Paul White plumbs new depths with KRK's recently introduced active sub‑bass package, designed to turn their nearfield monitors into full‑range systems.
The idea of combining nearfield monitors with a single sub‑bass cabinet to provide full‑range monitoring for small studios is very seductive, and done properly it can work exceptionally well. The theory is that at very long wavelengths, sound doesn't convey as many stereo clues as it does at higher frequencies, so if the bottom octave or so of the spectrum is in mono, nobody should really notice. Certain audio gurus have argued the contrary, stating that stereo separation is still important at low frequencies, but in a practical monitoring environment, mono sub‑bass systems have been demonstrated to give perfectly acceptable results as far as the majority of listeners are concerned.
The next problem is to decide how to power the sub‑bass speaker. Should it include a passive crossover so it can work off the same amplifiers as the main monitors, or should it have its own power amp and active crossover? Passive systems can and do work, but you can't feed a single mono speaker from a stereo power amp unless it has a fancy double voice coil, so you have to design a cabinet with two drivers and passive crossover components. With an active system, the crossover characteristics can be made tighter, less power is lost, and you can get by with a single bass driver. That's the road KRK have taken with the Rok Bottom, a substantial mono sub‑bass enclosure containing a 13‑inch woofer with a roll surround.
In keeping with other KRK designs, the Rok Bottoms are smooth with rounded corners and a textured paint finish. The MDF cabinet measures 18.75 x 17.75 x 21 inches, though it tapers towards the top, making it look like something from the Stargate film set. The shape, combined with a weight of 85lb and an absence of handles, makes lifting the unit about as easy as holding a hundredweight bar of soap!
Dual bass ports provide cabinet tuning, and a solid‑state power amplifier rated at 140W delivers the power. An integral four‑second delay inhibits power‑on thumps, and the amplifier includes DC output fault protection, high‑current limiting and thermal shutdown. A gain control, detented at unity gain, sets the sub‑bass drive level; a small toggle switch enables the sub‑bass to be turned off.
The Rok Bottom is specifically designed to be used with KRK's existing nearfield monitors, such as the K Roks or the smaller model 6000s, both of which were provided for the purpose of this review. All signals are fed first into the Rok Bottom, which has three XLRs for left, right and centre signals. The bass end of the spectrum is then filtered out, combined into mono and passed on to the internal amplifier to drive the woofer. In order to remove the low bass being fed to the nearfield 'satellite' system, there's a switchable high‑pass filter in each channel, after which the line‑level signals reappear at three more left/right/centre XLRs. The centre connections would only be used in a surround system with a centre front speaker — for regular stereo monitoring, only the left and right would be used. Of course, a power amplifier is still required for the main speakers, unless they are active models.
The secret of successful sub‑bass is not to use too much of it.
The frequency response of the Rok Bottom is around one octave, covering 32‑60Hz where the crossover frequency is set to 60Hz. The low‑pass filter feeding the woofer section has a steep 24dB response, while the high‑pass filter feeding the main monitors has a gentler 12dB/octave response.
All the electronics are mounted on a sub‑assembly, the main panel of which is sited directly below the amplifier heatsink on the rear of the enclosure. An EC connector takes mains into the unit; there are the six XLR inputs and outputs for left, right and centre; and the two toggle switches mute the sub‑bass and switch out the satellite high‑pass filters. That leaves only the detented sub‑bass gain control and a couple of status LEDs to tell anyone who happens to be behind the cabinet how the amp is coping with life. Obviously these might have been better placed on the front panel, but it could be argued that, given the places where people stick sub‑bass speakers, the lamps would probably be invisible no matter where you put them.
The secret of successful sub‑bass is not to use too much of it. Adding just enough sub‑bass to fill in that lower octave without swamping the music adds all the weight you need to feel TR909 kick drums, bass synths and acoustic drums, but without changing the inherent sound‑character of the monitoring. Using the Rok Bottom in this way really firms up the overall sound, but still leaves it sounding tight and well controlled. It also doesn't seem too critical where the Rok Bottom cabinet is placed; for optimum results, though, it should go mid‑way between the stereo speakers and be about the same distance from the listener. As you'd expect, turning the sub‑bass up too high makes your system sound like one of those ridiculous cars with high‑power audio systems that use the occupants as passive radiators!
Summing up, I think that KRK have come up with a good balance between size and true sub‑bass. So many alleged sub‑bass systems go down no further than the bottom end of a decent medium‑sized pair of 2‑way hi‑fi speakers, but the Rok Bottoms go right down to 30Hz, so the only things that will escape their attention are the fundamentals of very low organ pedal notes, and earthquakes. The tonal balance when they're used with other KRK monitors is pretty seamless, and siting doesn't seem at all critical — so in a small room you have some flexibility about where to put the unit. Furthermore, the low‑pass filtered output takes some of the low‑end strain off the main monitors, so the system is likely to be able to work at higher levels without distortion. If you're a KRK user and want to sink to new depths, you could do a lot worse than check out their Bottoms...
- PRE‑AMP SECTION: L, C, R summing input to woofer with switchable on/off high‑pass filter to satellite speakers.
- POWER: 140W maximum into 8Ω.
- DISTORTION: Less than 0.05% (THD or IMD).
- NOISE: 102dB below full output.
- FREQUENCY RESPONSE: 32‑60Hz (±0.5 dB).
- CROSSOVER (60Hz): 24dB/octave low‑pass, 12dB/octave high‑pass.
- DIMENSIONS: 18.75 x 17.75 x 21.0 inches (h x w x d).
- WEIGHT: 85lb.
- Easy to install and use.
- Includes centre connection for use in surround monitoring or playback applications.
- Reasonably compact considering its performance.
- As with all separate systems, getting the correct balance with
A practical, compact and effective way to add another octave of bass to your existing KRK monitors.