Mapleshade wood platform as the Carissa amp. Another benefit of the Opus 4 smallish chassis design is to enable the use of numerous amps when driving systems for multiple video needs that could use a separate small-sized amp for each channel.

There was a sense of extra control and projection of the sound into the room from the Quad 63 speakers with the greater 50 Watts of triode power from the quartet of KT120 tubes push/pull circuitry design used in each monoblock. The Opus 4 monoblock amps normally are supplied from the factory with KT88 tubes. I have had other amps that use the traditional KT88 power tube and would have liked to have been able to hear the Art Audio amp with some of the various makes that are available for the KT88 compared to the Tung-Sol KT120. I would think some fine-tuning and excellent tube rolling possibilities would be a welcome addition for a prospective owner of these fine amps although the KT120 tubes have a reputation for strength, clarity, and clear-sounding power.

The 63s had an excellent and exciting open, transparent feeling with the Opus 4 amps. In comparison to the Carissa stereo amplifier, the combination of the Quad 63 speakers with the Opus 4 monoblocks had quicker articulation and dynamics, faster and crisper transients, more openness with deeper layers in the soundscape, better control of the bass range, and an overall engaging and faster-paced sound. The Carissa brought a fuller warmth and smoothness to the presentation.

The Art Audio Carissa vacuum tube stereo amplifier was a pleasure to hear with chamber music and vocals, piano, and smaller ensembles that excelled with intimacy and musical colors. As I conjure up comparisons and memories of so many music recordings I heard, the 63s seemed closer in temperament and even closer to the Quad 57s with the SET amp. I unquestionably appreciated and enjoyed Carissa's 18 Watts of SET beauty and cohesiveness with the 63 speaker combination until I decided to push and challenge the amp with the realm of several big Philadelphia Orchestra performances where I had been at the recording sessions.

While the Carissa Amps could bring the Quad 63 speakers to sound levels of over 90dB, they were not opening and going further toward the dynamic impact impression of a live orchestra. Shostakovich Symphony 4 was a bit too much to ask them to handle with aplomb. The lower-end bass drum hits and timpani rolls became more rounded and laid back. It was still a nice presentation and not at all distorted, yet Art Audio's Carissa was more comfortable performing within the impressionistic realms of Ravel and Debussy symphonic music or portraying three-dimensional presentations of chamber music. The Quad 63s and Carissa stereo amp had me feeling like I was in the back row of a hall while the Opus 4 amps moved my listening location impressions to be closer to the stage and performers.

Thanks to a recommendation from my friend Kavi Alexander of Water Lily Acoustics, I have been listening to one of the earliest and best digital-era vinyl LPs produced by EMI. Andre Previn recorded Debussy's Images For Orchestra [Angel DS-37674]. The famous engineer Suvi Raj Grubb worked on the project in 1979. I also used several other EMI Angel records made with the Philadelphia Orchestra in the 1980s with Riccardo Muti where I was at the recording sessions. It is a family anecdote that my wife's cousin John Kurlander was the recording engineer for many of these recording sessions before he left EMI for an illustrious career in Hollywood studios. Now we occasionally see Cousin John's name on movie credits along with the back of LP covers.



Art Audio's Carissa stereo amplifier was a match made in heaven when paired with the Quad 57s. The 57s have a lighter power load to handle and the Carissa combination on the original ESL (Electro Static Loudspeaker) models was musical magic. The view at night with the glowing 845 tubes is simply breathtaking and mesmerizing. It was easy to be totally absorbed and lost listening way into the wee hours of the night. Art Audio's Carissa musical contributions created incremental excitement, wonderment, and a sense of getting closer to the musical moments from the performers along with their physical locations in the performing venue.

The difference in the musical experience was palpable when listening to jazz vocals from Billie, Ella, Sarah, or classical Schubert lieder with Dietrich Fischer-Dieskau. Kurt Weill sung by either Lotte Lenya, Teresa Stratas or Ute Lemper was a pleasure and enabled an appreciation and understanding of each of their voices and interpretations along with the textures, harmonies, and nuances of orchestrations for the various ensembles accompanying them.

Art Audio's Opus 4 monoblock amplifiers on the Quad 57 ESL brought a different balance of space, details, body, perspective, and sense of their musical portrayal. There was a bit of a more open and wider sense of the sound. The dynamics were noticeably clear and articulate. It was an exceptionally smooth, refined, and elegant presentation that kept the panels well-focused and balanced from bottom to top frequencies. However, I found less foot tapping and pretending to conduct along with the performers with the Opus 4 and 57 combinations. As lovely as the sound of the Opus 4 monoblock amps is, my heart and soul were more inclined to spend time listening to the Carissa amp.



The Sound of an Amp Impacting Speakers
The mixing and matching of the two Art Audio amplifiers through rotating the Quad 57 and 63 speakers created the realization of how much the quality of the speaker's sound is related to the amplifier choice being used within the system. The ageless riddle conundrum of asking which came first, the chicken or egg applies to asking which is more important, to the sound, the amplifier or speaker. The Art Audio amps were not the only possibilities I had when listening to the Quads. I also borrowed an older Rodger Modjeski RM9 amplifier for the 57s and used my large Conrad Johnson Premier 8A amps with the 63s. I was discovering how the same speakers could sound so different with the changes of amps.

The sound of the speakers changed significantly with each of the amplifiers. I found it fascinating to hear the results from the different power ratings and use of different tubes. It is essential to figure out the design and costs for the best amp possible to create and bring out the best sonic attributes of your speakers. While each of these amplifiers was excellently made, I eventually concluded which I liked best for the two types of Quad speakers I own.

I love listening to the Quad 57s. There is a reason they are mentioned on so many lists of the greatest speakers of all time and were considered such a breakthrough speaker design. The 57s sound is so emotionally captivating and engaging. They excel in a realistic and natural portrayal of musical sounds, especially with smaller ensembles of classical and jazz music that I dearly enjoy and play so much of the time. I will be honest, after being a member of the Philadelphia Orchestra for over 40 years and having a steady daily listening experience of live orchestra music, I like to hear other types of music and repertoire when I am relaxing at home. Yet at the same time, I like to challenge myself to also experience and hear just how well speakers and an amplifier work together to recreate the music on recordings, especially when they are performances where I was at the recording sessions.

In the realm of portraying big symphonic music renditions, the Quad 57s and Art Audio Carissa stereo amplifier offered a lovely romantic and glowing rendition of Shostakovich, Mahler, Stravinsky, Tchaikovsky, and other large symphonic composers. Although the true strength and power was a bit polite and laid back. I could certainly enjoy the music even when missing the big impressions that I had for years with the huge Magnepan 20.7 and their need for powerful 250 Watt amps.

The Quad 57s have so much resolution with transparency, clarity, and fullness in their sound with the gorgeous Carissa amplifier that I was always pulled into the musical vista. It is simply a dynamic, opulent, and realistic experience to hear the 57s and Carissa when it came to reproducing smaller jazz ensembles on Blue Note and Verve 50s and 60s era records.

The Art Audio Carissa stereo amplifier is as gorgeous to look at as it is to hear decked out with crystal venting chimneys and blue lights. The 845 vacuum tubes burn with an intense glow that is hypnotizing to see. I started with the stock Psvane 845 tubes. Then switched and preferred the extra touch of depth and musical attributes the premium tubes from Linlai contributed to the sound presentation.

I used Art Audio's Opus 4 monoblock push-pull triode amplifiers for the Quad 57s. These amps came fitted with optional KT120 tubes instead of the standard from the factory KT88s. I have found the Tung-Sol KT120 tubes offer a natural, open, and clean sound that has excellent control and strength. However, I also like the more diverse characteristics the KT88 tubes can offer since they are available from a variety of makers. Unfortunately, I was not able to get my hands on two sets of matched 88 quads to use for the review, and can only imagine the benefits and possibilities of using them with these distinguished amps.

Art Audio's Opus 4 mono amps are also available in an alternative factory set pentode version with a higher output rating. This configuration is recommended for audio-video systems set up with multiple-channel monoblock systems. Although I did not hear the switchable version of the amps, Scott offered his opinion that the factory triode version has the quietest and cleanest sound while the pentode is not quite as musically detailed. The Opus 4 is also available in a switchable model although the introduction of a switch will also introduce a very minor possibility of noise. I was pleased to have the triode version to use in my reference music system.

The Art Audio Quartet vacuum tube monoblock version of the similar-looking Carissa Amplifier was reviewed in November 2019 by Dr. Michael Bump. I suggest looking at that coverage in Enjoy the Music for another fascinating expose of another Art Audio amplifier. The original and earlier version of the Carissa was reviewed in Enjoy the Music.com as far back as November 2005 by Todd Warnke. I took the opportunity to ask the somewhat new US distributor Scott Bierfeldt how the older amp was revised and improved. Scott shared the following ideas regarding the new Copper Reference amplifier to illustrate and explain the changes from the older SE Edition.

1. The layout of the chassis has changed to optimize cooling.

2. Tom went from a single transformer powering the heater and audio circuits to two transformers, one High Tension for the Audio Circuits, and one Low Tension for tube heaters. This aids in cooling and makes the amp quieter.

3. The caps are from Mundorf now rather than Hovland.

4. A soft start circuit has been added. The main switch on the back of the chassis turns on the heater (and lights) if the amp has them, and the soft start switch on the front turns on the audio circuits.

5. All these changes mean the amp is delivering 18 Watts of power rather than 16 Watts and is far quieter than its older counterparts.



Perspectives On A Review Project
This review project was complex, challenging, and varied. I suppose it may have had something to do with the various combinations of two amps and two speakers. John Rahn (2000) used the term counterpoint to describe the interactions of multiple voices to create different combinations of expressive possibilities when combined with the sound of various musical instruments. Two amps and two speakers can provide a variety of combinations and I decided to check on the numerology of 4 and found an interesting answer.

The number 4 is a logical and rational number that brings a sense of security and consistency. It can be your life path, personality, soul, or karmic number, but wherever you find the number 4 in your numerology, it means you have something to learn about hard work, discipline, patience, order, and stability. That is what I learned about the various speaker/amp combinations.

The Quad 57 loudspeakers and Art Audio Opus 4 monoblock vacuum tube amps were always pleasing to hear with a good balance, impact, and refined sound. However, I preferred the more immersive and rounded sound of the Carissa 845 tube-based stereo amp for the Quad 57s. For the efficient rated Quad 57s, there were no issues or concerns with power handling from either amplifier. However, with the Quad 63s I preferred the strength, greater focus, and depth of the sound portrayed by the Opus 4 amps. The combination of the Opus 4 amps and the 63s was a delightful way to hear the other more power-demanding style of Quad speakers. While the Carissa power reserves managed to bring to life the Quad 63s, the Opus 4 combination had deeper, wider, and clearer layers to the musical images and imaginary soundstage.


"Art For The Eyes And Music To The Ears"
The quote in the title for this section is the motto of Tom Willis from Art Audio company. The quality designs, choice of parts, and visions of Tom Willis are readily apparent in the look and sound of both amplifiers. Both amps had lovely features and sound. They worked reliably for hours on end for many all-day and late-evening listening sessions. These amps were not overly colored and tubey in sound. They both simply portrayed and played beautiful music that was never fatiguing or tiresome to hear.

These days, there are many choices for amplifiers and decisions are not easy without personally listening to them in your own home. Overall if I did not already have another push/pull amp available at my home to use with the Quad 63s, I would have considered keeping the lovely Opus 4 monoblock amps. I suggest anybody seeking an amp of this design and style give it an audition in your home. It is quite a remarkable achievement that the 50 Watts triode power from the Opus 4 amps managed to portray so much strength and control for the Quad 63s.

Art Audio's Opus 4 monoblocks managed to convey an impression to me of nearly as much depth of overall musicality as the 275 Watts of ultra-linear power of my other reference system amp, the CJ Premier 8A. I ultimately decided to keep the Carissa amplifier as part of my system to use with the Quad 57s and as an alternative for the 63s when they are not connected to the big CJ amps. The power, control, and sound along with the lavish and lovely looks of this latest revised Art Audio 18 Watt SET amp as designed by Tom Willis is highly recommended to be seen and heard.

Art Audio's Carissa vacuum tube stereo amplifier has character, a distinctive beauty, and elegance that it continues to be part of my reference system. If you are looking for an elegant-looking design and musically refined, and lovely sounding amp in a more powerful rating range, then seek out an audition of the Art Audio Opus 4 monoblocks that have a diminutive look and weight yet produce a prodigious, powerful, and excellent quality output. Bravo to both Tom and Scott for their excellence in design, musical listening, and customer support throughout this evaluation period.




Carissa Opus 4


Sub-bass (10Hz - 60Hz)

Mid-bass (80Hz - 200Hz)

Midrange (200Hz - 3,000Hz)

High Frequencies (3,000Hz On Up)



Inner Resolution

Soundscape Width Front

Soundscape Width Rear
Soundscape Depth Behind Speakers

Soundscape Extension Into Room


Fit And Finish

Self Noise
Emotionally Engaging

Value For The Money




Carissa Stereo Vacuum Tube Amplifier 
Type: Vacuum tube stereo amplifier
Power: 18 Watts per channel
Frequency Response: 20Hz to 60kHz
Input Sensitivity: 750mv
Input Impedance: 180 kOhm
Output Impedance: 4 and 8 Ohm
Biasing: Automatic
Rectifier: Solid-state with two-stage choke filters
Driver Tubes: 6922 and 12BH7
Price: $13,499 for base model and $14,999 as reviewed



Opus 4 Vacuum Tube Monoblock Amplifier
Type: Vacuum tube monoblock power amplifier 
Output Power: 50 Watts
Frequency Response: 5Hz to 65kHz 
Input Sensitivity: 550 mv
Input Impedance: 180 kOhm
Output Impedance: 6 Ohms
Negative Feedback: 6dB
Biasing: Cathode
Rectifier: Solid-state
Driver Tubes: ECC88 and ECC82
Dimensions: 6.25" x 18.5" x 9.5" (WxDxH)
Weight: 26.4 lbs.
Price: $17,499 per pair




Art Audio UK LTD
Notingham, UK

Voice: +44 115 963 7795
E-mail: sales@artaudio.co.uk 
Website: http://www.artaudio.co.uk



United States Of America Distributor
Verdant Audio Inc
Far Hills, NJ 07931

Voice: (475) 350-9876
E-mail: info@verdantaudio.com 
Website: VerdantAudio.com















































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