SPL Control One & Marc One

SPL combine their acclaimed headphone amps with monitor control facilities and USB audio interfacing.

There are currently several affordable but good‑quality standalone monitor controllers on the market, but two recent additions come from the German manufacturer SPL. Part of their Series One range, the Control One and the Marc One are almost identical but the Marc One also includes a USB connection and stereo A‑D/D‑A converters; lots of monitor controllers have a D‑A and often USB connectivity to allow playback from a computer, but unlike most the Marc One can also serve as a two‑way line‑level audio interface. Both units also boast a good headphone amplifier, and a streamlined implementation of the crossfeed facility found on SPL’s Phonitor.

Overview

Both are desktop units with a vertical control panel on the front, and most signal and power connections plug into the rear; the single headphone output is sensibly placed on the front. They measure 210 x 50 x 220 mm and weigh around 1.5kg, so stay put on the desk well. A supplied wall‑wart PSU accepts any mains voltage between 100 and 240 V AC, outputting 12V DC at 1.5A via a coaxial plug. This is a Class‑2 power supply: there’s no direct connection between the monitor controller’s chassis or audio grounds and the mains safety earth, so ground loops shouldn’t be a problem, even with unbalanced connections. Internal DC‑DC converters transmogrify the 12V supply into ±17V rails for the audio circuitry, for a decent headroom margin. The coaxial power inlet is not a locking type and there’s no cable retainer either, but the power plug did remain securely in position during a week’s use on my desktop.

The rear‑panel audio connections comprise two stereo analogue line inputs, two stereo speaker outputs (one with a Sub output), plus a stereo line output. The Marc One model supplements these facilities with a USB B‑socket for connection to a computer, tablet, or smartphone.

All‑analogue connections are via quarter‑inch TRS sockets, but not all offer balanced operation. Line Input 1 and Speaker Output A (plus the associated ‘sub’ output) are all balanced, while Line Input 2, Speaker Output B and the Line outputs are all unbalanced. SPL argue there are technical benefits in using unbalanced (ie. ground‑referenced) connections in applications like a monitor controller, and there is some validity in that in specific circumstances. Yet, it also increases the risk of introducing ground‑loop problems, and I wonder if some potential purchasers might be put off at the idea of connecting their balanced‑input monitors to the Control One’s unbalanced outputs. However, in practice, sources and speakers with balanced I/O can usually be connected to unbalanced equipment without problems. The rear panel’s labelling indicates the use of TS plugs for the unbalanced I/O but, sensibly, they are TRS types with the ring terminals linked internally to the sleeve terminals (rather than just left floating), so TRS cables can be used reliably.

Plugging into just the left input sockets of the Line 1 or 2 inputs provides a dual‑mono signal, thanks to some internal normalling between corresponding left and right sockets — a useful convenience should you need to monitor any mono sources.

A pair of recessed DIP switches is located near the power inlet. The first attenuates the signal level to the speaker outputs by 10dB, which is a helpful option if using high‑sensitivity (or consumer) monitor speakers. The second DIP switch serves no function on the Control One but on the Marc One it determines the A‑D’s input source, and thus the signal that’s sent to the computer over the USB connection. The default mode routes Line Input 1 to the A‑D but operating the DIP switch sends a mix of both Line inputs 1 and 2 so that either source can be captured into the computer. The A‑D converter has a fixed alignment of 0dBFS = +15dBu, the idea being to allow budget analogue equipment to operate at moderate levels without incurring unwanted analogue saturation, coloration or distortion, yet still achieve a digital full‑scale signal. I did question the lack of options here for users expecting to work with professional headroom margins but SPL suggested that wasn’t their main target audience for this particular product.

The rear of the Marc One (bottom) is nearly identical to the Control One, but includes a USB connector for bidirectional audio interfacing.

The stereo line output sockets provide a buffered version of the selected monitoring signal (pre‑volume control). This is intended to feed an external headphone distribution amp or, potentially, a master stereo recorder. The two sets of stereo speaker outputs are self‑explanatory, but the Sub output associated with Speaker A is a mono‑summed (left plus right, ‑6dB) post‑volume‑control output. As there’s no low‑pass filtering involved, this is a full‑bandwidth mono signal, so it can be used to feed either a subwoofer (if it has its own low‑pass filter) or a mono‑check speaker.

On the front, a three‑way toggle switch selects the destination speakers: A+Sub, Off, or B — the Off mode is convenient when wanting to use the headphones without moving the main volume control, or for muting the system to answer a phone call, etc. (There is no speaker Dim button.) A second three‑way toggle selects the monitoring format which affects all outputs (speakers, headphones and line outputs): Mono (with 6dB summing attenuation), Stereo or Reverse Stereo. The last of these, an L/R swap function, was apparently requested by users who found it an enormous help when listening through sample libraries. I can see the convenience for that specific application, though I’d have liked also to have the ability to audition the stereo difference signal (Left minus Right), which I personally find much more useful for evaluating stereo imaging, mono compatibility, lossy‑codec damage, and aligning stereo channels and mic arrays.

A red power‑on LED sits in the centre of the panel, while over to the right are the input source selection and headphone controls. The headphone output is presented on a single quarter‑inch TRS socket accompanied by two small rotary controls to adjust the Crossfeed level and the overall headphone volume, independently of the loudspeaker’s Volume control.

The Crossfeed system employs the same Phonitor Matrix as is used on other SPL devices, but with preset values for the speaker angle (30 degrees) and centre level (‑1dB). When fully anti‑clockwise the Phonitor matrix is relay‑bypassed, but as the control is advanced the relay routes the headphone signal through the matrix with an increasing interaural level difference. Basically, this controls the amount of opposite‑channel bleed (with delay and high‑frequency attenuation characteristics to replicate typical head‑shadowing effects). This Crossfeed technique can be very helpful in making the headphone listening experience much more similar to that of listening to loudspeakers, with the aim not only of delivering a more pleasant listening experience, but also improving the translation of mixes made using only headphones. Once familiar with the benefits of the Phonitor Matrix for headphone listening, it’s not something you’d want to be without!

On the Control One, the desired monitoring source is selected with a three‑way toggle to the left of the Headphone Volume control, with options for Line 1, Line 2 or a (unity) mix of both. The Marc One takes a different approach, to accommodate the USB input: instead of a switch, it employs a small rotary control marked Monitor, which blends between a (unity) mix of the Line 1 and 2 analogue inputs at the anti‑clockwise end and the USB D‑A converter’s signal at the clockwise end. The Marc One also includes a couple of extra LEDs either side of the power‑on LED which illuminate if the internal A‑D converter is overloaded.

The Marc One... can accommodate PCM sample rates up to 768kHz, as well as the DSD64 to DSD256 formats.

Technicalities

The maximum line input and line output levels are specified as +22.5dBu, and although the Speaker outputs start to clip slightly earlier at +21dBu there’s plenty of headroom here. The frequency response measured ruler flat from 10Hz up to around 50kHz, falling by ‑0.5dB at the 80kHz limit of my AP measurement system.

Cheap rotary potentiometers used as volume controls typically suffer from poor channel‑matching, particularly at higher attenuations. Happily, then, the component employed in the Control One is remarkably good, maintaining excellent matching of ±0.1dB from the 11‑o’clock position upwards, and only degrading to around ±0.5dB for most lower settings.

The volume control’s close channel matching is impressive.

I measured the signal‑to‑noise ratio at 102dB (A‑wtd), with a 0dBu input and the Volume control at maximum, giving a total dynamic range capability of around 124dB, which is commendable. Crosstalk between channels measured an excellent ‑74dB at 10kHz, and even better at lower frequencies.

As with most USB interfaces, the Marc One is class‑compliant, and no driver is required for Mac OS or iOS. It will run on Windows without a driver at base sample rates (44.1 and 48 kHz) too, but an ASIO driver can be downloaded for Windows 7‑10 (32‑ and 64‑bit) and must be used when higher sample rates are required.

AKM’s Velvet Sound converter chips are used in the Marc One, specifically the AK5552 A‑D and AK4490 D‑A, which employ 32‑bit processing and can accommodate PCM sample rates up to 768kHz, as well as the DSD64 to DSD256 (DOP) formats. These are the highest sample‑rate formats in current use and few other products match this capability.

A peep inside the Marc One, with the interfacing daughter card visible bottom right.I find that AES17 dynamic range measurement is a useful indication of converter system quality and, running the test in an A‑D‑A configuration using my Audio Precision test set, the Marc One measured 112dB (A‑wtd), which is fractionally below the quoted specification of 113dB. I’ve measured other devices with the same AKM chips that achieved better results, so there’s probably room for improvement here, though it’s unlikely to be an issue in practice. The round‑trip total harmonic distortion and noise (THD+N) measured 0.0012 percent, which is a good result.

The headphone amplifier in both units is clean and capable, delivering around 400mW per channel into low‑impedance headphones (30 to 50 Ω) while still managing 330mW into 250Ω models. It certainly went more than loud enough with all the headphones I have here. The bypassable and variable Crossfeed feature is very useful, largely because I find it makes extended headphone listening less fatiguing.

Impressions

As we’ve come to expect from SPL, the Control One and Marc One are beautifully constructed and solidly engineered products, which feel reliable and long‑lasting under the fingers. They offer useful speaker‑selection facilities and that alone will satisfy many users, but as mid‑market monitoring controllers go the range of facilities is relatively basic.

The Control One in particular faces a very competitive marketplace and while few other monitor controllers offer the SPL’s direct output (which may be a critical requirement in some workflows) or the headphone Crossfeed facility, some other manufacturers offer products with similar technical performance and more facilities at a lower cost (see the 'Alternatives' box). That said, I can’t argue with the subjective sound quality of the SPL products: a monitor controller has to be completely transparent and these are exactly that!

The Marc One is in a different position because there are fewer affordable monitor controllers that include USB replay facilities... and fewer still that can serve as a bidirectional USB audio interface...

The Marc One is in a different position because there are fewer affordable monitor controllers that include USB replay facilities, let alone models which support such a wide range of formats, and fewer still that can serve as a bidirectional USB audio interface, even if its published technical performance isn’t as good as some interfaces that cost less. However, while the inclusion of USB replay is welcome, I found it slightly restrictive that the Marc One’s D‑A has no provision for external digital inputs, such as S/PDIF; I suppose many of us are using fewer devices with such outputs now (streamers, CD players, and the like) but I think plenty will still have that need.

Choosing a monitor controller is quite a personal matter because users naturally have different requirements and expectations. For me, while the Control One does perform well and sounds good, its relatively high price and restricted range of signal‑checking facilities make it difficult to recommend unconditionally in such a strongly competitive field. Having said that: it is a neutral, accurate and transparent‑sounding controller, which is its raison d’être; the Volume Control’s channel‑matching is very impressive, and the useful inclusion of direct outputs could make it a perfect solution for some. Its headphone amplifier is also extremely capable, and its crossfeed system is a genuinely useful selling point. The Marc One, on the other hand, has little direct competition at the price, not least because of the bidirectional USB interfacing, so it has obvious appeal.