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Schertler Arthur: Modular Mixer

Modular Analogue Mixer
Published December 2016
By Hugh Robjohns

Schertler Arthur: Modular Mixer

We built and then test-drove the world’s first commercially available fully modular console. Here’s what we thought...

Swiss company Schertler already have an enviable reputation for their PA systems, instrument pickups and amps, but with their latest product, the Arthur Format48, they’ve struck out in a radical new direction. There have been plenty of semi-modular mixers arranged around single-channel modules or ‘buckets’ of eight or more channels, and it has recently become fashionable to integrate slots for 500-series modules. But all these are based around a fixed chassis, even if different chassis sizes are available.

In complete contrast, the Arthur has been conceived to be fully scalable, all the way from a single input channel plus master output, to a sophisticated console with up to 80 different modules. The key to this flexibility is a mechanical ‘chassis’, formed as you fasten the modules together; the chassis grows as more channels are added! Thus, the customer can choose how many of which modules to include, and can assemble them in almost any order — and you can see a graphical preview of the specified system via the ‘Configurator’ on Scherter’s web site.

This ingenious mechanical design is only part of the Arthur story, though. Another key selling point is its claimed premium sound quality, courtesy of all-discrete, DC-coupled (there are no DC-blocking capacitors in the signal paths), Class-A transistorised circuitry, which has been designed with ‘zero negative feedback’ (NFB) throughout. It’s extremely difficult to design active circuitry that genuinely omits all negative feedback completely (current- as well as voltage-based), and in my experience most ‘zero NFB’ designs, including this one, turn out to be what I’d consider ’minimal NFB’ designs. But semantics aside, Schertler’s design intention is clear, and the absence of significant amounts of negative feedback is evident in both the mixer’s sound and technical specifications.

Every pro-audio product I’m tasked with reviewing usually has a clearly definable target market, and my review focuses on how well the product meets that market’s needs. Yet, while it’s clear from the outset that the Arthur’s design has been influenced strongly by a desire to satisfy the practical requirements of musicians and small bands (resulting in a relatively simple console architecture), this mixer’s nature means there are multiple and very diverse potential markets, some of which probably haven’t yet occurred to me! In between the one- and 80-channel extremes, there’s potential for building almost anything from a small, on-stage musician’s mixer, to a compact bespoke location-recording console, and there’s yet more potential in larger-scale live-sound, theatre, or studio-recording arenas. It also has attractive possibilities as a DAW front-end or summing mixer. By way of example of a smaller setup, Schertler tell me they’ve recently sold a simple dual input channel configuration to a guitarist who wanted a high-quality recording channel with monitoring facilities.

Here, then, I’ll leave you to imagine the many possibilities and evaluate the Arthur based on its merits in terms of technical performance, ergonomics, functionality, and, importantly for a modular product, ease of assembly.

Ins & Outs

The only mandatory module is the Master LR Output, which is the only unit containing the stereo mix-bus summing amps, and which also hosts the primary power inlet. For simple applications, where one aux is sufficient and separate monitoring unnecessary (such as for stage source selection or a DAW front-end) you won’t need, or need to pay for, the Aux Master module, but in practice most customers will want to add it: it expands the number of aux outputs and provides basic monitoring facilities.

On the input side, the module options presently comprise two different mono Microphone In channels, an Instrument channel and a Stereo Line input channel. Although systems with as many as 80 modules can be assembled, there is, as yet, no subgrouping facility on the input modules, nor any subgroup buses or bus-selection facility on the output modules. Consequently, even 40-input configurations would prove challenging to manage and control in a straight live-to-stereo mix situation.

However, an advantage of the modular approach is that Schertler can develop new modules that can be incorporated into existing setups. Designer Stephan Schertler tells me a subgroup facility will follow soon, which is welcome news (though I’m intrigued to see how this will work with the existing modules). I gather that there are also plans for (mono) spring reverb and stereo EQ modules, the latter intended for use with the Master LR Output module.

Three power-supply options are intended to energise consoles of up to 12, 25, or 80 modules. The two smaller PSUs are ‘line-lump’ switch-mode units, which are relatively low-cost options. The largest unit is a chunky linear design with three separate power outlets — the multiple outlets are because the integral power-bus connections between adjacent modules have a limited current capacity and so, to get around this practical limitation for large configurations (more than about 30 modules), additional bespoke Power Input modules are required. These interrupt the internal power-bussing system so that an additional PSU (or PSU output) can feed a subset of modules to the left of a Power Input module.

Sensibly, the power rails of each module are double-filtered with Darlington-transistor regulators, providing mains-harmonic filtering of around -80dB at 100Hz and -60dB at 50Hz. Nonetheless, Schertler recommend the large linear PSU even for small consoles, as, compared with the line-lump options, this reduces the console’s noise floor by 10dB.

The Arthur provides direct channel routing only to the main stereo mix bus, along with three independent aux sends plus a channel PFL, which can be employed alternatively as a fourth fixed-level aux send. The mic and instrument modules are equipped with versatile preamp sections, three-band EQ plus some specialised filtering options, an unbalanced insert point (which can be configured as a direct output), three independently switchable pre/post aux sends, a pan-pot, a long-throw fader, and simple ‘traffic-light’ style metering.

The Master LR Output module features a power input socket, long-throw faders for the stereo mix outputs, and a stereo LED bar-graph meter with a VU response, which permanently monitors the mix bus. It also provides the aux 1 master output and has a mono effects return feeding the mix bus, and each of these has its own short fader. The separate Aux Master module is required to access the other two aux sends and the PFL bus, and this also provides headphone and control-room outputs, plus a talkback facility.

Optional accessories include wooden or black console side-cheeks, and spare fader knobs, control knobs and assembly hardware are available. The literature also mentions a wooden armrest, though this wasn’t yet available when writing this review.

Possible configurations range from small consoles, like this, to 80-module behemoths!Possible configurations range from small consoles, like this, to 80-module behemoths!

Teething Problems

As I received one of the very first production ‘kits’ to leave the factory, it’s perhaps not surprising that I encountered a few ‘teething problems’. For example, there were a few translation errors and some inconsistencies between the printed and online literature, the diagrams, and the physical products, and some issues with the solvents used in the packaging. Thankfully, such frustrations are minor and quick and easy to fix — I’m assured that most already have been. A few issues relate to design decisions, and some of these may take rather longer to resolve, though I’ve been greatly impressed with Schertler’s response to my findings, and I’m told many improvements have already been made in the latest batch of production units.

Because of all this, unless I’ve stated otherwise, this review features my own test-bench measurements and specifications derived from the early production modules, rather than those published by Schertler; future production units may well exhibit improved specifications.

Construction & Assembly

The ‘Format48’ name is derived from the 48cm length of the modules. I understand a more sophisticated console with larger, 60cm-deep modules is already on the drawing board, and presumably that will be the Arthur Format 60. Each module weighs 0.5kg and measures 36 by 475 mm. Impressively, the channel modules are remarkably shallow, at just 45mm to the top plate, and 58mm to the tops of the knobs, and the mechanical construction is both sturdy and elegant. In each module, the top and bottom end caps form the core mechanical strength. These are CNC-machined from aluminium, which is black-anodised. Stretching between these caps are a laser-printed silver aluminium faceplate and a black baseplate, supported by three rubber feet. Each plate is folded into a U-channel, making the whole structure strong enough to support all the controls, connectors and electronics. There’s no sign of any flexing in use.

A single, horizontal circuit board runs the length of the baseplate, and the rotary controls, connectors and buttons are mounted directly on it. The majority of components are surface-mount devices (SMDs), although there are lots of through-hole capacitors too. The faders appear to be ALPS N-Type carbon-track units and the pots all have metal shafts. All the control buttons are illuminated, and most change colour to indicate status but, curiously, most seem to have been installed ‘upside down’ — the illumination status is much more obvious if viewed from the connector rather than the fader end, but perhaps that’s intentional? All audio-signal and DC-power connections are located on the top panels.

Assembly was obvious and easy; I put together the review console in under 10 minutes without guidance. (I only discovered later that full assembly instructions are included in the Master LR Output module’s manual!) Adjacent modules are fixed together by inserting short metal tabs into slots in the top and bottom end-caps, and securing them with large countersunk hex screws. It’s a simple, yet immensely strong system, and the completed console felt extremely solid. Importantly, this construction method makes it is equally simple to split the console to add, move or remove modules.

Cleverly, all the necessary electrical connections between modules are made automatically, thanks to pairs of compact 16-way connectors on each module’s PCB near the bottom of the fader. The right-hand side of each module carries a socket and the left the corresponding plug, and these daisy-chain all the power lines, grounds and mix buses across the entire console. All the sockets aligned perfectly.

Warming Up

The Arthur’s manual mentions that the internal power regulators need about two minutes to stabilise and recommends about 30 minutes before doing serious work. This matches my experience — the faders and other controls often sounded quite ‘scratchy’ and ‘noisy’ immediately after powering-up, probably due to unwanted DC voltages appearing across the controls before the circuitry had fully stabilised — that’s inevitable, given the deliberate omission of DC-blocking capacitors and the ‘zero negative feedback’ design. As the circuitry’s DC conditions stabilised over the first 20-30 minutes, though, these unwanted noise issues resolved themselves.

It’s perhaps no surprise, given the discrete-transistor nature of the circuitry, but it’s worth mentioning that the Arthur gets pretty hot. I measured consistent module top-panel surface temperatures of around 40 degrees Celsius (104 degrees Fahrenheit) after an hour, and with just six input and two output modules here I could sense the heat on my face as I worked. Schertler acknowledge this heat but point out both that it does no damage to the circuitry and that the temperature doesn’t rise with the addition of more modules, even in 80-module configurations. These are fair points, but a larger console inherently means a larger radiating surface!

Microphone Inputs

The Instrument In module includes a  rare and useful facility: variable input impedance.The Instrument In module includes a rare and useful facility: variable input impedance.The two mono Microphone In modules differ in both price and technical performance. Externally, both versions appear identical other than a ULN logo near the fader of one. The standard module’s mic preamp has an Equivalent Input Noise (EIN) of 122.5dB, which is okay but nothing really to write home about. For €60 ($60-70) more the ultra-low noise (ULN) variant manages a far more impressive EIN of 128.7dB (both were measured with 60dB gain and a 150Ω source). So the standard module will be adequate for most live-sound applications but the ULN is far better for high-quality recordings.

An input XLR socket at the very top is joined by a number of illuminated buttons and controls below, which configure the input section, insert point, and channel EQ. Most buttons have white background illumination when the channel is turned on and change colour when activated. The first of two buttons provides polarity inversion (green when active) and a ‘+15dB gain boost’ (green), which alters the gain structure of the preamp itself (more on this later). Three more buttons enable phantom power (orange), a high-pass filter (orange) and the insert return (red). On the review units, a fourth button (green) was labelled Mic/Line. Actually, there isn’t a separate line input or even a dedicated line mode as such, and it doesn’t work like an input pad, either. Much like the +15dB button, what this feature does is alter the preamp’s gain structure, this time to better accommodate high-level input signals. Schertler tell me that on the latest production models that button’s label is ‘High/Low Gain’, which more closely reflects what it does.

The high-pass filter imposes a 12dB/octave slope with a -3dB turnover point at 90Hz. With the filter bypassed the basic channel’s LF response is -3dB at about 15Hz. In fact, the overall channel bandwidth rolls off at 6dB/octave below 15Hz and above about 75kHz, which is very good. (I measured the -1dB bandwidth limits, relative to 1kHz, at 25Hz and 60kHz.)

Very unusually — it’s been a long time since I’ve measured a mixing console that didn’t have a ruler-flat frequency response — the mid-range exhibits a distinctly non-flat frequency response, with a modest HF plateau and some mild LF scooping. Schertler tell me this is due to the absence of NFB, and that they believe the mildly uneven response (since improved to ±0.3dB) is quite acceptable given all the other benefits of their ‘zero-NFB’ circuitry. I wouldn’t disagree, but it’s worth noting that this modestly non-flat response lends a very subtly ‘forward’ or ‘lean-sounding’ character.This AP plot shows the frequency response from a  ULN mic/line channel to the Arthur’s main outputs, and shows a  mild but audible HF plateau and slightly recessed lower mid-range. The two traces are with and without the channel HPF engaged.This AP plot shows the frequency response from a ULN mic/line channel to the Arthur’s main outputs, and shows a mild but audible HF plateau and slightly recessed lower mid-range. The two traces are with and without the channel HPF engaged.

I measured the phantom power voltage as a little low off-load but still within spec and, importantly, it remained so even at full load, confirming a solid power supply. When selected, the mic module’s phantom power is present on the input XLR regardless of the preamp gain settings, though; a lot of line-level equipment doesn’t respond well to having 48V across its outputs, so I strongly advise double-checking the phantom status before connecting line sources.

This AP plot illustrates the ULM mic/line module’s equaliser responses, with high (blue) and low (red) EQ bands at maximum boost and cut settings. The black line is the response with the EQ section inactive.This AP plot illustrates the ULM mic/line module’s equaliser responses, with high (blue) and low (red) EQ bands at maximum boost and cut settings. The black line is the response with the EQ section inactive.The preamp’s rotary gain control has no numerical scale markings because the gain range varies according to the Mic/Line (high/low gain) and +15dB button settings. I measured the standard gain range as +17 to +45 dB in normal Mic (now High) mode, extending to +17 to +60 dB with the +15dB button activated and reducing to -8 to +20 dB with the Line (now Low) button pressed. Confusingly, the +15dB button has no effect when the Line/Low mode is selected. Like the majority of continuously adjustable mic preamps, the Arthur exhibits some gain-bunching at the loud end of the gain control, particularly in the normal (High) and +15dB settings.

To put some practical signal-source level extremes on all this, when switched to +15dB, with the channel and master faders both at their 0dB marks, the lowest mic signal that can be raised to a nominal +4dBu at the mixer output is -56dBu, and the loudest is -12.5dBu. In Low/Line the numbers change to -17dBu and +13dBu. So most typical sources can be accommodated without difficulty.

A TRS socket provides access to the unbalanced direct output/insert point, but because this is located below the module’s input section the connecting cable ends up draped across some controls —this could be awkward if using the console to feed a number of direct outputs to an external recorder. The position of this socket is a consequence of the circuit-board layout. Schertler acknowledge this may be less than ideal in some installations, and now offer module variations in which all input and output connections are accessible via fixed cables wired directly to the module PCBs, as well as via the physical connectors — this way, the cables can be wired to an external patchbay.

Each input-channel module, irrespective of the type, features three dedicated aux sends, plus a PFL which doubles up as a fourth (pre-fade) aux send.Each input-channel module, irrespective of the type, features three dedicated aux sends, plus a PFL which doubles up as a fourth (pre-fade) aux send.

Regardless of this minor interfacing issue, the ability to bypass the insert return point is very nice, as it allows the insert socket to be employed safely as an unbalanced direct output (post-input stage and HPF, but pre-EQ) without having to resort to the often unreliable ‘plug half-out’ trick. The direct/insert send output level is the same as that reaching the main outputs when the channel and main faders are at their 0dB marks.

Just above the mic modules’ EQ section, there’s another unusual feature: the bespoke ‘Reson’ notch-filter helps control feedback resonances from acoustic instruments, which is something Schertler are very experienced with. The notch’s depth is adjustable up to about -8dB and you can choose between 150 and 240 Hz centre frequencies. The bandwidth is fixed at about one octave, and the Reson filter is not bypassed with the main EQ.

Speaking of which, the three-band EQ is activated with another illuminated (green) button. The high and low bands effectively have shelf responses with -3dB points at 3.5kHz and 100Hz, and their boost/cut range is ±15dB, with centre detents at unity gain. The mid-band section has a slightly smaller ±12dB range with symmetrical boost/cut curves. A continuously adjustable centre-frequency control spans 250Hz to 3.2kHz, with its middle position at 500Hz.The responses of the ULN mic/line channel EQ’s mid band, with the frequency control at the minimum, centre and maximum positions corresponding to 250, 500, and 3200 Hz. The control range is roughly ±12dBThe responses of the ULN mic/line channel EQ’s mid band, with the frequency control at the minimum, centre and maximum positions corresponding to 250, 500, and 3200 Hz. The control range is roughly ±12dBBasic LF and HF bell sections for the stereo channel EQ.Basic LF and HF bell sections for the stereo channel EQ.

A ‘Unit On’ button on the mic module lights green when active. When off, there’s around 74dB of attenuation at 1kHz and 54dB at 10kHz. That’s not great and some HF crosstalk may be audible, but closing the fader produces a substantially better (75dB) attenuation at 10kHz. When the channel is turned off, the stereo mix-bus and aux outputs are muted but the PFL output remains available and the channel’s LED metering continues to work normally. To further aid recognition of this ‘off’ state, the illumination of most buttons in the module’s top half (controlling the input section and EQ) are extinguished, with only the insert button remaining lit.

Progressing down the module, the three aux-send controls each have an illuminated pre/post button (blue for pre orange for post). Experimenting with these aux sends I noticed a marked level difference between the pre and post modes when the fader was at the 0dB mark — the post-fade mode was 6dB quieter than pre-fade. Schertler assure me that this has been addressed in current production modules.

Next comes the channel pan-pot feeding the stereo mix bus, and on the review modules I found this had a very strange operation: any pan settings between the nine and three o’clock positions were barely noticeable, and all the image movement occurred in a rush over the last few degrees of rotation. As this is clearly not very practical other than for hard-panning sources, I raised the matter with Stephan Schertler, who explained that the original pan-pot circuitry was designed with minimising noise and distortion uppermost in mind, but he agreed that its behaviour was not ideal. Soon after, he told me he’d conceived of a far better arrangement using a complex ganged log/anti-log potentiometer, saying the resulting pan-pot law is now “perfect”. I’ve not been able to test this design yet but the theory works, and it should be implemented in all new input modules by the time you read this review.

Completing the module is a 100mm ALPS channel fader, with a smooth and light action. The 6dB gain margin above the 0dB mark is less than most consoles provide (10dB) but I doubt anyone will find that problematic. Near the top of the fader is a PFL button (lit white at rest and red when activated), which routes a pre-fade signal to the Aux Master module, from where it can be auditioned at the headphone output or via a dedicated output socket.

Alongside the fader, a column of three square LEDs are marked ‘Signal’, ‘Nominal’ and ‘Peak’. The first starts to illuminate green for signals around -30dBu (at the output), getting progressively brighter until around -6dBu when the Nominal light (also green) starts to glow. The red channel peak light comes on at +4dBu, which is rather lower than I’d expect, but it does encourage the maintenance of a healthy headroom margin! Again, Schertler tell me this relatively sensitive ‘peak’ metering is intentional; it uses an averaging VU response and so tends to miss brief transients on music signals.

Stereo Line Input

The Stereo Line Input module has balanced TRS and unbalanced RCA-phono sockets, wired in parallel, for the left and right signals. Independent left and right gain controls and (orange) buttons to select fixed unity gain are provided. I measured a gain range of -15 to +23 dB relative to the unity gain mode, with the lowest balanced input level able to deliver +4dBu at the outputs (with the channel and main faders at the 0dB marks) being -23dBu. The highest input to produce +4dBu at the output (faders at 0dB) is +16dBu. The unity mode delivers +4dBu at the output for an input level of 0dBu. Switching to the phono sockets, a +4dBu output is achieved with an input of -2dBV when the unity button is pressed, or anything between +14dBV and -24dBV if the gain control is used.

A two-band stereo EQ comprises bell sections centred at 60Hz and 8kHz, and a Unit On button again mutes the main stereo and aux outputs. The different control layout of the Stereo Line module places this button much further up than on the mic modules, helping to differentiate the module types visually. The level of attenuation with the channel turned off was just 61dB at 1kHz and 41dB at 10kHz, again rather lower than I’d expected. Closing the faders produced -74 and -61 dB at 1kHz and 10kHz respectively, which suggests some crosstalk problems on the circuit board around the Unit On switch.A set of frequency responses from the Instrument channel module, with HPF (red) and Warm filters (blue) compared to the flat channel (black).A set of frequency responses from the Instrument channel module, with HPF (red) and Warm filters (blue) compared to the flat channel (black).

There’s more unconventionality in the aux sends, because the left and right inputs each have independent controls, allowing routing to the three aux sends at different levels, and with different pre/post settings — I’ve not seen this elsewhere. An unavoidable consequence of this flexible arrangement is that the aux controls don’t align with those of the mic modules, which is a potential source of confusion.

The separate faders for each channel of the stereo signal are useful, since there’s no balance control; any necessary stereo image adjustments must be done either with the input gain controls or by offsetting the faders. A stereo PFL button is joined alongside the fader by a mono-sum button, but the summing is actually performed directly after the input gain stage, so it affects the pre and post aux sends too. A lack of space around the faders prevents the inclusion of a meter, so channel levels can only be set visually by fading the stereo channel up to use the main output meters.

Instrument Input

A third type of input module sports the name Yellow, but has a distinctive blue-anodised colour scheme and is dedicated to mono instrument sources. Most controls and the panel layout are as the mic modules, with only the input facilities above the EQ section being altered to suit the different source. The input connector is an unbalanced TS socket, and there are illuminated buttons for polarity inversion, a -15dB input pad, and enabling a 10V DC power supply for Schertler’s electrostatic pickups and unbalanced electret mics. All three buttons light green when active. (Red might have been better for the 10V supply, which could have a deleterious effect on some guitar pickups.)

The only orange-lit button on this module activates a 100Hz, second-order high-pass filter, but the module’s native low-frequency turnover was around 40Hz. That’s surprisingly high and will affect the strength of the lowest notes of bass guitars and keyboards — at the time of writing, Stephan was investigating the possibility of using a larger input capacitor to extend the LF bandwidth. In contrast to the mic module, the Instrument module measured flat, with -3dB points at 40Hz and above 80kHz.

I measured an input gain range of +8 to +41 dB, allowing a maximum input level of +10dBu with the pad engaged (for +4dBu out with faders at 0dB), and a minimum of -38dBu without the pad. A useful variable input-impedance control provides a continuously adjustable load of between 22kΩ and 1MΩ. The lowest value is intended for active and electronic instruments, while mid-range settings (250-500 kΩ) suit standard electric guitars, and the highest provision will load piezo pickups correctly. This is a very thoughtful and pragmatic facility — it’s a shame more manufacturers don’t offer similar amenities.

An unbalanced insert point again serves as a direct output by default, with a button (red light) to activate the insert return. The channel EQ, aux sends, fader and metering are all exactly the same as on the mic modules, except that in place of the ‘Reson’ filter there’s a button labelled ‘Warm’. Active only when the EQ section is engaged, this applies a 6dB/octave low-pass filter to roll off the high-end above 2kHz. The intention is to polish out the nasty high harmonics which some instruments generate so unhelpfully!

LR Master Output

All Arthur configurations will require the LR Master output module (left), but most users will also want the Aux Master module too, which provides both the effects returns and basic monitoring facilities.All Arthur configurations will require the LR Master output module (left), but most users will also want the Aux Master module too, which provides both the effects returns and basic monitoring facilities.The LR Master output module contains the stereo mix-bus summing amps as well as the summing amp for aux 1, and the primary DC power inlet. The last is a mini-DIN type, located at the top of the module and capable of running up to 36 modules. Frustratingly, the DC plug’s close proximity to the output XLRs made it difficult to access the XLR release buttons; Schertler tell me this will be remedied by mounting the XLR sockets the other way up.

Intriguingly, it’s possible to install more than one LR Master output module. Although it’s not a sub-grouping facility (additional masters can’t be accessed directly by input modules) this allows duplicate, independently controllable stereo outputs. When used in this way, a switch on each additional LR Master module’s circuit board must be operated to disable its internal bus-summing functions and reconfigure the signal path to pick up a buffered signal from the primary LR Master output module’s summing amps instead.

Returning to the module’s socketry, directly below the output XLRs are two TRS sockets, one providing a balanced mono effects return to the stereo mix bus, and the other the balanced aux 1 output. Two eight-segment LED bar-graph VU meters dominate the module, scaled from +6 to -25, with the top red ‘+6’ segment lighting when there’s +10dBu at the outputs, implying a standard nominal calibration where ‘zero’ on the meter equates to +4dBu at the outputs (as per a standard VU meter). However, the lower meter segments appear to illuminate roughly 3dB higher than their panel markings, so the scale should probably read +6, 0, -5, -7, -10, -12, -14 and -20, rather than the current +6, 0, -6, -9, -12, -15, -18 and -25.

Below these meters, two 60mm faders control the mono effects return level into the stereo mix-bus and the aux 1 master output level. The effects return fader has 8dB of gain available (early modules’ panel markings suggested 6dB). A 0dBu input to the effects return socket delivers +8dBu at the output with the fader at zero. The panel legends for both faders are obscured when the fader knobs rest at their 0dB marks — although the effects-return knob is black and the Aux 1 send red, making identification straightforward with familiarity.

Just above the left/right long-throw mix output faders are two illuminated (green) buttons which turn on or mute their respective outputs, and the main faders can apply 6dB of gain.

Aux Master

Although the console will work perfectly well with just the LR Master output module, most customers will also want the Aux Master module. Six balanced TRS connectors provide balanced outputs for aux 2 and aux 3, PFL, talkback, and left and right control-room monitors. The control-room signal is derived from the stereo mix bus before the main faders; there are no facilities to select any external inputs or the PFL signal. In fact, the only facilities associated with the control-room outputs are a mono button and a volume knob, which is a massively missed opportunity. A fully equipped monitor control section, with external inputs and selectable outputs for multiple monitors, as well as facilities to mute L/R, monitor the Sides signal, introduce a polarity reverse, switch mono to a single speaker, mute a subwoofer, and so on, would have made the Arthur console a really attractive proposition to ‘book-end’ a DAW installation, combining high-quality direct recording inputs, artist cues mixes, and comprehensive monitoring facilities in one neat, scalable unit. That’s not currently a viable scheme, but perhaps Schertler will develop a more comprehensive monitoring module option in the future?

A separate headphone output is provided, with a standard TRS stereo headphone socket half-way down the panel. Again, it has its own independent volume control and defaults to reproducing the pre-fader stereo mix-bus output, but an illuminated button selects the PFL signal as an alternate source. That’s a little clumsy: hitting a PFL button on most consoles routes the PFL signal directly into the monitoring system, but the Arthur requires you to press two buttons, one on the PFL’d channel and another on the headphone monitor. This can be avoided if the dedicated PFL output is connected to an external monitor speaker, which has some merit, but that’s a level of complexity and expense that won’t appeal to many.

As with the LR Master module, the aux 2 and aux 3 outputs are controlled by short-throw faders, which are aligned horizontally with the Aux 1 output fader on the LR Master module. Their identifying panel legends are similarly obscured when their fader knobs are at the 0dB marks. Finally, a talkback section comprises a built-in mic, a gain control and a latching on button (green). When the talkback signal is activated it appears at a dedicated output socket at the top of the module, while a second button (orange) routes this signal to the aux 2 output too. Another short-throw fader controls the PFL signal’s output level via the socket at the top. As mentioned earlier, this is intended to allow the PFL bus to serve as a fourth (permanently pre-fader) aux send. Importantly, this doesn’t affect the PFL signal level reaching the headphone monitor.

Sound Quality

Schertler promote the use of discrete-transistor circuitry entirely free of coupling capacitors and with ‘zero NFB’ as one of the Arthur’s major selling points. Indeed, Stephan Schertler claims, “it will be hard to find a better sounding mixer,” and, overall, the Arthur does indeed sound very good. It has an open, airy, and detailed character, which certainly puts it a step above typical budget mixer designs, which often sound a little congested, especially when the output levels are pushed a bit! In contrast, the Arthur has good internal headroom margins, and the overall tonal character develops a musical richness and sense of drive and body when pushed, rather than sounding strained, or closed-in. Transients tend to sound a little sharper and crisper than in many conventional consoles too, and all-in-all it’s a very musically complimentary console. It may not have the best distortion specifications on paper but it’s certainly not bland or anaemic, and that may be a strong part of its appeal for many.Schertler Arthur: Modular Mixer.

Bench Tests

In my bench tests, using an Audio Precision analyser, I found that below about -18dBu at the output the THD figure stayed comfortably below 0.1 percent with a very even distribution of harmonic distortion products. As the output level was raised, the balance of harmonics shifted towards a more dominant second harmonic and the THD figure reached 0.15 percent at an output level of 0dBu. Pushing on to +10dBu at the output caused the THD figure to blossom to around 0.25 percent, and then onwards to 0.5 percent by +18dBu at the output, finally crossing the one percent THD threshold by +24dBu. In practice this means that the tonality varies slightly with signal level, as the balance and level of harmonic distortion components changes.

The output noise floor is essentially a smooth pink noise, sitting around -83dBu with the main faders closed and rising to -74dBu with the output faders up but all input channels closed. Fading up one Mic ULN channel set to 30dB of gain raises the noise floor to around -70dBu. While there are quieter consoles, I doubt anyone will find the Arthur’s noise floor intrusive.

I made some FFT plots (print-edition readers can find these in the online version of the article) of the mixer’s noise floor, first with no input signals and all channels faded down and second with a single ULN mic module faded up, with the preamp set to maximum gain. Both are quite impressive and revealing, in their own way, but note that many of the spikes visible on the second plot are due to mains harmonics being picked up on the input cable rather than by the mixer itself.

An FFT plot of the Arthur’s noise floor with all input channels faded down.An FFT plot of the Arthur’s noise floor with all input channels faded down.The specifications cite a maximum output level of +27dBu (with 0.5 percent THD) and this is achievable with the master faders raised to their +6dB end-stops. With the faders at their nominal 0dB marks the maximum output reduces to +22.6dBu. An inevitable side-effect of using output drivers with ‘zero negative feedback’ is a strong sensitivity to the destination’s impedance and consequently the Arthur really doesn’t like driving very low impedances. The measurements above were achieved with a load impedance of 200kΩ, but connect the Arthur to, say, a vintage Pultec EQ with 600Ω input impedance and the maximum output level falls dramatically to around +1dBu (at 0.5 percent THD).

This emergent distortion results in a ‘classically analogue’ tonality that’s reminiscent of good analogue tape in some ways: there’s a subtly rich mid-range with some musical weight, gradually becoming stronger and more obvious as the output level is pushed harder. The mic modules’ input stage can also be deliberately overdriven with a strong enough signal, and that too introduces an attractive musical grittiness and bite. To facilitate (ab)using the console in this way, it would help if Schertler were to tweak the input modules’ metering thresholds — currently, the red lights illuminate with just +10dBu at the outputs, making it difficult to know how much headroom is really left.

I didn’t notice any particular sonic benefit accruing from the absence of coupling capacitors in the signal path; other than the Yellow (instrument) module, the Arthur’s overall LF roll-off is much the same as for any other conventional analogue console, and any potential reduction in capacitor dielectric distortion is overwhelmed by the relatively high THD. The overall phase response is also not significantly different from conventional consoles, reaching around +95 degrees at 20Hz, and -25 degrees at 20kHz.The same as above, but with one ULN mic channel faded up and set with maximum mic input gain. Many of the spikes are mains harmonics picked up on the input cable, rather than by the mixer itself.The same as above, but with one ULN mic channel faded up and set with maximum mic input gain. Many of the spikes are mains harmonics picked up on the input cable, rather than by the mixer itself.The overall phase response measured via a  ULN mic input channel at full gain to the master output. The phase shift ranges from +95 degrees at 20Hz to -25 degrees at 20kHz.The overall phase response measured via a ULN mic input channel at full gain to the master output. The phase shift ranges from +95 degrees at 20Hz to -25 degrees at 20kHz.

Arthurian Legend?

Schertler’s Arthur Format48 is a very interesting console in so many ways. Mechanically, the modular construction works extremely well and this genuinely unique approach allows the user to build a bespoke mixer to meet almost any requirement, no matter how specific or unusual. It also enables changes to be made to the number and position of modules at a later date with considerable ease, which is equally attractive. This flexibility sets this innovative design apart from all others, and also means Schertler can add new modules to expand the scope and capabilities of the system.

A downside of the modularity and the discrete electronics is the price. I drew a sharp intake of breath when I discovered the €2816 list price of the review console, comprising as it did just four mic modules (two standard and two ULN), one Instrument and one Stereo channel, the LR Master and Aux Output modules, wooden side pieces and the Pro PSU. If you don’t require the Arthur’s matchlessly customisable modularity, an off-the-peg console might offer a more affordable alternative — if, that is, the fixed configuration and size meets your needs. Despite that, I’m quite sure the Arthur’s unique strengths will win it a body of keen supporters.

For me, the elephant in the room is the number of ergonomic, functional, and technical signal-path issues I encountered on the early review system — it strikes me that this intriguing design is still really under development. Obviously, some of the points I’ve raised are comparatively trivial and won’t concern everyone (although they probably take on more weight given the module prices), but others are quite significant and will be challenging to resolve. But although the Arthur could be considered a work-in-progress, I certainly don’t want to leave you with an overall negative impression. I’ve been greatly encouraged by Schertler’s willingness to listen to and discuss feedback and, indeed, to implement changes swiftly where possible. There have been several sensible revisions already, and this all bodes well for the future of the Arthur range. Furthermore, it is already a good-sounding and very musical console, with genuinely unique strengths and attractive selling points.

The biggest question any potential customer must ask themselves, then, is whether the ability to specify (and alter) the number, type, and location of what are high-quality yet quite basic input and output modules justifies the price. Regardless, Schertler have struck out in a brave new direction with the Arthur Format48, and this is definitely one to watch in the months and years to come.

Published December 2016