Review: A Sonnet Stack: Hermes Streamer, Pasithea DAC, Kratos Mono Amplifiers

If we want to understand the sound a hifi designer is after, what better way than to sample a stack of their gear. Here we’re going to be digging into the Kratos Mono Power Amplifiers, Pasithea DAC, and Hermes Streamer from Cees Ruijtenberg of Sonnet Digital Audio.

I’ve already reviewed the front ends—Sonnet Pasithea DAC (review) and Hermes Streamer (review)—so I suggest heading to those reviews for their fuller story. One new twist for this stack-O-Sonnet review is the use of the I2S connection between the Hermes and Pasithea, a connection that I found bettered the other digital inputs on the lower priced, but still impressive, Sonnet Morpheus DAC. The same holds for the Pasithea and I used its I2S  input throughout this review.

In brief, the Sonnet Pasithea is a non-oversampling resistor ladder DAC built around the company’s SDA-3 (Sonnet Digital Audio) R2R DAC modules, while the Roon-Ready Hermes streamer is built on a Raspberry Pi 4 foundation. The Pasithea also includes a defeatable volume control which I did not defeat, as it also served as preamplifier in this Sonnet system. An RJ45 cable is included to connect the two via I2S and I leashed the Pasithea’s analog XLR outputs to the Kratos Mono Amps with lengths of AudioQuest Firebird XLR interconnects. The Sonnets were all plugged into an AudioQuest Niagara 3000 power conditioner using AQ Blizzard power cables.

The Kratos Mono Amplifiers employ a FET input and Class A/B MOSFET output for 50 Watts of power into into 8 Ohms (95 Watts into 4). The Kratos are fully balanced and fairly straight forward with a single RCA or XLR in and a single pair of speaker binding posts out on each mono block. Included cables allow you connect the ‘Remote Switch’ inputs on each amplifier for use with the nice aluminum remote that comes, optionally, with the Pasithea DAC allowing one-click control of the stack. One functional oddity is two recessed switches reside on each amps underbelly—one is to select between the RCA and XLR inputs, and the other is to engage “music sense” which automatically puts the Kratos in standby mode after 30 minutes of silence.

The DeVore Fidelity O/96 and review Perlisten S5t speakers (more info) took turns as Sonnet stack partners, with lengths of AudioQuest Robin Hood speaker cable.

In hifi, sometimes not being surprised is the preferable outcome and the Sonnet stack announced itself with the familiar fluidity, exceptional clarity, and silky resolution that made the Pasithea DAC such a pleasure to listen through. Here, with the Kratos powering the DeVore O/96, I was treated to this same easy to enjoy sound coupled with fairly convincing control of the O/96 that made them sound nicely fit and full bodied. In the Pasithea review I commented about how it struck me as having a perfectly balanced resolution-to-engagement ratio, sounding at once highly resolving yet not overly etched or artificial and these same qualities apply to the Kratos amps.

Kelly Lee Owens’ recent EP LP8.2 was a case of love at first listen, all rich, mysterious and inviting. Electronic bass throbs, ethereal sounds hover, while all manner of clicks, clacks, and chatter create a sound world for Owen’s vocals to soar over, all menace and lace. The Sonnet stack did a fine job of building this sound world in Barn from the bass foundation, the skeletal core like sonic scaffolding, with processed vocals all expanding beyond the speakers. There was also room to breathe in and around all of these parts, but there was a bit less atmosphere than I’m used to hearing through other amplification partners and the O/96. The Sonnet stack’s strengths were still on display with this music offering that wonderful combination of fluidity and resolution but something sounded off—music was a bit flat, a bit dark, and a tad closed in.

poolblood’s mole from January of this year is Ontario-based artist Maryam Said’s first full length and its a lovely blend of acoustic guitar, vocals, and oddities with other sounds and less than obvious melodic twists and turns. It’s no surprise to learn that she sites Cat Stevens and a steady diet of punk rock and hardcore as influences even though this record mostly leans towards a kinda of hardened folk. The Sonnet stack did a fine job of grabbing hold of all of these seemingly disparate sonic elements and presenting them laid out across a spacious sound image. I have heard better image specificity, with distinct elements sounding more solidly placed in space and in relation to one another where the Sonnet stack presented an ever so slightly more amorphous sound image. More specifically, the scale of various elements wasn’t quite as clearly defined as other amplification I’m used to like the resident Leben CS600 or the recently reviewed, and much more expensive Audia Flight FLS 10 (review).

Sonnet talks about how the Kratos’ FET input stage shares sonic similarities with hybrid designs that use tubes in the input stage and while I do hear a lovely sense of the space of the recording, atmosphere, I will say I have heard airier presentations from hybrid designs like the much more expensive Riviera Labs gear (review) and of course a full tube design like the Line Magnetic LM-845iA Integrated Amplifier (review) that emphasizes timbral richness and gut punching power.

I first heard pianist Krystian Zimmerman many years ago at a hifi show, one of those lovely and unfortunately kinda rare events where the music playing in room grabbed my attention way down deep. Regular readers know of my affinity for Schubert and piano sonatas so this record is a well worn favorite (the bits don’t mind) that I’ve listened to on many a system. The Sonnet stack, which I will add adds up to $11,300 in round numbers, presented a rich and robust sound capturing the movements and mood(s) of Zimmerman’s Schubert. I have heard a more incisive sound, with greater impact ringing off the keyboard and more bite from the upper registers that to my way of hearing makes this music a bit more thrilling. My sense is the Sonnet stack may be a better partner for a different speaker.

There, that’s better. Much, much better. The review Perlisten S5t, rated at 4 Ohms (nominal) impedance,  house the company’s proprietary DPC-Array (Directivity Pattern Control) for Mid/High frequencies with a 28mm Beryllium dome tweeter sitting in between dual 28mm TPCD (thin poly carbonate diaphragm) ultra-lightweight mid-range drivers and a pair of 7″ checkerboard patterned Textreme TPCD woofers. I’ve favorably reviewed the current flagship Perlisten S7t (review) and the more wallet friendly S5t (review) so I think I have good grip on the Perlisten sound. One of their sonic strengths is related to that DPC Array and its super refined super precision that puts a fine point, not too sharp mind you, on upper frequencies so as you might imagine if you’ve been reading along is they pair up perfectly with the Sonnet stack. Strength meeting and mating strength.

What this translates into is more tinkle from Zimmerman’s right hand when he’s reaching farther right, and a greater sense of incisiveness and attack. More drama! Side note: when reading a review, especially of amplifiers and speakers, you want to read about a few different partners to get a more complete picture because we’re always reviewing systems (in rooms!). If I’d let my Sonnet stack review experience begin and end with my beloved DeVore’s, I’d have come away with an incomplete picture of the Sonnet stuff leaving me, and you, with a half baked view.

I also swung back to Kelly Lee Owens where I found more excitement from “Moebius”, with more boom and bloom to the monstrous bass, greater glints of light from those clicks, clacks, and chatters, all unraveling in a larger and more spacious sound world. More drama! My best guess is the O/96’s higher impedance load (10 Ohms) makes them a less than stellar partner for the Sonnet Kratos amps. I’ll also share that this is not the first amplifier that reacted in a similar manner to the O/96’s higher impedance load, sounding a bit flat, a bit dark, and a tad closed in.

Endure from Special Interest is a wild raucous ride passing through disco, punk, funk, and more. A veritable carnival of sound. With the Sonnet stack doing the streaming, D to A’ing, and amplifying the Perlisten S5t, the Barn morphed into club with happiness, big ass bass, crunch, and Alli Logout’s fierce vocals barking with lifelike force, grit, and power bouncing off the walls. The Sonnet’s supreme fluidity and resolving powers were still on display but with the Perlisten converting waves into sound energy the Barn lit up with infused, driving music-making joy. Fun(k)!

With the happier Perlisten / Sonnet stack combo in place, I ran through a bunch of favorite test tracks that highlight different aspects of system performance including low level resolution (Tom Waits’ “I’m Not Here”), spatial resolution (Einstürzende Neubauten’s “Ring My Bell”), rage and growl (Corpus I’s “Spit”), grace and eerie beauty (Antony & the Johnsons’ “One Dove”), magical interpay (Mal Waldron “Warm Canto”) and on and on through the worlds of emotions, moods, and moves that only music contains and communicates through physical form in air. And if I had a check list, and a clipboard (and a pocket protector), the Sonnet/Perlisten combo was checking all the right boxes, making all the right moves. Physicality? Check. Timbral richness? Check. Resolution? Double check. And so on down the line.

Harry Partch’s The Bewitched is a magical mystery expedition of the highest order. Last year, the Composers Recordings, Inc. (CRI) re-released, digital-only, their classic recording from the University of Illinois’ Champaign-Urbana campus in 1957. Here’s Partch on The Bewitched:

It is a seeking for release—through satire, whimsy, magic, ribaldry—from the catharsis of tragedy. It is an essay toward a miraculous abeyance of civilized rigidity, in the feeling that the modern spirit might thereby find some ancient and magical sense of rebirth.

Chorus and more instruments from around the globe than you can name, The Bewitched is a wild beast of a record and if you can strap yourself in for its full jaunty ride you may come out the other side speaking in tongues (in a good way). Gongs, kithara, chamber bowls, clarinet, koto, cello, and more (lions, tigers, and bears oh my), this recording, mostly from the original 15-ips mono master tapes, is a sonic joy ride and the Sonnet stack/Perlisten combo acted as the perfect tour guide offering up its worldly sonic riches in full bloom from micro minutiae to macro boogie. Magical.

It is a real treat to live with and experience a hifi designer’s vision in sound. While I’ve enjoyed my time with all of Cees Ruijtenberg’s DACs that have come into my life over the years, I am very happy to report that the Sonnet stack— Hermes Streamer, Pasithea DAC, and Kratos Mono Amplifiers—added up to a fuller range picture of the Sonnet house sound which is very capable of delivering a compelling and moving musical experience. Beyond checking off the usual performance parameters we expect from a hifi, the Sonnet sound adds a refined natural ease that draws us into music’s finer moments.

Sonnet Kratos Mono Power Amplifiers
Price: $5195/pair


50 Watt into 8 Ohms, 95 Watt into 4 Ohms
Class A/B
Distortion 0.040% THD
Output impedance 0.10 Ohms
110/115V AC 220/230V AC 60/50Hz
Output Noise 310µV RMS A weighting
Frequency Response 10 – 230.000 Hz – 3dB
Slew Rate 100V / µSec
Input Impedance RCA 15 KOhm, XLR 60 KOhm
Gain: 22dB
Max Power consumption 400 Watt
Main Voltage 110/115V AC, 220/230V, 60/50Hz
Dimensions 290 x 250 x 60 mm
Weight 3.9KG

Sonnet Digital Audio Pasithea DAC
Price: $4995 | $5285 w/MQA Module


Balanced, Non oversampling DAC
Four SDA-3 DAC modules per channel, Eight total, in differential mode
Power supply: 30VA 110/115V AC 220/230V AC 60/50Hz. Power required max 16 Watts
Input: 1x optical, 1x coaxial and 1x AES/EBU, 1x I2S and 1x USB input.
Output: 1x stereo pair single ended 2 Volts RMS. In -10dB mode 0.66 Volts RMS
Output: 1x stereo pair XLR balanced 4 Volts RMS In -10dB mode 1.33 Volts RMS
Frequency Response: 44.1 kHz sampling 1Hz – 20 khz -1dB
Frequency Response: 192 kHz sampling 1Hz – 65 kHz – 3dB and 384kHz (USB)
Distortion: 0.001% THD
Channel separation 125dB
Noise Floor: -160 dB related to 4 Volt RMS
Output impedance: RCA 100 Ohm, XLR 16 Ohms, Physical -10dB setting 8 Ohms
Sampling rate: Optical 44.1 – 96 kHz
Sampling rate: Coax and AES/EBU: 44.1 -192 kHz
Sampling rate: USB 44.1 – 384kHz (Windows)

Aluminum six button remote control included.
Possible options: MQA module (standard not included)

Sonnet Hermes Digital Bridge
: $1,199.00


Working principle: Linux based ROON end point using precision clocks
Power supply: 10 VA. Power consumption: Standby < 1 Watt. Full operation max 6 Watts
Power requirement: 110-230 V AC 60/50Hz
Inputs: Lan (local area network) max. 100 Mbps
Outputs: 1x optical Toslink, 1x RCA coax, 1x AES/EBU and I2S.
Sampling frequency:

Optical : 44.1 – 96 kHz sampling
Coaxial: 44.1 – 192 kHz sampling
I2S: 44.1- 192 kHz sampling. Max cable length in case of UTP CAT5e 20 meters

Dimensions (H x W x D): 19 x 6 x 25 cm.
Weight: 2500 gram

Company Website: Sonnet Digital Audio
US Distributor’s Website: Audio Art Cable