Rupert Neve Designs MBT

For the first time, the Red and Blue bands of RND’s popular Silk circuit can be used together in the same device — but there’s more to the MBT than purple Silk!

Rupert Neve Designs (RND) are probably best known for their mixers, mic preamps, EQs and dynamics processors, but they’ve also built up a significant range of master‑bus related products, and their latest release is the MBT, or the Master Bus Transformer to give it its fully expanded title. The MBT is a stereo device that’s designed to give the user control over a range of potentially pleasing processes, including tone‑shaping, compression, stereo width enhancement and harmonic distortion.

Construction & Controls

The fascia of its substantial, 2U 19‑inch rackmountable, black‑painted steel chassis offers plenty of space between the controls, and the white legending is large enough to be easily readable from a distance (something that can often present a challenge to spectacle wearers such as myself!). On the back, you’ll find XLR connectors for the electronically balanced inputs and transformer balanced outputs, along with a combination IEC connector and mains power on/off switch. And on the inside, as with all the RND products I’ve reviewed, the PCB design, layout, components and build are all of the highest quality. The main PCB is dominated by the two channels’ custom output and inter‑stage transformers, and power to both it and the front‑panel daughterboard is provided by a pair of high‑quality switch‑mode power supplies.

The electronically balanced inputs are on Neutrik Combo connectors, while the transformer‑balanced outputs are on XLRs.The front‑panel control sections feature 31‑detent potentiometers throughout. Those with dark blue knobs set operational parameters, while those with red knobs (with one exception) control gain. All function in/out switching is carried out using relays activated by buttons that (with a few exceptions) illuminate green when active. A non‑illuminated button selects between the low and high compression ratios.

The input section features a ±12dB trim control and a relay‑switched high‑pass filter, whose corner frequency can be set within a range of 15‑100 Hz, allowing you to remove unwanted low or subsonic frequencies below the cutoff point. The post‑trim/pre‑EQ stereo signal level is displayed on a pair of LEDs that turn green at ‑20dBu and red at +23dBu.

The relay‑switched EQ that follows the input is a simple two‑band shelving setup with up to ±9dB gain per band. The low‑frequency band covers 30‑240 Hz and its high‑frequency counterpart 3‑24 kHz but, as you’d expect, these curves reach deeper into the frequency spectrum than those figures might suggest, and the bands overlap somewhat between approximately 350Hz and 2.5kHz.

The curves of the shelving filters can reach well up/down into the audible band, even at their most extreme settings.On leaving the EQ section, the stereo signal enters the MBT’s relay‑switched Color Comp. This programme‑dependent optical compressor has a fixed 20ms attack time and simplified control set, with threshold (0‑24 dBu), release (100ms to 1.5s), a simple button switch to select a low (2:1) or high (5:1) ratio, and up to 20dB of Class‑A make‑up gain. A high‑pass side‑chain filter with a variable corner frequency (20‑350 Hz) allows you to prevent low frequencies from triggering unwanted compression. A green LED illuminates when gain reduction is taking place, and a dry/wet Blend control makes dialling in just the right amount of parallel compression a breeze. The coloration that gives this Color Comp section its name comes, according to RND, from the specific kinds of low‑order distortion and other non‑linearities that are generated when using an LDR (light‑dependent resistor) for compression.

A potential side‑effect of compressing a stereo source can be a narrowing of its soundfield, and the relay‑switched Width section that follows allows you to compensate for this, but it also gives you the ability to creatively widen your mix’s stereo image. It’s not the simple M‑S balance control you might have expected, though. It operates on a sum of the L‑R signal, which is high‑pass filtered and then passes through a gain stage. The signal is then multed, with a polarity inverted version fed to one of the L‑R channels and a non‑inverted one to the other. The Width knob controls the gain stage, while the always active high‑pass filter has a user‑adjustable corner frequency range of 50‑800 Hz, so you can remove low and low‑mid frequencies up to 800Hz from this ‘synthetic Sides’ signal to avoid any low‑frequency anomalies that result from the process.

Super Silk

The Super Silk section allows you to add more distortion than in previous products, and to blend the Red and Blue Silk characters.Super Silk is an updated implementation of RND’s signature Silk Texture function that allows you to add a variable level of second‑ and third‑order harmonic transformer‑based distortion into the output signal. In other products, there are two colours, Red Silk and Blue Silk, and these are both present on the MBT. But, I believe for the first time in an RND product, they can be accessed individually or in combination, an arrangement that the indicator LED suggests we might call Purple Silk! Each Silk has its own individual level control knob (a very dark blue for Blue Silk and the standard red for Red Silk), allowing you to create the perfect harmonic distortion balance for your mix. The Super Silk section’s Harmonics control determines the overall amount of low‑order harmonic distortion.

Super Silk utilises a dedicated L‑R pair of inter‑stage transformers that can be pushed close to core saturation without affecting the output level. A Drive LED sitting between the Blue Silk and Red Silk level controls illuminates when the signal level entering the inter‑stage transformers reaches the optimal range for the production of the Silk harmonic distortion. This new Super Silk circuit topology allows you to achieve much higher levels of harmonic distortion than were possible under previous iterations of the Silk Texture function. In addition, an amber‑illuminated button activates Zener diode‑based soft‑clip circuitry that sits after the inter‑stage transformers and adds emphasis to the Silks’ harmonic distortion.

In the final front section there’s ±12dB of output trim available. This area also hosts a red‑backlit button that relay‑switches the MBT into hard bypass, as well as a twin 16‑segment LED output level meter, scaled from 0‑24 dBu, and a green power‑on LED. The evenly spaced meter scaling is worthy of mention: with 0‑12 dBu being covered in two 6dB steps, 12‑16 dBu in a single 4dB step and 16‑24dBu in four 2dB steps, there’s helpfully higher resolution across the most critical area.

This new Super Silk circuit topology allows you to achieve much higher levels of harmonic distortion than were possible under previous iterations of the Silk Texture function.

On Test

Once you’ve inserted the MBT on your master bus, everything that follows requires only your ears, intuition and creativity. The high‑pass filter on the input is an extremely effective subsonic filter and I found myself leaving it permanently in circuit, set at approximately 25Hz. The equaliser is smooth, transparent, effective and sounds extremely musical to my ears. The high‑frequency band’s extension up to 24kHz and beyond offers the ability to add an airy, silky high end to a track, whilst the low‑frequency extension meant that with judicious use, and careful balancing with the high‑pass filter, I could boost a track’s low‑end foundations and still maintain definition.

The optical Color Comp compressor features both a side‑chain high‑pass filter and a parallel compression Blend control.

As a master bus compressor, the MBT’s Color Comp does the business too. Set as I like my master bus compressor to be (a 2:1 ratio with the side‑chain HPF engaged to keep unwanted low‑frequency‑driven compression in check, with a long release time and the threshold set so the LED flickers to indicate just 2‑3 dB of gain reduction) it sounded good. Really, really good. With tracks that needed some compressor ‘glue’ to hold them together, increasing the amount of compression and, where necessary, increasing the ratio to 5:1, it gave me exactly what I wanted. But there’s more to it than that. The higher compression level built up the track’s weight and warmth with the increased low‑frequency harmonic content. And, of course, the Blend control made setting up parallel compression simple and intuitive when required.

If I have a favourite section on the MBT, it’s the Width processor...

If I have a favourite section on the MBT, though, it’s the Width processor. I’m something of a convert to M‑S mastering but on heavier mixes I found that I preferred the sound of the MBT’s Width process. With the Width control set at approximately 60 percent of its travel and the HPF at about 100Hz, the stereo soundfield of one of my favourite tracks (Colin Blunstone’s live performance of ‘She’s Not There’ from the Night of the Proms ’93 CD) was lifted out of my monitors in stunning fashion. I did experiment with boosting the Sides signal in my DAW to see how close this would get me, but while it gave me a result that I wouldn’t have been reluctant to go with it just didn’t feel quite as exciting as what I’d achieved using the MBT’s Width control.

The Silk harmonic distortion facility has always appealed to me, but the increased level range available within Super Silk’s two frequency bands (in broad terms, across low to low‑mid for Blue Silk and high‑mid to high for Red Silk) mean you can now make things really saturated if you feel the need. As I’ve found in the past, deciding on the amounts of either Silk is a matter of using your ears rather than anything else. Since the MBT offers the ability to run both colours separately or in combination, and the Harmonics knob gives you control over just how much of that Silking drives the inter‑stage transformers, you might find you want to spend a little longer listening out for exactly what this section is doing.

That said, you might want to keep an eye on what the Drive LED is doing, since that gives you a measure of what’s happening inside the inter‑stage transformers. A solid (and pretty bright) LED indicates that you’re getting very close to core saturation, while for mixing and/or mastering if the Drive LED is flickering and the Red and Blue Silks and/or the Harmonics master are in the first 20 percent of their travel, you should be in a fairly safe zone, where transients aren’t being reshaped too much.

Switching in the transient‑shaping Zener diode soft‑clipping acts to increase the overall harmonic saturation and bring out further the impact of the Silk mix that you have created. Personally, then, I think this all adds up to make the MBT as good a candidate for processing things like drum, synth and guitar subgroups as for master bus processing or mastering.

Finally, it isn’t often that the output stage of a piece of outboard gets singled out for mention, other than in relation to any potential transformer‑related distortion, but in the case of the MBT the ±12dB output trim control is a critical component. Sitting in the signal path are five potential sources of gain — input trim, EQ boost, the Color Comp’s make‑up gain, and the additive contributions of the Width and Super Silk sections — so in practice the output trim sees pretty much constant use, as you balance the effects of the first four gain stages, the amount of Silk you add, and the input level requirements of the next unit in your signal chain.

Ride The Bus?

Rupert Neve Designs’ Master Bus Transformer is an extremely impressive device that delivers exactly what its designers intended it should, and although the price tag puts it out of the reach of all but professional studios and wealthy project‑studio owners, its audio quality and performance easily justify that cost. Obviously, signal coloration is a matter of taste, but if your production direction and technique involve adding copious amounts of harmonic distortion to your subgroups, stems or mixes, the MBT is definitely going to appeal. Special mention also goes to the Width function, which does broadly what M‑S based stereo widening can do but in a slightly more exciting fashion, especially when processing heavier tracks. I’d urge anyone who has both the need for bus or mastering colour and the budget available to audition an MBT at the earliest opportunity. I’ll be very sorry to see this one leave!