Theta Digital Prometheus monoblock power amplifier Page 2

I listened to the Thetas for seven months, alternating them with a Mark Levinson No.334 and a pair of MBL Corona C15s, the latter supplied by JA, who has them on long-term loan. The amplifiers drove my Quad ESL-989 and Revel Ultima Salon2 speakers full range, and were reconfigured to reproduced frequencies above 80Hz when Revel's Rhythm2 subwoofer was in circuit.

Although setting up a pair of monoblocks seldom requires instructions, I enjoyed reading Theta Digital's well-written and detailed manual. It revealed that the small rear-panel switch that controls the AC mains supply must be flipped on before the front-panel switch can be used to cycle the amp from Standby to full on. This year, I've traveled frequently. When away for more than a few days, I turned off the amp completely with the rear-panel switch. Otherwise I left it in Standby, that LED glowing red, to ensure that the Prometheus was always at its best for listening. As recommended by Jeff Hipps, I played music through the Thetas at a low level for one week before doing any serious listening.

The most striking things about the Prometheus's sound were its huge dynamic range and bass impact. This was even evident during that first, burn-in week, when I played the timpani passage from the recording of Stravinsky's Rite of Spring by Eiji Oue and the Minnesota Orchestra (24-bit/176.4kHz DVD, Reference HRX RR-70). Don Dorsey's synthesizer-based "Ascent," from the Time Warp sampler (CD, Telarc CD-80106), erupted with enormous bass pulses through the Theta amps, followed by a mix of softer pulses, bleeps, whooshes, and high-pitched tones that pan back and forth across the soundstage. The Thetas' high power and dynamics were heard in the stunning synthesizer percussion of struck chimes and thunderous bass drum that tighten the suspense in "Assault on Ryan's House," from James Horner's score for Patriot Games (RCA 66051-2); and the kickdrum at the end of the sleepy opening of David Bowie's "Putting Out Fire," from the Cat People soundtrack (CD, MCA MCAD-1498), exploded at a more energetic and urgent level than I'd heard before.

The Prometheuses projected a broad, detailed, involving, three-dimensional soundstage. I heard this clearly playing one of my 2015 Records to Die For: the live recording of Beethoven's Symphony 9, performed by multiple ensembles and conducted by Leonard Bernstein shortly after the fall of the Berlin Wall (CD, Deutsche Grammophon 429 861-2). The separation of instruments, the passion of the soloists and choirs, and the spaciousness of the soundstage make this recording my favorite Beethoven 9th for choral qualities.

Two other (mostly) orchestral works benefited greatly from the Prometheuses' dynamic range and soundstaging. Pass Labs' XA60.5 monoblocks had proven their dynamic range with Benjamin Zander and the Philharmonia Orchestra's performance of Mahler's Symphony 2 (24/192 FLAC, Linn CKD 452). JA had noted that the "Resurrection" has huge orchestral climaxes contrasted with much softer sections in which a single instrument plays. The Thetas handled both extremely well, reproducing the solo instrumental sections clearly, then easily managing the huge climax 10 minutes into the third movement, In ruhig fliessender Bewegung. The second orchestral recording was our Recording of the Month" for October 2014: the performance of Bruckner's Symphony 9 mentioned at the beginning. In the Scherzo, the Thetas reproduced the full, surging power of the violins' emphatic, fierce fast pattern, coupled with pounding brass and drums. This is one of my favorite movements in the classical repertoire, and the Thetas' reproduction of it was the best I've heard in my listening room.

The Prometheuses produced excellent imaging and soundstaging with "Breathe," from Pink Floyd's Dark Side of the Moon (SACD/CD, EMI 82136-2). Driving the Revel Salon2s, the Thetas projected the throbbing helicopter rotors, jackhammers, footsteps running across my listening room, dive bombers, and PA announcements on a wide soundstage to cinematic effect. With John Rutter's The Lord Is My Light and My Salvation, from Requiem, performed by Timothy Seelig and the Turtle Creek Chorale (CD, Reference RR-57CD), the sounds of the clarinet, organ, and harp were separate and precisely positioned, and each rank of choristers was spread out in a clearly "visible" arc. A Gaelic Prayer, also from Requiem, had new clarity and balance, with superb pitch definition of the pipe organ's pedal notes.

The Prometheus's bass response was outstanding. Driving the Revel Salon2 towers with the Revel Rhythm2 subwoofer turned on, the Thetas produced tuneful, solid, punchy bass with great pace and jump factor. In the Toccata of Widor's Organ Symphony 5, as recorded by JA (24/88.2 AIFF file), the deepest bass notes were reproduced with unexpected power and mass. The 32Hz pipes produced immense weight, solidity, and room lock. The mountainous, deep pedal note that ends James Busby's performance of Herbert Howells's Master Tallis's Testament, from Pipes Rhode Island (CD, Riago 101), produced incredible room lock. I heard and felt the various pedal ranks underpinning Gnomus, from Jean Guillou's performance of his own transcription of Mussorgsky's Pictures at an Exhibition (CD, Dorian DOR-90117).

The midrange response blossomed when my speakers were driven by the Thetas. Whether the volume was low or high, I heard delicate differences in tone and timbre in male voices, piano, and percussion instruments. The light, clear quality of Keith Jarrett's piano in "Heartland," from his Concerts: Bregenz München (24/96 AIFF, ECM 1227–29)7 was not disrupted by his thudding foot stomps. The lilting tenors of the vocal group Cantus in Edie Hill's A Sound Like This, from their While You Are Alive, recorded by John Atkinson (24/88.2 digital file or CD, Cantus CTS-1208), also exhibited wonderful clarity and timbre.

Similarly, the Prometheus transmitted more of the timbres and harmonics of solo male voices, without tubbiness or midbass emphasis. Chris Martin's plaintive falsetto singing of the simple, intensely romantic words that I find so captivating and urgent in "Yellow," from Coldplay's Parachutes (CD, Parlophone 5 40504 2), appeared behind and separated cleanly from the dense mix of guitar, bass, and drums that supports the song's intoxicatingly slow, dark melody. And José Carreras's light, lyrical tenor remained pure during the Kyrie of Ariel Ramirez's Misa Criolla (CD, Philips 420 955-2).

The Prometheus's reproduction of the upper midrange and treble was especially commendable, especially with cymbals. I heard the harmonic overtones from different areas of single cymbals, as well as from different types of cymbals, in Mark Walker's drum solo in "Nardis," from Patricia Barber's Café Blue (SACD/CD, Premonition/Blue Note/Mobile Fidelity Sound Lab UDSACD 2002)—and the metallic cymbal sheen that opens "The Mooche," from the Jerome Harris Quintet's Rendezvous (CD, Stereophile STPH013-2), was rendered as a shimmering rather than as soft, hissing static.

While auditioning the Theta Prometheus monoblocks, I also listened to my Mark Levinson No.334 ($5900 in 1999) and the MBL Corona C15 monoblocks ($25,000/pair). A single Prometheus or Corona C15 tips the scale at less than half the weight of the No.334. The No.334 has softer-, warmer-sounding bass, and delivers 200W less at clipping into 4 ohms than either class-D amp. This might be a problem with less sensitive speakers and bigger rooms. More important, the Levinson, which I've thoroughly enjoyed for 15 years, seemed compressed, shut down, and dark, with a two-dimensional soundstage. In comparison, both class-D amps were faster, more open, more transparent, and produced a greater sense of three-dimensionality. Compared to the Prometheus, the MBL seemed leaner in the midrange and upper bass when driving the Revel Salon2s. The Prometheus had a much more successful integration with the Revel Rhythm2 subwoofer than the other two amps, being able to produce a full, driving midrange, and to reveal layers of three-dimensional detail. Overall, for me, the Theta Prometheus delivered the most emotionally involving sound.

The Theta Digital Prometheus's class-D output stage performed as predicted by theory: The monoblocks were still running coolly in my room after hours of full output to Revel's Salon2s, and delivered twice the power of my Mark Levinson No.334 at slightly less than the ML's weight. The Theta's price of $12,000/pair—though less than half the price of the MBL Coronas—puts it in the high-priced range. But the Prometheuses delivered better midrange and treble definition, greater depth of field, greater jump factor, and better imaging than any other amplifier I've heard in years. It let the Revel Salon2s sound more alive and more dynamic than ever, with faster, deeper bass response, and made it possible for the combo of Revel Salon2s and Revel Rhythm2 subwoofer to deliver better synergy than I'd heard before. The Theta Prometheus even delivered sweet, open highs and solid bass from my electrostatic Quad ESL-989s.

My last impression of the Prometheus was the same as my first: It's one of the best-sounding amplifiers I've heard in my listening room. Seven months after I began listening to it, its soundstaging, imaging, depiction of detail and speed and slam, and its exciting dynamic contrasts remain totally addictive. I give it my strongest recommendation for inclusion in Class A of Stereophile's "Recommended Components."

Theta Digital/ATI
1749 Chapin Road
Montebello, CA 90640
(323) 278-0001

remlab's picture

..and I think he is a genius, but when all these companies are using the same basic modules(I think that Jeff Rowland uses it also), aren't they all building pretty much the same basic amp?

dce22's picture

sys2722 has AES17 filters use those to test Class D amps stop using AUX-0025 on AP2.
Theta Digital Prometheus Distortion 0.01 at 15 watts?
Is that a joke? John Fix Your Measurements Chain!
I have tested alot of ICE power amps and Stereophile data always has more than 10x distortion than the real world data.
AP1 and AP2 inputs will not distort from switching residual only the Autorange circut will trigger wrong scale and you use AES17 filter to fix that, and for frequency response you do not use filters of any kind only Distortion Measurments needs AES17.

PS. IM graph is not correct and Freq Response is from the AUX-0025 not the amp.

John Atkinson's picture
Thank you for your comments, especially concerning the auto ranging. I have tried measuring both with and without the AES17 brick wall filters. The frequency response is genuine, BTW, and is not due to the AUX0025.

John Atkinson
Editor, Stereophile

dce22's picture

John i did a prototype with NC1200 modules and got identical graph as Hypex datasheet i dont use AUX0025 at all only AES17 the switching residual will not induce slew rate distortion (unless it has so badly design filter that will not pass FCC regulation), maybe Theta Digital has that bad input stage that adds high frequency distortion, noise and roll off ill try to borrow one to test it.
The distortion nubmers on 4 ohm are terrible for this kind of amp maybe the power supply transformer is too small and the noise performance is crap.
I can upload images but they are the same as the official datasheet. Cheers J.

PS. I need to mention for non technical audiophiles when using AES17 filter distortion data are only valid up to 6 khz because AES17 filter is a 20khz filter design to measure Sigma Delta DAC's, from 6 - 50khz is covered by 19+20kHz Intermod distortion measurments (you can use 19+20khz IMD data as equivalent to 20khz sine wave distorion measurments only instead of looking at the spectrum above 20k the distorion commponents will mirror back inside the audio band and you can use 20khz bandwidth for less noise there is no need to use some new "super hyper oscilloscope" thats the beauty of it) and switching residual is not somekind of wierd noise, it looks like a parabolic shapes up and down string together something like sinewave but with parabolic halfs and is in milivolts the tweeters are up to hundreds of ohms at 400khz cant even fell anything let alone distort so no worries.
When you Measure the amp with AP2 and you start at microvolts the Autorange circut will see mili volts and yell to his buddy DSP hey there is voltage comming get going change the scale! and the low level data is not usefull, there is no distortion no wierd thing going on people need to relax AP recommends AUX passive filter because there is a nutjobs that run Class D without filters and try to measure it that way that amp my frends is broken, that can be a good test for John if it need AUX-0025 That amp is Broken :) and AP1 needs AUX-0025 because it does not have proper AES17 module slot. I forgot to trasmit 400khz you need 750 meters of speaker cable so dont worry about RF offcourse there are horrible designs that wipeout all the radio staions in the area but it is not because off the speakers cable emmits at 400khz its because amp pushes Mhz's into it, amps modules like B&O Ice power and Hypex have non of the problems.

dmusoke's picture

John.. thanks for the detailed review. Its nice to know that Class D technology has matured to the point of being considered in high end audiophile amplifiers. I have one concern in your measurements, but by placing an external 20kHz filter that doesn't exist in real life, you make the measurements of the amplifier look better than they should be. The effect of these high energy ultra-sonics on the distortion numbers is masked. What effect would they have on actual loudspeaker systems? Why not make the measurements with the default 80kHz bandwidth of the AP test system?

John Atkinson's picture
Why not make the measurements with the default 80kHz bandwidth of the AP test system?

First, because with hundreds of millivolts of ultrasonic noise present, any THD+Noise measurement will be dominated by that noise, even with the 80kHz measurement bandwidth. Second, that ultrasonic noise will drive the AP's input stage into slew-rate limiting, generating additional distortion.

John Atkinson
Editor, Stereophile

dmusoke's picture

So should we have no concerns for these ultra sonics on our loudspeaker systems or radiated emissions to other electronics in our AV systems? BTW, what is the frequency of this ultrasonic noise, which I presume would be the switching frequency of the amplifier?

John Atkinson's picture
dmusoke wrote:
So should we have no concerns for these ultra sonics on our loudspeaker systems or radiated emissions to other electronics in our AV systems?

Not necessarily. Remember that hooked up to the amplifier's class-D output stage is an antenna - the speaker cable. This, of course, is loaded by the speaker, which presents a low impedance at audio frequencies. But above 100kHz, the speaker may well present a very high impedance, due to the voice-coil inductance of the tweeter. So depending on the length of the cable and the ultrasonic impedance of the speaker, the residual switching noise may well be re-radiated into the environment, as well as finding its way into the system ground, which will depend on the amplifier design and circuit layout. All these factors are unpredictable.

dmusoke wrote:
BTW, what is the frequency of this ultrasonic noise, which I presume would be the switching frequency of the amplifier?

It tends to be between 400kHz and 500kHz.

John Atkinson
Editor, Stereophile

dmusoke's picture

I appreciate the answers John. Thank you :)

IgAK's picture

An interesting article, thank you. A comparison to the similarly priced Merrill amp using the same rather popular N-Core module would have been an interesting implementation comparison.

MerrillAudio's picture

I would be happy to provide a pair of the Merrill Audio VERITAS Monoblocks for the comparison. I have also offered Stereophile the Merrill Audio VERITAS Monoblocks for review.

Perhaps there could be an independent, neutral place where the A/B comparison is done side by side. I understand some have done the side by side comparison and hope they will share their findings.

dcvibe's picture

Would love to get a non biased opinion about these two amps?