Review written by Ian Dunmore (@Torq)
The “SPL Phonitor x” is a premium, feature-rich, headphone amplifier, and pre-amp, billed as having features and performance that lend it equal facility in professional/studio settings as well as in the role of an audiophile headphone system. The unit I am reviewing is on kind loan from “Headphones.com” via the “Community Review Program” as part of “The HEADPHONE Community” forum for audition/review purposes.
SPL (“Sound Performance Lab”) is a German firm that is probably better known in professional audio circles than it is in the audiophile and/or headphone world (something I expect will change in relatively short order). They’ve been on the periphery of my radar for a while, but not something I’d gotten to spend proper listening time with until I was offered the opportunity to audition this unit at home.
The unit provides both balanced and single-ended source inputs and headphone outputs. A configurable cross-feed/speaker simulation feature, a granular balance/stereo/mono control, VU meters with configurable sensitivity and enough power to drive almost any headphone with power to spare.
Review Equipment & Material
Sources (DACs) used in this review include the Chord DAVE, Chord Hugo 2, Schiit Audio Yggdrasil Analog 2, RME ADI-2 DAC, Massdrop x Airist Audio R-2R DAC and the built-in SPL “DAC192”.
Headphones/IEMs used for this evaluation include: Focal Utopia, JPS Labs Abyss AB-1266 Phi, Audeze LCD-4, Sennheiser HD800S, HD660S & HD650, Sony MDR-Z1R, Fostex TH900 Mk2, ZMF Eikon (Padauk), Empire Ears Zeus XR (ADEL) and Etymotic ER4XR.
The majority of the music I use in my evaluations is in “Red Book” CD format (16 bit, 44.1 kHz), most of which comes from CD rips; an initial playlist for my audition listening can be found here. Where appropriate/referenced I utilize a number of high-quality, high-resolution, albums, needle-drops, and also some native DSD content.
In addition to “classic black”, the Phonitor x can also be had (along with the rest of the matching line of SPL components), in two additional colors … a classy looking silver and, perhaps my (guilty) favorite, a metallic red finish.
It is worth noting that it is only the faceplate of the unit that is colored. The main body of the case is finished in black, regardless of what color unit you choose. You can see this below, and this was the primary factor in my ultimate preference for this unit in an all-over black finish … if the whole body had been red, well, that’s where I would have gone:
An optional DAC module, the “DAC192”, with USB, TOSLINK and COAX S/PDIF inputs, can be added to the unit (at time of purchase) for all-in-one operation. The unit I am reviewing is equipped with this module, which will be included in this evaluation.
The Phonitor x has a very solid feel to it; all of the switchgear is wonderfully tactile and engages firmly and positively with a satisfyingly solid “thunk”. The large, central, illuminated volume dial is buttery-smooth. All sockets mate solidly with both input and headphone connectors. Labels are clear and concise, with the functions they indicate being entirely unambiguous. The overall impression is that this is a very high-quality and entirely “premium” device.
I have no doubt that this will hold up to the rigors of studio use, which are generally much more demanding than at-home/audiophile listening scenarios.
A nice touch is that the input connections on the rear of the unit also have normal and inverted labels, so you can see what you’re doing, when peering over the back of the thing from the front of the unit:
Power is supplied via a standard, switched, IEC socket; so you can use the substantial included AC cable or substitute your own after-market/upgrade cable easily.
The only minor quibble I could cite with the build is that the casework can exhibit a little bit of flex towards the rear of the unit. This is only going to be apparent if you’re moving the unit around a lot; even then it’s a minor point and unless you grab the unit “just so” you’ll probably never notice it.
The Phonitor x has a number of interesting features, some of which herald from the professional market, some from the audiophile world and some are relatively uncommon, but quite welcome, in either:
- SPL make a point of calling out their “VOLTAiR” high-voltage power-rail capability (also known as the “SPL 120V Rail Technology”). This provides for a lot of voltage-swing in the power-supply, lots of available power and significantly improved dynamic range (claimed >135 dB) vs. more typical 30v and 36v supply-rail implementations.
- Headphone output is via either 4-pin XLR (balanced) and 1/4” (6.35mm) TRS jacks (single-ended), and can deliver up to 3.7 watts into 120 ohms. In practice this is enough to drive every headphone I own with total authority.
- In common with a lot of high-end headphone amplifiers, the Phonitor x has both balanced (3-pin XLR) and single-ended (RCA) inputs. Unlike most of the competition, however, these inputs can accept high professional-gear signal levels without clipping. Pre-amp outputs are also available as both balanced (3-pin XLR) and single-ended (RCA) connections, which can be independently configured to be fixed-level (volume bypass) or to track the volume dial.
- An intriguing, and nicely implemented, “Matrix” function is available. This is essentially a combination of an adjustable crossfeed capability and selectable simulated speaker-placement settings for the headphone outputs.
- A “Laterality” control is also provided; essentially a very fine-grained balance control, it can be bypassed completely, put into “mono” mode or used to correct channel imbalances in headphones.
- Remote volume control is available; the main volume dial being motorized with an illuminated position indicator. It is very easy and quick to program (nothing like the nonsense most programmable TV/remotes put you through) and works with pretty much any infra-red remote control. I used an Apple TV remote control.
- Finally, besides the marine/aircraft/ham-shack front-panel appearance (an aesthetic that I find hugely appealing), probably the most visible “feature” is the pair of illuminated VU meters, which have a selectable 10 dB attenuator for the 0 dB level (useful with professional-level outputs and some audiophile sources that have “hot” outputs, like the Chord DAVE or Hugo 2). At a practical level these are probably more interesting for studio use, but they’re pretty and quite entertaining even when just sitting and listening to music for the fun of it!
The $2,499 question is, of course, “How does the Phonitor x sound?”
In a word … fantastic!
For a start, it is clearly both an entirely neutral and highly transparent performer. Using a variety of sources and headphones there’s no tonal shift beyond what those components impart by themselves.
Much of my listening was performed using a Chord DAVE DAC as the source. That has a built-in headphone output that, by definition, offers the most transparent delivery possible from that unit. With some headphones, I feel DAVE lacks sufficient raw grunt to get the job done convincingly and really needs an external amplifier to deliver their best (e.g. Audeze LCD-4, JPS Labs Abyss AB-1266 Phi). The Phonitor x is the only solid-state amplifier I’ve paired with/compared to DAVE that does NOT result in a noticeable loss in transparency or resolution.
It covers up nothing, makes excuses for nothing, and lets the source show its true colors without omission or editorialization.
It is, for all intents and purposes, the proverbial “wire with gain”.
Lots of gain, if you want it …
There is an addictive sense of lurking power, absolutely effortless delivery and this is accompanied by an impression of “scale” to music that I’m not used to outside of either very-capable speaker systems or state-of-the-art tube-based headphone rigs. This is the case regardless of what gain setting you are using (set via DIP switches under the unit), though I did not need to switch from the lowest setting in any of my listening, and doing so experimentally didn’t change any impressions other than to make things louder for a given position on the volume dial.
Huge dynamic swings in the most powerful musical performances don’t even begin to phase this thing, while micro-dynamic nuances are fully resolved, even when they’re occurring together. Whether it’s micro-variations in the pressure or drag of a bow over a violin string or subtle reverberation in the deepest and softest vocalizations in a singers voice, it’s all rendered clearly and cleanly.
Transient response, perhaps most subtly exemplified by plucked strings and most vividly by rapid-fire electronic percussion is, to say the least, first class. Paired with Focal Utopia, HD800S or Abyss AB-1266 Phi, transients are lightning fast, and that remains the case with somewhat more laid back cans like the Audeze LCD-4. In fact this is one of the few amps I’ve felt works well enough with the LCD-4 that Audeze’s “Reveal” plug-in/EQ isn’t required.
Fire up something like “Who Needs Love (Like That)”, Erasure (Pop!) … indeed most of that album, and it will be immediately clear that nothing is getting rounded off, rolled off, or otherwise fiddled with. It’s fast, precise and … impactful.
Listening to one of my favorite instruments, the piano, both from commercial and my own recordings (of my own instrument), illustrates a pure, unwavering tonality. No emphasis, no bias, just a pure and honest reproduction of what the unit is being fed. This is one of the hardest instruments I find for any piece of stereo equipment to convincingly reproduce for me, but the Phonitor x does it as perfectly as I’ve ever heard.
Treble is delightfully smooth, but without any loss of detail and maintains an excellent sense of air, space and, where called for, delivers any sparkle present in the source material. I heard no rough spots nor glare during any of my auditioning, even when using a variety of “torture” tracks, some of which are just samples of complex real-world sounds rather than music.
Bass is reproduced with excellent drive, slam and control and yet retains its texture. Tunes played in the lowest bass registers are ably communicated. Some of this is no doubt due to the significant available power, combined with larger than typical voltage drive, which seems to make for particularly effective pairings with higher-impedance headphones. Bass performance with cans like the HD650, HD660S and HD800S was better than I’m used to, if still not quite on par with the best planar designs in that realm. Though that's down to the headphones and not the amp, in this case.
Both upper and lower registers blend perfectly, seamlessly, into a fully detailed and present midrange. Once again I was struck by how transparent the performance of the Phonitor x was, even compared to the direct outputs of the DACs I’ve been feeding it with. Instruments sound the way they are supposed to with no artificial edges added nor any softening of their natural bite. The neutrality of the unit’s presentation means there’s no added warmth or liquidity added to vocals or the main body of the music, but what is there from the original material, or from your source, comes through with absolute clarity.
Detail and resolution are superlative; where brush strokes on cymbals or drum-skins are so well rendered that you can hear when the wire bundle is split or part of it clips the rim of the drum head. The inadvertent twisting of a bow as a violinist seems to shift the hold mid-note, minor utterances, odd, tiny, shifts in a vocalist’s tone, changes in the depth and pace of the artists breathing, environmental noises (chairs moving, pages turning in music, other sounds common to live recordings) are all clearly audible1. These can get lost, as they’re often very low-level in nature, with some amplifiers, but they’re faithfully reproduced here.
Complex layering is easy to hear through and excellent separation make focusing on a given instrument, or voice, amazingly easy … either spatially (especially with the right headphones and the “Matrix” function in play) or by being able to isolate that one performer/instrument via its sound and how it is being played. If you want to pick a mix apart or focus on part of an orchestra, the Phonitor x is an excellent tool. This is both a boon for listening to complex music as well as when fine-tuning complex tracking and mixing in a professional/studio environment.
Overall, music is portrayed with a distinct sense of “substance” and notes carry an interesting feeling of “weight”, despite the result never feeling anything other than realistic, natural and transparent.
This is set against a dead-silent, carbon-black, background. Only by engaging all +24 dB of headphone output boost, turning the volume to maximum, and using my most sensitive, closed-back, headphones, via the balanced output, could I hear even the slightest noise from the unit. At no point, in normal operation, was the background anything other than completely silent. And it is, perhaps, in part due to the absolute void against which the music is delivered that results in the presentation being one of absolute clarity and vividity.
There’s no apparent coloration of the sound, and this remains true with feeds from the pre-outs as well. Somehow, the Phonitor x manages to yield this uncolored-delivery without resulting in an analytical or sterile sound … it’s still producing music … and is thoroughly engaging to listen through.
So … I said this thing sounds “fantastic”. But I’m not sure “it” actually “sounds” at all. Unless you engage the “Matrix” function, it just seems to disappear completely … serving only bring what you feed it up to an appropriate level to drive your chosen headphones. Which is about the highest praise I can give an amplifier, unless I’m looking to have it color, or in some way alter, the final sound.
The “Matrix” processing function of the SPL Phonitor x is intended to create a more speaker-like listening experience when using headphones. It is really an advanced, analog-domain, crossfeed2 system that allows simulation of interaural time and volume differences at various user-selectable levels.
You can read a full explanation of the theory and settings in the Phonitor x User Manual (refer to page 11), but for this review, I’ll focus on the actual effects of the system rather than what’s behind it.
For purists, it is worth noting that a) you can completely disable this feature; it is only engaged when the “Matrix” switch is set to “on”, b) it only affects the headphone outputs … the processing is never applied to the pre-outs and c) the whole thing is handled with purely analog filters.
Once enabled, even on the “minimum” settings, I noticed both an immediate, and obvious, change in both the spatial presentation of music (recordings with hard-panned elements benefit more in this regard), as well as a small tonal shift -mostly a slight reduction in bass presence.
I would not say the “Matrix” feature necessarily replicates the same apparent width and depth of a stereo recording played on a properly setup speaker system. Nor was it always a beneficial change. It isn’t my preference for “critical” listening, such as I do when reviewing gear, but from a “musical enjoyment” perspective, over several weeks of listening, I found myself using it more than I expected - especially for “background listening".
For active listening, I have found the “Matrix” option to be peculiarly enjoyable … especially with relatively simple, stereo-mic, recordings of small-club jazz and other acoustic/vocal music. Mary Black’s “Columbus” (Looking Back) yields a particularly good example of the soundstage expansion/rendering the “Matrix” feature can deliver.
Experimenting with the settings can produce more, or less, realistic portrayal of soundstage width as well as instrument location. This varies, significantly, based on the headphones you are using. With the Sennheiser HD800 and JPS Labs Abyss AB-1266 Phi the effect is quite pronounced; in the best cases, the effect restores some perception of depth, not just width, to the stage.
With cans like the Utopia, that have a naturally compressed and flat stage, the effect tends to be somewhat less pronounced, but can be particularly welcome as a result.
All in, however, I suspect this feature is going to be something of a “Marmite”3 thing for most users.
DAC Module (DAC192)
SPL offer the Phonitor x with an optional, internal, DAC module (called “DAC192”), which was installed in the unit that I auditioned and used for this review.
When present, the “Digital” position on the “Source” selector engages the built-in DAC, and then the “Digital” selector lets you choose between the three selectable digital inputs; S/PDIF via optical (TOSLINK) and coaxial (RCA) connections, and a class-compliant4 USB 2.0 asynchronous input.
The DAC module is based on the AKM 4396 converter and offers support for PCM samples rates from 44.1 to 192 kHz at bit depths from 16 to 24 bits. There is no support for DSD, native or otherwise - you’ll need to convert to PCM in your player.
The DAC module, in keeping with the overall signature of the Phonitor X, is largely neutral in it’s presentation. Unlike many DACs using more recent iterations of the AKM chipset (particularly the 449X series), the DAC192’s bass delivery is natural with just a very slight perceptual boost in the lower/mid-bass - but there’s no sense of bloom or bloat … it remains taut and controlled.
There is a slight tendency to emphasize sibilance if it is present in the original recording. This was quite noticeable in “Heart’s” Heart album, as well as with some tracks on “Julia Fordham’s” first offering. I put this down to the DAC rather than the amplifier, as it is NOT present when using external DACs via the unit’s analog inputs.
In general the midrange exhibits some congestion, but is otherwise nicely integrated with the upper registers, and the overall balance is good. There’s a suitable sense of air, and space, to the top-end. Dynamics are good, micro dynamics somewhat less so. For example, it loses something in rendering the gravel in Leonard Cohen’s voice, especially when he hits lower tones and quieter phrases.
Overall tonality is natural, timbre is conveyed effectively, but the presentation lacks some weight, even with the tonal density that the Phonitor x amplifier stage, itself, provides.
I would say that the internal DAC module is competent, broadly neutral, and convenient, but is otherwise uninspiring. As a built-in option for use when monitoring or when simplicity is paramount, it’s not an unreasonable proposition. And for those purposes, I’m ordering my unit with that option installed.
That said, for general “listening to music for pleasure”, if I didn’t already have myriad DAC/source options, I would be spending my $300 somewhere else. The iFi Audio Nano iDSD Black Label or Schiit Audio Modi MB would be excellent places to start.
While I didn’t find any combination of source gear/headphones that didn’t sound excellent with the Phonitor x, as I explored myriad combinations of same I did find some particular synergies. These I would define as combinations of gear that provide a “better than expected” or “performs above their price points” combination, or that show a particular component either in a “new light” or let them perform the best I have heard them to date. I thought it might be interesting to call out the most notable of those, here.
RME ADI-2 DAC
The RME ADI-2 DAC has the same kind of fundamentally neutral, transparent and highly-resolving presentation as the Phonitor x. As a pair they provide an uncommonly revealing lens through which to explore music. You’ll hit limits with your transducers long before you’ll find them with this DAC and amp pairing.
The combined transparency and neutrally also means that this pairing does nothing to taint the raw character of your headphones. This yields a particularly effective combination for hearing your cans without upstream editorialization. And if you’re the type the likes to EQ your headphones, then having a totally neutral source to do it from is exactly what you want.
The Phonitor x easily makes up for the shortcomings in the RME unit’s built-in headphone amplifier - and the rendering has more tonal weight and gravity than ADI-2 DAC alone. Comparisons are easy, since you can readily swap between the ADI-2 and the Phonitor x connections. And what you’ll find is the somewhat lacking dynamics of the native output on the RME are suddenly restored and yield the full swing and nuance that the DAC portion of that unit is actually capable of. Such a comparison also makes it very plain that the SPL amp is not coloring the sound at all.
I’d consider this a “reference” pairing.
Pairing a $2,499 amplifier with a $300-400 headphone (depending on where/when you buy) is probably not the most obvious setup. The HD650 is known for its ability to scale, and to make the most of better sources and amplification, and with the Phonitor x they really come alive and sing.
There is absolutely no sense of “veil” with the HD650 driven out of the SPL unit. This is true whether using a pair that hails from their year of introduction, or a more recent copy (early 2016). Micro-detail and raw resolution are brought out fully, bass delivery is tighter than expected (quite welcome) and seems to delve deeper. Percussion is crisp, plucked strings have fantastic attack, and the top-end is rendered in a very and natural smooth manner - yet with a good sense of air, appropriate sparkle and no sense of being rounded nor rolled off.
The somewhat intimate stage of the HD650 can be expanded, usefully, with the Phonitor x’s “matrix” function, and the effect here is more immediately obvious than with some other headphones that have a naturally more expansive image.
This is the best I’ve heard the HD650 from any solid state amplifier.
To get better I think you have to look to higher end tube-based amplifiers, particularly some of the more capable OTL designs (that are at their best with higher impedance cans like the Sennheisers).
ZMF Eikon (Padauk)
In combination with the RME ADI-2 DAC (in particular), the ZMF Eikon delivers a performance from the Phonitor x that made me look at them in a different light. Somehow, the normally slightly warm and softer, mellower, sound of the Eikon (vs. something like the Sennheiser HD800S or Focal Utopia), gave way to a cleaner bottom end, with less mid-range bleed-in and that comes across with less emphasis. Additionally, the Eikon’s apparent resolution improves and the end result is a presentation with greater clarity than I expect from these cans.
Even with other DACs, the Eikon opens up and provides a more coherent overall experience with the Phonitor x than I have found when pairing it with other solid state amps (e.g. the iFi Audio Pro iCAN). The RME/SPL/ZMF trio threatened to consume a disproportionate portion of my overall listening time and I had to exercise some discipline to continue evaluating the amp with a broader set of headphones and sources (a third, back-to-back play through of Kate Bush’ “Before the Dawn”, was probably a good indication that I _really_ liked this setup).
The SPL/Focal pairing really demonstrates just how resolving and dynamic both pieces are. And it does so without exciting the Utopia in undesirable ways - with some other very resolving solid-state amplifiers, I find the Utopia can get a bit hot in the treble. The presentation here remains clean, detailed, dynamic, fast and even with some very tense female vocal work, there is no loss of composure or harshness evident.
The Utopias are known for projecting a very minimal stage, especially if you compare them to headphones like the Sennheiser HD800(S). Again, the “Matrix” function on the Phonitor x allows you to compensate for this when you want to. It’s not a perfect solution, but with the right music, it is surprisingly effective.
Feeding the Phonitor x with a Chord DAVE, and then driving the Utopia, yielded no apparent loss of resolution or transparency and is, at this juncture, the overall best all-solid-state/dynamic headphone combination I’ve heard to date.
Additional Thoughts, Quirks, and Issues
A few things came to mind or were uncovered, during my auditioning/testing of the Phonitor x. These don’t relate to how the unit sounds, at all, but are more aligned with the operation of the unit and/or it’s configuration:
- It would be nice to be able to set the VU meter attenuation independently by input. The internal DAC calls for it to be in its default, unattenuated setting or the needle barely moves. With an external, balanced, studio DAC, I need the 10 dB attenuator engaged or the needle spends most of its time buried in the opposite side of the scale. Since the control for this is via a dip-switch underneath the unit, changing it on the fly is not really practical.
- The optional DAC module seems to have issues with players that do automatic sample rate-switching under macOS 10.13.5 (High Sierra) when using the USB input. Setting the bit-depth and sample rate manually, via the Audio-MIDI Setup tool, prior to playback, alleviates this issue. The TOSLINK and COAX inputs are not affected and switch rates seamlessly and reliably. This is not really an issue for studio use but will affect music-lovers with collections that include music of different sample rates.
- You MUST turn down the volume or, better still, engage the “mute” function when connecting/disconnecting single-ended headphones via the standard 1/4” (6.35mm) jack.
Simply put, the SPL Phonitor x is the best “sounding” solid-state headphone amplifier I’ve heard.
It maintains a neutral, accurate, extremely transparent rendering while providing ample drive to the most demanding headphones I own. It yields a huge sense of scale and power with effortless delivery, has excellent tonality AND tonal weight, transient response is superb and it plies its trade against a dead-silent, void-black, background.
It is as close to the notionally ideal wire-with-gain as I am able to audibly assess, and yet, unlike other amplifiers that might qualify for a similar description, does its thing without robbing music of life and enjoyment.
Its features, and sonic performance make it easy to integrate into studio scenarios and still able to satisfy audiophile sensibilities.
And it is, so far, the only solid-state amplifier I’ve paired with the Chord DAVE, for situations where DAVE’s built-in headphone amplifier doesn’t quite have the drive/grunt I want, that doesn’t result in any noticeable loss of transparency.
The optional DAC module is a useful convenience feature, but I’d reserve its use for monitoring purposes. Its performance is not out of line for $300, but that same amount, spent elsewhere, will almost certainly yield better results - more in keeping with the capabilities of the amplifier section - and with no rate-switching quirks.
If you’re looking at high-end headphone amplification, then the SPL Phonitor x really deserves to be on your audition list.
I am sufficiently enamored with the Phonitor x that I have decided to buy one; which is the highest recommendation I can give any component. It’ll replace the iFi Pro iCAN that I was previously using for solid-state duties in my primary rig.
-Ian Dunmore (@Torq)
Buy the SPL Phonitor X on Headphones.com here at the best price available.
- I have an EMI recording of Saint-Saëns “Introduction et Rondo Capriccioso, Op.28 for Violin and Orchestra in B minor”, that is an amazingly vivid demonstration of this ability (of course, you’ll need a suitably resolving source, too). ↩︎
- Similar, but much less sophisticated, systems have existed in some headphone amplifiers for decades … the first I encountered was in the HeadRoom Supreme. ↩︎
- A common, English, very strongly flavored, savory, yeast-extract spread, often applied to toast. It is generally regarded as something you either love or hate. ↩︎
- No drivers required for OS X/macOS, Windows 10 (Creators Edition or later) or Linux. ↩︎