June 02, 2019 12 min read

Suolo Mono Review
By Samuel Rosen

How it all began:
At the end of January, I reached out to Justin, the owner of Ampsandsound, to order an Agartha 2.0. He asked me why I wanted an Agartha and what headphones I was planning to use with it. After hearing what I was planning to do, he immediately told me the Agartha was not what I was looking for. For my use case it would be too noisy. He then mentioned to me that he was working on something special, the Suolo Monos. He described them as a Kenzie on steroids, it would take the best parts of the Mogwai SE (which I currently own) and combine it with the mid-range of the Kenzie/Kenzie Encore. It would also be Justin’s best built amp, using turret boards and point to point wiring. I was immediately sold on the idea and sent Justin the deposit for unit number 1. In the middle of March the Suolo Monos (Rev 1) arrived.

Justin describes the construction of the Suolo Mono as “jewel like” and I have to agree with him. The finish is beautiful, the case work is lovely, the tube sockets are snug and secure, and the wiring is a work of art. When I plugged in the amps and listening for the first time I was shocked to discover that Justin delivered on his promise of sound quality. The Suolo Mono took the Kenzies excellent mid-range and gave it the Mogwai SE dynamics and grunt creating a near perfect amp. I say near perfect because the revision 1 design had one flaw, noise. The Revision 1 of the Suolo Mono was not a good pairing for high efficiency headphones like the Utopia, Stellia, and Vérité. Don’t get me wrong, the headphones sounded great, but if nothing was playing you knew the amp was on.

Most manufactures would have said “this is the risk you take by being owner number 1, it performed within my spec and it is good enough.” Instead, when I gave Justin this feedback, he said “let me see what I can do.” A few weeks later I got the news that he was able to dramatically reduce the noise floor with a change in the circuit (revision 2). A week later I was informed that there was one other minor tweak that helped the noise floor even more (revision 2b). At Justin’s request, I sent the amp back to him and he upgraded my unit from Rev 1 to Rev 2b. Rev 2b is the final production model and it features lower noise than the original Kenzie. It also improved the bass response significantly because part of the update was a new choke and a dramatic increase to power supply capacitance. This review will focus on the Revision 2b as this is the final production model of the amplifier. I want to thank Justin for the great customer service and his commitment to going above and beyond to deliver what he promises.

Review Equipment:
Amp: Suolo Mono
Tubes: Raytheon Windmill Getter 5751, Tung-sol 1626, Mullard EZ81
DAC: Auralic Vega G2 + Leo GX
Source: Roon (Qobuz, Tidal, and lossless FLAC)
Cabling: Wireworld Equinox Interconnects, and Nano Eclipse Headphone cables
Power Conditioner: Dual 480 watt balanced power conditioners (one for each amp, also built by Ampsandsound)
Other Equipment: XLR to RCA outboard Input Transformer built by Ampsandsound

Headphones: Focal Utopia, Focal Stellia, ZMF Vérité

For this review, the Suolo Monos were set at a volume of 25%, and volume control was managed by the Auralic Vega G2 (which has an excellent analogue volume control implementation). This configuration gave me the most range on the Vega G2s volume control (listenable from about 40 - 85 on a scale of 100) without blowing my ears out.

Unboxing and First Impressions:
The Suolo Monos represent a beginning of a new era for Justin as far as shipping and packaging. These are the first amps to get Seahorse cases by default. This means each amp shows up in a black plastic hard case, with a foam insert hand cut to accommodate each amp and the standard tube complement. Not only does this ensure safe shipping, but it also makes it much easier to justify to the other members of your household why you need to keep the cases.

Upon opening the cases I was greeted by the amp, standard power cord, and the standard tube set (JJ 5751, NOS 1626, and EH 6CA4). You also get the lovely hand written spec sheet showing the amps power output and noise output.

My amps spec sheet is below:
Input Sensitivity: 940 mV RMS
Max Power (8 ohms): 400 mW RMS
Max Power (32 ohms): 300 mW RMS
Max Power (300 ohms): 350 mW RMS
Noise (8 ohms) 440 micro Volts RMS
Noise (32 ohms): 540 microVolts RMS
Noise (300 ohms): 780 microVolts RMS*

*Justin has improved the 300 ohm tap and reduced noise to 640 microVolts. I did not choose for my amp to receive that update as it was not necessary for my application and would have resulting in a very reasonable but unnecessary expense.

For those who have seen other amps made by Ampsandsound then you know what to expect. The walnut cabinet is lovely and can be customized with a simple phone call to Justin before ordering. The finish of the top plate is excellent. The tube sockets are labelled, and the overall layout of the top plate is logical and functional. The fit and finish where the wood cabinet meets the top plate is more or less perfect. Unlike the Kenzie, all 3 headphone taps (8, 32, and 300) are placed directly in the top front of the unit, and there is a pair of binding posts on the back of the amp, because yes, it can drive speakers as well.

Justin had originally intended to make the Suolo Monos as true copies of one another (i.e. the exact same chassis for the left and right) but I was able to convince him to offer customers the ability to get them mirrored. Personally I think mirrored monos look great sitting next to each other and I was very happy to see that Justin was able to make this work when they arrived.

Each amp has a decent weight to them, as a set it is quite a large setup, but individually the amps can be described as cute. Volume is controlled on the monos with two stepped volume attenuator (one on each amp). This is nice because you can volume match by counting the steps, but you still have freedom to raise or lower either channel in the event that you have gain differences.

The amp also ships with a dual mono to stereo cable, in my case it was a dual mono to 4 pin balanced female connector as all my headphones are terminated for 4 pin balanced amps. The cable is beautifully built and while I love my Wireworld cables, Justin’s fit and finish is just better.

Listening impressions:
As I noted above all listening impressions were done with the Vega G2 and Leo GX clock with the balanced output of the Vega G2 going to an XLR -> RCA input transformer and then into the Suolo Monos. The volume is managed by the G2, and the monos are fixed at 25%.

Starting with the Focal Utopias, I really enjoy these headphones, there is something special about how they render tone, speed, and dynamics. The first time I really understood that was when I listened to the Utopias on the Mogwai SE. Unlike the Schiit Mjolnir 2 or Woo WA5 there was a richness that the Mogwai SE provided that took the headphones to the next level. Not only were they detailed but they were also fun to listen to. As I have written before about Justin’s amps, all of his amps have a house sound. He describes them as a modern take on 50’s and 60’s American Hifi. While I cannot comment on if he achieved that, I can comment that his amps are both accurate and fun to listen to. If you have a good source behind them, they will let all of the details through to the headphones and they will also add the slightest hint of fun. In my opinion this combination is exactly what you want a good amp to do.

Listening to the Utopias through the Suolo Monos is an entirely different experience than listening to them with the Mogwai SE. The Mogwai SE is an amazing amp because it put you (the listener) directly in front of the microphone. It is invigorating and exciting but the Suolo Monos put you in the front row. You get all the excitement of the Mogwai SE with visibility of the entire stage. Artists have distance and space around them in a way the Mogwai SE was not able to render. For those who have listened to Justin’s Kenzie amps, the monos render space similar to how the Kenzie presents space, however I would argue the Kenzie places you about 10-15 rows back in the audience. You will have all the visibility and space however, you miss some of that energy and excitement. The Suolo Monos take the best aspects of the Kenzie and Mogwai SE, combine them together, and add one more thing to the mix...bass. People who believe that tube amps cannot give them good bass need to listen to Justin’s amps. The entire line performs well in this respect but the Suolo Monos outperform his entire line in this area. The bass is deeper, faster, and harder hitting than the Mogwai SE. It is detailed and textured in a way that I have not heard before; this is made apparent on Boz Scaggs Radiator 110. The drum and bass guitar track drives the song forward pushing the experience into your head. It is visceral, it is fun, and it feels real.

Justin describes the 1626 as a poor man’s 300b. The 300b is legendary, it has an amazing mid range and when properly implemented can produce amazing sound from 20hz - 20khz. However, few people will ever hear a Western Electric 300b due to their rarity and prohibitive price tag. Likewise, even the higher end reproductions can cost as much as $2500 per pair. The 1626 delivers the vast majority of what the 300b does but does it at a price closer to $20 a tube. The $20 tubes are the original tubes made in 1943 not a reproduction (there currently are not any companies making reproductions of this tube).

With the 1626, the Suolo Monos have just the right amount of mid range bloom. Voices feel real (whether they are male or female) and you can with the right recordings, feel the pressure of the air from the artists mouth on your ears. This is a feeling few amps have been able to reproduce. To put it simply, I have no complaints about the mid range, it is exactly what I wanted; it is clean, clear, detailed, with just a slight hint of fun.

The high frequency extension is fantastic and smooth. Listening to Vampire Weekend’s “Father of the Bride” which is full of high frequency extension that can and does become grating on lesser systems, I find all of the detail is retained but I never find myself turning the volume down or becoming tired with the high frequencies. This is rare and exciting for me. I am actually quite sensitive to high frequency noise, but also enjoy what it adds to music. I find music lacking defined and extended highs boring. Finding an amp that produces that perfect balance in the high frequencies was a surprise and to say the Suolo Monos got this just right would be an understatement.

The last things I want to touch on is sound stage, you may be able to infer from the above text what I will say here, but the stage is well defined and solid. The amp makes me feel like I am in the first row and I can see the entire band. There is space between each player and varying distances between them and me creating a realistic and pleasant layering of music. Even the Utopias, which are not known for their sound stage, produce convincing space in a way that no other amp has been able to do for them. Coming back to Vampire Weekend’s “Father of the Bride,” streamed from Qobuz at 96/24, the music produces a stage full of space. Sounds move around the space in a cohesive manner, that at times was almost too real. This is both a compliment to the recording and the amps for being able to bring such a sense of movement that I actually found myself getting motion sick (something that I have not been able to reproduce on any of my other amps with the same equipment).

Listening with my other headphones:
The Focal Stellia is essentially a perfect match with the 32ohm output of the Suolo Monos. These headphones are so efficient (106db) that they allow me to spot noise from an amp with ease. Try as I might, with the Rev 2b I could only pick up the faintest of hums. This hum is so faint that the only way I can identify it is to put the headphones on, take them off, and put them on again. Upon putting them on a second time, my ears are able to detect the slight difference between the noise in the room and the noise in the headphone. Hopefully this is clear but if it is not I will say it directly, the Suolo Monos are incredibly quiet and match well with high efficiency headphones like the Stellia and Utopia.

All things that I have said about the Utopias and the Suolo Monos carry over to the Stellia and the Suolo Monos with a few exceptions:
Bass is deeper and a bit more powerful with the Stellia, which is incredibly enjoyable and expected given the Stellias closed back nature. Likewise, the Stellia images better than the Utopia. The sound stage is just better it has more space, more distance, and more layering. I highly suggest that people who buy the Suolo Monos or really any of Justin’s amps audition the Stellia, it is an excellent match.

The ZMF Vérité, with its 300ohm impedance, is a perfect match for the 300ohm tap. It also works well on the 32ohm tap. I find that on the 8ohm tap they sound a bit thin, but that is to be expected given the impedance difference. For my listening, I tend to listen to the Vérités on the 300ohm tap. Like the Stellia above, the ZMF inherits all of the good of the Utopias. Like the Stellia it has more bass than the Utopia and the sound stage improves. Given the ZMFs slightly warm tilt, I would say the big difference between running the Suolo Monos with the Focals vs the ZMF is that the Focals result in a very neutral presentation, where the ZMF results in a slightly warm presentation. Which you will prefer depends on your tastes and mood, which is why I own all three. They all have their time and place. However I will say that the ZMF Vérité produces the best Vocals of all three headphones.

The key here though is that all three headphones take on a similar set of characteristics when connected to this amp, they all produce good sound stages, they all have smooth highs, deep lows, and cohesive mid-ranges. But most importantly, all three are fun to listen to, something that is just not true especially for the Utopias on other amplifiers. This is a very important point about all amps from Ampsandsound. Justin has managed to strike the perfect balanced between detail retrieval and enjoyment. I not only can place the musicians on the stage, but I also have the inclination to get up and start dancing with them. In a nutshell this is the house sound of Ampsandsound and the reason I continue to look here first before going elsewhere.

Tube Rolling:
When I originally decided to purchase the Suolo Monos I thought tube rolling would be pretty limited, especially given hat there were only two power tube choices. Surprisingly, the Suolo Monos are a tube rollers dream. For driver tubes, I have run the amp with 5751s, 12AX7s, 12AT7s, 6414 (and other related variants). For power tubes, I have run the amps with is the 1626 and the 12b4a. For rectifiers, I have stuck with the European EZ81. The amp responds quite well to tube rolling and can change its base sound and characteristics, you want nearly a watt of output power, that is fine roll in a 12AX7 and a 12B4A. You will maintain most of the romance of the 1626 and produce nearly double the power. The resulting amp will also be more linear from top to bottom. Likewise, if you like the 1626 but want the amp to lean more neutral or neutral cold, replace the 5751 with a Telefunken 12AT7 or even an 802S (I have both but the 802S is much better and worth looking for). If you want the amp to lean a bit warmer, grab a pair of RCA Command Series 5751s and you will be very happy. Personally, I settled on Raytheon Windmill Getter 5751s which some say are the best 5751s ever made. While I have personally not listened to all 5751s every made, I have listened to many, and I will say the Raytheons are worth searching for. They have the extension of the Teles in both directions, with the mid-range of an RCA. For those looking for a cheaper alternative, look for the Raytheon 6414, cheap tube, lower gain (mu of 40 vs 70) but sounds fairly good in the circuit with high efficiency headphones.

The one combo that I do not suggest to future owners is a 12AX7 and 1626, the 1626 can be overdriven by the 12AX7, some cause distortion and some drive them just to the edge. You will be happier with the 5751 or 12AT7 equivalent if you plan to stick with the 1626.

At $5,500.00 these amps are incredibly expensive but they are worth the money. They are built like tanks with attention to detail that does not come from large manufacturers. The Suolo Monos are backed by a lifetime warranty and a company whose owner cares what his customers think, and much more importantly cares if his customers are happy. The amps are fun. While the spec sheet may not suggest it, they have enough power to drive pretty much any headphone you can throw at it, and even some high efficiency speakers. These are true reference devices and for me represent the end of my headphone amp journey. There is not a single thing more I want out my headphone amplifiers and as someone who has spent the past 10 years looking, that says a lot.

Great Job Justin Weber, the Suolo Monos are a keeper and come with my strongest recommendation.

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