Solid State Power Amp Reviews

The first surprise using the Delta Monos was that the meters showed that I almost never used an average level greater than 6W to drive the Vimberg Mino loudspeakers that I reviewed in April. (The meters actually maxed out at "3W," but as they are calibrated in 8 ohm watts and the Mino is a 4 ohm speaker, the indicated power needed to be doubled.) A power of 6W into 4 ohms is equivalent to a voltage of 4.9V RMS. If the music I typically listen to has a high crest factor of 10dB, that means the instantaneous peak voltage would be 15.5V. This is just below the amplifier's maximum voltage available in class-A into 8 ohms, which is 16.73V, equivalent to 35W. It is safe to say, therefore, that throughout the month I auditioned the Delta Monos, the amplifiers only rarely moved from class-A to class-AB operation.

The second surprise was how hard it was to get a handle on the Delta Mono's sound character. You know how it is with reviewers: We set the newcomer up in the reference system and cue up the test tracks that have served us well through the years at telling us what is going on.

My favorite test track for dynamics is "Fit Song" from Cornelius's Sensuous: la musique du 21st siécle (16/44.1k ALAC file, ripped from CD, Everloving/Warner Bros. EVE016). With the Delta Monos driven by the Vimberg speakers, the beautifully clean drum samples on "Fit Song" had me dancing in my seat. And when the kickdrum was doubled by dropped-bass synth notes, the low frequencies effortlessly filled my room. Similarly, when I played the test track I use to judge bass extension and weight, my recording of Jonas Nordwall performing Widor's Organ Symphony No.5 (24/88.2 AIFF file), which has high-level spectral content down almost to 20Hz, nothing was missing from the sound produced by the Classé/Vimberg combo.

However, with the iron-fisted control exerted by the Delta Monos, I did feel I had to remove the plugs from the speakers' uppermost ports in order to add back a little midbass bloom with the Widor. This didn't detract from the clarity of the repeated 16th-note bass line in "Last Train Home," from Pat Metheny's Still Life (Talking) (16/44.1k ALAC file, ripped from CD, Geffen GEFD 24145-2).

I like to use solo piano recordings to examine an amplifier's midrange articulation and evenness of balance. I cued up Evelina Vorontsova's recording of Rachmaninoff's Moments Musicaux (16/44.1 ALAC file ripped from CD, STH Quality Classics CD 1416092). The third of these six short pieces, in B minor, is my favorite, with its plaintive, almost gloomy melody leading into the march-like bridge section. The midrange transparency of the Classé amplifiers was impressive, Vorontsova's Steinway sounding appropriately harmonically complex. At the same time, the differences between my usual PS Audio DirectStream DAC and the Weiss DAC502 that I have in for review were laid bare with this recording. (More on that in my Weiss review next month.)

Though the Vimberg Mino's on-axis behavior is superbly flat, when I measured its spatially averaged response in my listening room (footnote 2), I found that there was slightly more energy in the mid-treble than was strictly accurate due to the Mino's off-axis behavior. While I was aware of this characteristic with the Classé amplifiers driving the speakers, it wasn't exacerbated. However, the two octaves above this region did sound slightly suppressed; I suspect that the transparency of the Classé amps made what the speakers were doing more apparent. A favorite track of mine from electric guitar virtuoso Jeff Beck is "Cause We've Ended as Lovers" from Performing This Week . . . Live at Ronnie Scott's (16/44.1k FLAC, Tidal/Eagle Records), not only because of Beck's masterfully lyrical playing but also because this was the first recording on which I heard young Australian bass guitarist Tal Wilkenfeld demonstrate her mastery of the instrument. Beck and Wilkenfeld sounded magnificent, of course, but Vinnie Colaiuta's cymbals sounded a touch darker than I was used to. Even so, the Delta Mono's high frequencies were impressively clean and smooth.

As you'd expect from a pair of monoblocks, the Delta Monos' stereo imaging was superbly stable and well-defined. When I recorded male-voice choir Cantus performing Eric Whitacre's "Lux Aurumque" in Indiana's Goshen College in June 2007—it was released on While You Are Alive (CD Baby 5637240534)—the nine singers were positioned in an arc in front of the array of six microphones. Before I started capturing the performance, I got each singer in turn to say his name. That way, when I prepared the mixdown, I could make sure that I wasn't distorting the stereo image. However, I accepted a slight broadening of the images of the singers at the edges of the soundstage in order to preserve enough of the bloom of the hall's glorious ambience. Listening to the MQA-encoded 24-bit, 88.2kHz master file of "Lux Aurumque" with the Classé-driven Vimbergs, that is exactly what I heard: tightly focused images of the singers in the center and slightly more diffuse images to the sides, with excellent soundstage depth overall. To draw a photographic analogy, the Delta Monos offer superb image acuity.

Comparing
The Classé Delta Monos replaced the $16,990/pair Parasound Halo JC 1+s that I reviewed in the June 2020 issue. Like the Delta Mono, the Parasound is heavily biased into partial class-A operation, though with 25W available in class-A rather than the Delta's 35W. I wrote in my review that "the Parasound's high frequencies sounded more like what I experience from a good tube amplifier." With levels matched at 1kHz—straightforward to achieve because the Parasound and Classé have almost the same 29dB gain—I played an album I had mentioned in my Parasound review: Christian Tetzlaff performing the Beethoven Violin Concerto with Robin Ticciati conducting the Deutsches Symphonie-Orchester Berlin (24/48k FLAC, Ondine 1334).

Listening first to the Delta Monos, there was excellent depth to the orchestra, and there was palpable space around repeated notes on the timps that open the work's first movement. The image of Tetzlaff's violin was stable, narrow, and appropriately presented in front of the orchestra. Turning to the Parasounds—easier to write than to do with four large, heavy amplifiers—after allowing them to warm up, the Halo JC 1+s did have a tubelike presentation, in that the lower midrange had a touch more bloom than through the Delta Monos. The Parasound's highs were a tad lighter-balanced than the Classé's, but soundstage depth and definition were very close. There was slightly more space around the images of the violin and timps with the Halo JC 1+s. In the bass, the Jonas Nordwall organ recording revealed that the Delta Monos exerted slightly more control over the Mino's woofers than the JC 1+s'—and again, there was a little more space apparent with the Parasounds. Though the Delta Mono was less "tubelike" than the JC 1+, it excelled in the articulation and spatial definition of the various ranks of organ pipes Nordwall uses.

My long-term reference amplifiers are a pair of Lamm M1.2 Reference monoblocks ($33,990/pair), which, although less powerful than the Classés and the Parasounds, also operate in class-A at the bottom of their dynamic range. With levels again matched with a 1kHz tone—the Lamm is 2.6dB more sensitive than the Classé—the organ recording sounded more laid back than it did with either of the other amplifiers, and more upper-bass bloom was audible. This was also noticeable with the bass guitar and the lower-pitched toms on the Jeff Beck track, though the Lamm's top-octave balance was similar to the Delta Mono's. Overall, I felt the more expensive Lamm was a slightly better match for the Vimberg Mino speakers, though I suspect the Classé Delta Mono would work better with a sweeter-balanced speaker such as the Magico M2 I reviewed in February 2020. A close-run thing, nevertheless.

More listening
I worked on this review during the first month of New York's stay-at-home, social distancing mandate. Maintaining a daily routine was an important part of coping with the feeling of isolation, and I got into the habit of listening weekday nights at 6pm EST to BBC Radios 3's Night Tracks immersive mashups, courtesy of Roon's Live Radio function. But one Saturday evening in April, instead of Night Tracks, the BBC was rebroadcasting the 2015 performance of Max Richter's eight-hour epic, Sleep (footnote 3), because, in their words, "when hours seemingly stretch into the distance, Sleep offers a mindful way to forget everything going on around us and to enter another world."

The first 19 minutes of Sleep consist of a solo piano hypnotically vamping a simple chord sequence, punctuated with deep synth-bass pedal notes. Even though the audio was encoded as a 128kbps MP3 stream, the sense of space with the PS Audio/Classé/Vimberg system was impressive and the low bass notes sounded magnificent. I was pulled into Richter's vision—a great audio system in service to the emotional relief offered by music.

I could only listen to the first 90 minutes of the Sleep broadcast, through excursions into sections for string quartet, solo soprano, and looped samples, so the following day I streamed an album of excerpts (24/96 MQA FLAC, DG/ Tidal). The recording venue was different—AIR Studios London rather than the Reading Room of the Wellcome Collection in London—but the transparent window into music offered by the Delta Monos allowed me to clearly hear that despite the low bit rate, the broadcast was superior in terms of space and depth to the commercial release, though the piano was not as well in tune.

Summing up
I greatly enjoyed my time with the Classé Delta Monos. Their transparency, coupled with smooth high frequencies and excellent low-frequency control and articulation, were addictive. Highly recommended.

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Footnote 2: See fig.7 here.

Footnote 3: This performance was available for streaming here until May 11 2020.