Why might you need a dedicated monitor controller — and what separates the good models from the bad?

In days gone by, analogue consoles included a monitor section that, at the very least, provided a volume control (usually the largest knob on the desk) and a source selector, typically to switch between monitoring the mix output and one or two external two‑track recorder playbacks. Today, most of the studio action centres around a computer, DAW software and an audio interface. Without a console, you’ll either need your DAW and audio interface to perform those duties — or a monitor controller.

Can I Just Use My Interface?

In its most basic form, a monitor controller controls the listening volume of a single set of speakers. Most also include some means of switching between a few sources, and of sending the selected source to different loudspeaker and headphone systems. More elaborate units enable you to check various aspects of the monitored signal and possibly control artists’ cue headphones.

The combination of DAW and audio interface is capable of performing the same tasks, and can be a very cost‑effective solution: the signal path from interface to speakers is direct and simple, with nothing in between to degrade the signal; a multi‑channel interface can cater for multiple sources and destinations; and your DAW can be used for various signal checks.

The fact that most interfaces adjust the monitoring volume digitally seems to concern some people: for each 6dB of digital attenuation you use one less bit of the source’s word length through the D‑A converter, so that must mean less ‘resolution’, right? No! The signal‑to‑noise ratio does decrease as the signal is attenuated, but the quietest elements disappear gracefully into the (dithered) noise floor, just as in the analogue world: there’s absolutely no ‘loss of resolution’ relative to an analogue volume control.

What is important, though, is to work with a properly optimised gain structure throughout the monitoring signal path. This ensures that the interface’s noise floor is kept as low as possible. Most budget interfaces’ D‑A converters are capable of 115dB or more dynamic range, and most nearfield monitors can produce peaks at 1m of around 112dB SPL or more. So, if the peak digital output level is aligned to match the peak SPL capability of the speakers, the interface’s noise floor will lie comfortably below the threshold of hearing: ‑3dB SPL, with the above figures. I covered the optimisation of the monitoring path gain structure in detail in a 2014 article, which you’ll find on the SOS website: https://sosm.ag/reference-monitoring

What most interfaces don’t offer is any signal‑conditioning functions so, in such a setup, these must typically be performed in the DAW software, and some DAWs make that easier than others. For example, Cubase Pro and Nuendo include a Control Room facility for switching between different speaker and headphone sets, setting and recalling reference levels and hosting plug‑ins in the monitoring path.

That all sounds very promising, doesn’t it? So why bother with a dedicated monitor controller? Actually, there are various reasons. First, although computer crashes are rarer today than in the early days of DAWs they still happen, and when they do there’s a chance that the audio interface crashes too, generating full‑level noises that aren’t good for speakers or the listener’s ears. Having a reliable volume knob or mute switch a hand‑stretch away can be an ‘ear‑saver’.

...a standalone controller makes it possible to select and audition external sources when the computer is powered down.

Another consideration is that a standalone controller makes it possible to select and audition external sources when the computer is powered down. For example, you can connect a CD player, or take a minijack feed from a phone. Or perhaps you want to play a synth or through a hardware amp modeller without booting up your PC.

Furthermore, as a hardware controller is a self‑contained entity, there’s no need to dedicate space on your screen for monitoring functions, or to switch screen views to get at them — something which I find can become particularly tiresome if working on a laptop.

Finally, using a monitor controller means that you can employ multiple speaker sets without using up dedicated audio interface I/O, and that can mean that you have a wider range of audio interface options within your budget.

Active Or Passive?

The simplest monitor controllers are passive volume controls comprising a potentiometer or adjustable attenuator in a desktop box. However, passive controllers are not always cheap and basic. Many include alternative input selection and multiple loudspeaker output routing, and some very expensive passive designs employ elaborate switched attenuators as precision volume controls. Some even use bespoke multi‑tap transformers. However, it’s difficult to include complex signal‑conditioning options in a passive controller.

Many people believe that passive controllers are inherently better than active ones simply because there are no electronics in the signal path to add noise or distortion, but the reality is more nuanced. Provided the connecting cables are kept reasonably short (a couple of metres or less), they usually work perfectly well, so at the budget end of the market, they can offer good value. A potential downside, though, is that as the volume control is adjusted the source device ‘sees’ a varying input impedance, while the loudspeakers receive a signal from a varying source impedance. With good design and short cables these variations aren’t usually problematic, but if working with vintage impedance‑matched equipment or long cables, they can cause a loss of high frequencies and introduce distortion. Radio‑frequency interference (RFI) is often a concern too, particularly with budget passive controllers, because many don’t maintain an accurate impedance balance between the hot and cold sides of the balanced signal paths, which substantially degrades the common‑mode rejection ratio (CMRR).

Active designs employ electronics to buffer the inputs and outputs, to ensure stable and properly optimised input and output impedances and good CMRRs, so long cables or unusual source and destination impedances can be accommodated. With good design, the noise and distortion introduced by the active circuitry is far below audibility, often bordering on being unmeasurable. Active designs also make it easier to incorporate additional functionality like headphone amps, talkback and metering, as well as making it easier to implement signal‑check facilities. Low‑level, unbalanced consumer signal sources can also be accommodated because the necessary gain is easy to provide.

Naturally, active monitor controllers cost more to make than simple passive types, and most upmarket monitor controllers are active. But not all good ones are expensive: the ‘sweet spot’ for home studios is probably in the £200‑500 $300‑600 range, where you’ll find technically well‑specified and well‑equipped products from the likes of Drawmer, Audient and SPL. The bottom line, whether a model is active or passive, is that if the volume is set to unity and you can hear a quality difference when the monitor controller is plugged in or bypassed, it’s not good enough: a monitor controller’s role is to allow you to hear only what is presented by the source signal, so it is essential that it adds nothing and takes nothing away.

Inputs & Outputs

A basic requirement of a monitor controller is to allow comparison between a current work‑in‑progress and a reference: assuming you want to work with an external reference source like a CD player, that means at least two stereo inputs. Most studio equipment has balanced analogue line‑level outputs, but the ability to accept signals from unbalanced consumer equipment is often important too. A mini‑jack socket, for example, allows many smartphones to be hooked up. Many more advanced monitor controllers also include digital and/or USB inputs and a reference‑grade D‑A converter, and some models now cater for Bluetooth streaming. When comparing sources, it’s usually important that they have the same perceived volume, so the better‑equipped monitor controllers include facilities to adjust or ‘trim’ the level of selected inputs.

It’s often helpful to be able to audition signals on a couple of different monitoring systems. The primary reference would typically be a set of full‑range, high‑quality speakers, perhaps with headphones as a secondary option. In addition, checking a mix on a set of compact and limited‑bandwidth speakers gives a useful impression of what someone listening on a TV or laptop computer might hear. Where multiple speaker outputs are provided, the monitor controller usually incorporates facilities to adjust the level of the additional outputs relative to the primary set, so that the SPL (Sound Pressure Level) in the room doesn’t change significantly when switching between systems.

In installations involving a subwoofer, it can be helpful if the sub can be switched on and off from the monitoring controller (that may require bass‑management facilities in the controller rather than the sub), and in multi‑channel rooms it’s generally necessary to be able to switch between the surround speaker set and a stereo set, to check downmixes. It’s preferable to check mono compatibility on a single dedicated mono speaker too, so an output configurable for that purpose can be useful.

Soloing the Sides channel makes it quick and easy to align channel gains on stereo sources or stereo mic arrays, as well as to assess what’s being lost in the summed‑mono signal.

On The Level

It’s important not to disturb the main volume control, because we automatically acquire an acoustic frame of reference when working at a consistent listening volume; if that volume changes, our frame of reference changes too, and mixing decisions made with the different monitor levels come out differently. More sophisticated monitor controllers usually allow a ‘reference’ listening level to be recalled at the press of a button. With less elaborate systems, we rely on using a calibration mark around the volume control — I often use a wax pencil mark. Ideally, this would be around the 1 or 2 o’clock positions, where ganged rotary pot controls offer the most precise gain adjustment and accurate tracking between channels, leaving an extra 10‑12 dB of gain for checking low‑level sounds.

After the volume control and source/speaker selection, I find that the most useful controls are the Dim and Mute buttons. Mute kills sound whereas Dim reduces the level, typically by 20dB, and they’re useful if you need to quickly quash loud sounds (such as someone moving or unplugging a mic) or have a discussion in the control room, without shouting over the music or changing the volume setting. Importantly, a Dim button allows you to check a mix at low levels — which can be very helpful in identifying balance problems — without disturbing the volume control. But it also neatly side‑steps the problem that most ganged potentiometers suffer from very poor channel tracking at low volume settings, causing the image balance to be offset and/or move around wildly with small changes in volume.

Signal‑check Facilities

Most monitor controllers cover the basic signal‑switching and conditioning facilities described above, but when auditioning music or other recorded signals, other tools can help you identify common problems. The most important is undoubtedly the ability to check mono compatibility, which requires listening to the sum of the left and right channels. This summed signal is likely to be louder than either channel individually, so most (not all) mono‑summation circuits introduce an overall attenuation of 3‑6 dB, to maintain a consistent listening level. This summed‑mono signal is typically routed to both speakers, which is useful for checking the location and focus of the phantom image. In a correctly setup system that should manifest as a narrow, sharply‑focused sound source precisely mid‑way between the two speakers. If not, one speaker may be defective or set up incorrectly, or there could be problems with local reflective surfaces. Some monitor controllers also include a balance control to shift the image centre left or right, to compensate for the individual listener’s hearing; different people have different sensitivity in each ear, and so perceive ‘the middle’ differently.

Listening to summed mono on two speakers is the default condition, but it creates a very different impression, particularly of the low end, from listening to mono on a single speaker. More capable monitor controllers allow the summed‑mono signal to be routed to a single speaker in a stereo pair — traditionally the left one — and some can be configured to route the summed‑mono signal to a separate dedicated ‘mono‑check’ loudspeaker. If this facility isn’t provided, an acceptable alternative is to send summed mono to both speakers, then mute the right speaker, which requires individual channel mute controls in addition to the overall mute function. Usefully, though, this facility doubles as a channel solo: auditioning each channel in isolation can help you identify problems on a single channel, which can be difficult to detect with both channels running.

The next most useful feature is a polarity reverse (often mislabelled ‘phase’). It’s usually applied to the right channel but occasionally each channel can be inverted individually. Flipping the polarity of one channel provides a helpful ‘sanity check’ when auditioning very wide or ‘phasey’ mixes: if the mix becomes more stable and focused with the polarity reversed, there’s probably an unwanted polarity inversion in the recorded signal.

Most acoustic signals have a distinct natural polarity, typically with more positive energy (compression) than negative (rarefaction). Ideally, the monitor system should reproduce sound in the same polarity, often called absolute phase, and flipping the polarity of both channels simultaneously allows that condition to be checked. Many monitor systems will sound quite different if, for example, kick drum transients cause the woofers to move inwards instead of out!

Polarity reverse functions should be implemented before the mono sum circuitry, so that when the right‑channel polarity reverse and summed‑mono facilities are both employed the output is L‑R instead of L+R. This is the ‘stereo difference’ signal, or Sides channel. Soloing the Sides channel makes it quick and easy to align channel gains on stereo sources or stereo mic arrays, as well as to assess what’s being lost in the summed‑mono signal. The stereo difference signal is very revealing of the damage imposed by lossy codec formats such as MP3: lower bit‑rates affect stereo imaging and reverberation character significantly.

Other useful facilities are left‑right swap and mono left/mono right: the mono left/right modes route the selected channel to both monitor speakers, while the left‑right swap exchanges the two channels. The former is helpful when working with single‑channel material or assessing the quality of each channel independently. The latter is useful for identifying where different elements are in the stereo image, and confirming whether a channel swap has occurred somewhere in the plugging. Taken together with the polarity inversion functions, these facilities allow individual auditioning of the Mid, Sides, Left and Right elements of a stereo programme.

Many monitor controllers are intended to serve as the central control hub of a project studio, with facilities for artist headphone monitoring (with independent source selection and talkback). Metering is sometimes included internally too, but more often, a dedicated output is provided for an external metering system. Sometimes the DAW input is made available at buffered outputs to feed external hardware recorders.

I would prioritise the ability to set a reliable reference listening level above all else, followed by having dim and mute buttons easily to hand, being able to check summed mono on a single speaker, and access to the stereo difference signal...

Decisions...

Different virtual or hardware monitor controllers include different subsets of the features and facilities described above. Selection of an appropriate model depends on the physical installation requirements, and your normal workflow patterns and personal preferences. I would prioritise the ability to set a reliable reference listening level above all else, followed by having dim and mute buttons easily to hand, being able to check summed mono on a single speaker, and access to the stereo difference signal — these are the functions I use all the time.

If your budget is tight and most of what you want to check emanates from the DAW, you could consider a hybrid approach whereby your DAW provides sophisticated signal‑check facilities, and feeds a simple monitor controller offering external inputs, a level control and mute/dim.