This unusual compressor was specifically designed to process complex material, with dedicated bus-compression applications in mind.
Bus compression is used extensively in popular music production, for treating submixes (such as drums), processing complete mixes at the final mix stage, and during the mastering process. In this last application, the sonic character of the compressor plays a very important role, and when it comes to compressor design, the way the attack and release envelopes work can be as important as — if not more so than — the subtle distortions added by the gain-control element.
The solid-state Roll Music compressor is a dedicated stereo unit designed to process complex material, and was created specifically with bus compression in mind. I have to admit to not knowing much about the company, other than that they're based in Minneapolis in the US, but their design concept seems to be based on a combination of good circuit knowledge and an appreciation of sound character born of extensive studio experience. Though simple, the RMS755's controls provide all the variation needed for mix processing, and there are no gates, de-essing filters or other sophistications to get in the way.
The manufacturers claim that the RMS755 provides precise control over attack and release times, and fundamental to the compressor's sonic character is a dual release mode that imposes a slower release time on sustained loud passages, while still controlling the recovery from fast transients such as drum hits. This is described as being unique, although I'm sure the Aphex Compellor was designed to do something similar and I've also come across complex programme-dependent release systems in some Drawmer products. Of course, every designer approaches the problem in a slightly different way, which is why each compressor has its own sonic signature. A switchable 150Hz high-pass filter in the side-chain helps avoid level pumping on deep kick-drum beats. The rest of the circuitry is designed to sound as clean and transparent as possible.
Constructionally, the RMS755 is packed in a 1U, stainless-steel rack case, with a brushed-finish front panel and seriously chunky metal knobs. I'm not certain, but I think the front panel is lacquered, as it doesn't seem to show up fingerprints like the rest of the case. All the legend is laser-etched, so it is unlikely to wear off. Overall, the RMS755 Super Stereo looks neither modern nor retro, but has a slightly quirky, individual vibe (which is not a bad thing at all!).
An illuminated, horizontal, VU-style meter monitors gain reduction. The Ratio, Attack and Release times are switchable, rather than continuously variable, and ratios of 1.5:1, 2:1, 4:1 and 10:1 are available. Attack time can be set to 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20 or 50ms and release time to 0.1, 0.15, 0.2, 0.3, 0.4, 0.8. 1.2 or 1.6 seconds. Engaging the Dual button brings in the programme-dependent release mode mentioned earlier, so that the release time is shorter following brief transients than it is following continuous passages of loud material. A Gain make-up control (-14dB to +20dB) is provided to restore any level lost through compression, and the Threshold is fully variable, as you'd expect. Engaging the 150Hz filter attenuates the low end feeding the side-chain, making the compressor less sensitive to low frequencies. When Bypass is pressed, the inputs are connected directly to the outputs, so in this respect it is a true hard bypass.
Signal connection is via balanced XLRs only, with the capability to handle a maximum input level of +23dBu. The maximum output level is +29dBu and the outputs have a low 100Ω impedance, enabling them to drive long lines. The quoted frequency response is from 12Hz to 32kHz within a quarter of a dB, or 2Hz to 150kHz ±3dB. Where unbalanced use is necessary, the input should ideally be connected between pin 2 (hot) and pin 3 (the inverse phase leg, which can be connected to the incoming signal screen) to minimise the risk of ground loops. To use the output unbalanced, pins 1 and 2 should be used, with pin 3 left unconnected. To complete the 'specification figures' section, crosstalk is quoted as being better than -90dB at 1kHz.
Power comes into the RMS755 via the usual IEC socket with front-panel power switch, and the voltage is internally set, from a choice of 230V AC or 117V AC, via a slide-switch that's accessible once the top cover of the unit has been removed. Removing the top cover also reveals a single, neatly built, glass-fibre circuit board. Two of the respected THAT 2181LA voltage-control amplifier chips are used as gain-control elements, along with a THAT 2181LC in the side-chain control circuit. Critical amplification chores are carried out by four Analogue Devices OP275 dual low-current op-amps. These feature a 'Butler Amplifier' front-end, combining both bipolar and JFET transistors, to attain the best characteristics of both: low noise and speed. These chips are mounted in sockets, and a cursory scan around the PCB shows that all the other components, including the potentiometers and capacitors, are of a very high standard. An encapsulated torroidal transformer forms the heart of the regulated power supply and all connections carrying mains voltage are correctly sleeved. The ground plane of the PCB is wired directly to the mains ground/chassis and pin 1 of each XLR is connected to the chassis.
I tested the RMS755 mainly as a mix compressor, as that is its prime purpose in life. True to its promise, it does manage to keep a check on levels without allowing the bass elements to dominate and without taking away the impact of the brighter components of the mix. With very bass-heavy material the 150Hz filter really helps, as it stops the kick drum or bass synth from causing everything else to 'suck out'. The gain-reduction meter shows just how progressive the two-stage release is; the RMS755 reins in any excessive peaks fairly quickly, but at the same time it behaves in a more relaxed way when it comes to average level.
I tend to prefer lower compression ratios for bus compression or for mastering, but there's enough range on hand to allow you to become more obvious and assertive where the situation demands it. There's a nice little legend printed on the rear panel that says 'Overcompression Ruins Music', which can act as a timely reminder: you should always remember to compress your music because it needs it, rather than because you happen to have a spare compressor not doing anything! If you want the RMS755 to apply obvious compression as an effect, it can do that too, in a very musical way, without choking the life out of your sound. However, for most bus-compression applications I think the key to effectively exploiting its magic is to use this compressor sparingly. For most broad-spectrum mixed material, leaving the filter switched in and the dual mode on seems to be the most effective approach, but if you want the compressor to start making its presence known, you can get into 'compression as an effect' territory by turning them off.
The RMS755 is actually very flexible and is as much at home compressing a stereo piano track as it is a submix or mix. What I like about this compressor is that when used sensibly it adds density and increases the average level, which is what you'd expect, but, more importantly, it homogenises the various elements within a mix or submix in a way that doesn't compromise definition or separation. Dare I say that it makes digital mixes sound more analogue?
By way of value, the RMS755 isn't going to appeal to those after a budget bargain, but at the same time it stands comparison with more expensive compressors and has already attracted the attention of a number of big hitters on the US music-production scene. For the quality-conscious studio owner who understands the contribution that well-designed analogue equipment can make, the RMS755 is a very attractive proposition — but at the lower end of the food chain, you can probably buy a complete DAW system, including the computer, for the price of this compressor, so it clearly isn't for everyone. The RMS755 is most definitely aimed at the discerning professional and deserves to be judged accordingly.
The RMS755 is being pitched at the same potential buyers as other high-quality bus compressors such as the Aphex Compellor and SSL X-Rack Bus Compressor. Though not cheap as such, it competes well on price in this area of the market.
- Extremely clean, quiet signal path.
- Musical character.
- Really helps to blend sounds together.
- Too costly for most project studio owners.
A high-quality 'boutique' compressor that does what it sets out to do, but at a boutique price.
£1293 including VAT.
KMR Audio +44 (0)20 8445 2446.
+44 (0)20 8369 5529.