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Abacus C‑Box Series

Active Monitors By Mike Senior
Published March 2024

Abacus C‑Box Series

Can these small two‑way monitors really deliver accurate sub‑bass information at the mix?

There’s a widespread belief among project‑studio owners that genuinely insightful sub‑50Hz monitoring is simply beyond the capabilities of any affordable two‑way nearfield design. Yet, in principle, even a small woofer can generate those kinds of sub‑bass frequencies — it’s just that you won’t get much listening level before the driver reaches its excursion limits and distortion creeps in. In response to this inherent volume cap, pretty much all manufacturers in this space now design their speaker cabinets to resonate at low frequencies, thereby significantly boosting the low‑frequency acoustic output before the woofer‑cone excursions start maxing out.

The resonance is usually created using a frequency‑tuned reflex port or passive radiator, both of which involve some unwelcome sonic trade‑offs. Typically the bass level of resonant two‑way nearfields falls off rapidly below 50Hz, such that the lowest octave all but disappears and you’ll struggle to judge the relative balance of low‑frequency components either side of the fall‑off point. Low‑frequency time‑smearing is another a common problem, making kick transients sluggish and generally smudging low‑frequency instrument layers together so that it’s tough to distinguish between them.

But what if a manufacturer decided not to accept these trade‑offs, and pursued superior low‑frequency accuracy instead, at the expense of sheer output welly? Well, that’s exactly what the German company Abacus Electronics have been doing for years, and it’s their current C‑Box active nearfield range that’s the subject of this review.

Home On The Range

The C‑Bass 10 is based around a 10‑inch woofer.The C‑Bass 10 is based around a 10‑inch woofer.There are three speakers in the range. The C‑Box 3 and C‑Box 4 are two‑way closed‑box designs, which have similar phase‑plug tweeters but woofers of different diameters (10cm and 14cm respectively). Despite their diminutive dimensions, the loudspeakers boast low‑frequency extension down to 35Hz and 32Hz respectively at the ‑6dB point, and useful audibility of energy well below that because of the comparatively gentle low‑end roll‑off characteristics of closed‑box cabinets. Where you need higher listening levels, these speakers can be joined by the C‑Bass 10, a closed‑box subwoofer based around a long‑throw 10‑inch driver. Simple 2.1 bass management is built into the sub’s cabinet, with controls for level and phase, for the crossover’s high‑pass and low‑pass filter frequencies, and for a sub‑bass cut EQ that helps compensate for low‑frequency ‘room gain’ in small studios.

There’s plenty more technical information on the Abacus website, so I won’t bother parroting that here, but there is one aspect of the hardware that studio users definitely need to be aware of: all the C‑Series audio connections are on unbalanced RCA phonos. I didn’t encounter any problems at all in my own studio tests (and the speakers have very low self‑noise too), but I do use a filtered mains supply and I kept all my cables as short as possible, so I can’t say how much unwanted electromagnetic interference these speakers might pick up under less favourable conditions.

Talking Loud

Let’s get one crucial question out of the way first: how loud are these speakers? Well, this depends on the bass content of the mix you’re listening to. The worst‑case scenario is anything with the kind of powerful sub‑50Hz kick/bass fundamentals that most quickly max out the woofer’s clean driver excursion — tracks like Arizona Zervas’s ‘Roxanne’, Justin Bieber’s ‘Boyfriend’, or Stormzy’s ‘Big For Your Boots’, say. For the C‑Box 4, in practical terms that means keeping the listening volume low enough that you can easily have a conversation over the top without raising your voice. While this feels loud enough for mixing purposes, it won’t give you much of a physical bass sensation, so you have to get used to judging low‑end balances by what you hear rather than what you feel. Nor will this kind of playback volume impress visiting clients, hype up a band fresh from their first live‑room take, or inspire a room full of musical collaborators — all scenarios where monitoring wallop usually pays dividends.

For the smaller C‑Box 3, mixing LF‑heavy productions is unquestionably a quiet listening experience. This is a speaker that should be within a metre of your head to maximise what you can hear, and you’ll want to minimise background noise in your workspace too. Under those conditions it’s still just loud enough for professional‑level work, in my opinion — but it’s right on the cusp!

The C‑Box monitors feature built‑in high‑pass filtering (which can come in handy when you need a little more level at the expense of bass extension), and are fed from unbalanced phono inputs.The C‑Box monitors feature built‑in high‑pass filtering (which can come in handy when you need a little more level at the expense of bass extension), and are fed from unbalanced phono inputs.Whichever C‑Boxes you use, though, you need to take care with your monitoring volume to avoid unwanted distortion. Unfortunately, the speakers provide no visual overload indication to guide you in this respect, so there’s an element of trial and error involved here. I found that experimenting with sine‑wave tones helped me develop a sense of how much clean headroom was on offer at different frequencies, but despite this I regularly found myself pulling down my monitoring volume to check whether some bass harmonic I was hearing was in the mix itself or whether I was just...

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