The Master Buss Converter is a stereo analog signal processor and modern digital-to-analog converter that allows an engineer/producer to customize the crucial A/D conversion finishing process to suit the style and sound of their audio production. Using dual-path topology, the MBC’s pristine design maintains the highest possible quality audio before A/D conversion. Furthermore, the front panel controls and super-precision metering allow for instantaneous adjustments that encourage maximizing and/or experimenting with the process.
The 1U MBC has two TLA (Transformer-Like Amplifiers), line-level, XLR analog input channels called A and B, each with its own separate cluster of controls and metering facilities. Each channel has a 22-segment LED meter that shows the analog input level going into the AKM AK5397EQ A/D converter chip. The meters measure from -60 dBFS to 0 dBFS, and I found their wide dynamic range valuable to alert me to low-level stereo bus noise and hum that I may not actually hear at normal monitoring levels.
Just underneath the main meters are individual eight-LED gain reduction meters with a range of 14 to 0.5 dB when the limiter is activated. The rear panel has a pair of small switches for setting the meter’s hold time (one or three seconds), and a recessed trim pot on the front panel adjusts overall brightness of the meters.
All front panel control pots are 31-detent models made by either Alps or Noble, and I liked the feel of these—perfect for easily matching settings by feeling and/or counting the detents.
Each channel has a single, continuous-frequency control knob for the limiter’s side-chain highpass filter—a 20 Hz to 250 Hz, 12dB/octave Bessel filter. At the straight-up 12-noon position, it is -3 dB at 125Hz. This proved to be a very effective filter for full mixes and a big part of the sound of the MBC. When a HPF is inserted in the limiter’s side-chain, low-frequency peaks are less likely to trigger unwanted gain reductions, which is more important than ever with today’s low frequency–heavy Pop music. An HPF on/off push-button lights up to indicate when the HPF is inserted.
All push-buttons on the MBC have LED-lit centers when activated, and all switches toggle sealed Panasonic miniature relays inside the unit. No audio signals are going directly through switch contacts, which can become dirty and intermittent over time.
The Gain control ranges from 0 dB to +20 dB in 0.75dB steps. You can use Gain to increase the input signal (if needed) until it reaches the selected setting of the limiter’s Threshold control. It also sets the level going into the transformer/Silk circuitry (if activated) that feeds the A/D converter. Gain can be thought of as makeup gain and is only in play when the limiter is active. If the Gain knob is increased and/or the input signal itself exceeds the Threshold setting, limiting will start at a fixed 10:1 ratio.
The Threshold control covers from -14 dBFS (full CCW), straight up at 12-noon is -6 dBFS, and the 3 o’clock position is 0 dBFs. Full clockwise is called OVER, a clever way to keep Gain control and the limiter in circuit but without any limiting. I always start with Threshold at full CW.
The Limiter uses a pair of THAT Corp 2181A VCA chips and finishes with the Release time control that is adjustable from 50 ms to 1 second. Attack time is fixed at 0.5 ms, and both the limiter’s Gain and Threshold settings are dependent on the ADC Calibration selected in what I will call the Center Section of the MBC.
The Center Section
The middle-third of the MBC’s front panel is split into two parts. On the left side is a stack of LEDs to indicate the selected sample rate, be it 44.1, 48, 88.2 96, 176.4 or 192 kHz. The MBC builds on the converter design within the Rupert Neve Designs RMP-D8 8-Channel Dante Mic Pre.
The MBC can act as master word clock or will synchronize to incoming clock signals with a pair of Word Clock In/Out BNC coaxial connectors on the rear panel. Once sync is obtained with an incoming word clock signal, a green LED lights up and a buffered clock signal is sent back out the word clock output BNC.
Next on the panel is a push-button to toggle through the MBC’s choices of various analog-to-digital converter digital reference calibration standards, or ADC Calibration. They are: -14 dBFS, -16 dBFS, -18 dBFS, and -20 dBFS and are referenced to +4dBu analog level, or 1.228 volts (RMS).
All four of my Pro Tools HD system’s interfaces are set and calibrated to -18 dBFS, where a 1kHz tone coming in at +4 dBU would read -18 dB on the DAW’s meters. The stereo bus of a professional mixing console or summing system would have no problem making these operating levels. Being able to change the ADC Cal “at will” is an interesting feature; if you’re using the MBC to master a recorded stereo mix file that is recorded too low, you may try re-calibrating downward from -18 dBFS or upward to -20 dBFS to deal with hotter, more dynamic mix files.
Also in the Center section is the Limiter In/Out, L/R link buttons, and the transformer in/out button and Silk transformer saturation mode toggle switch. Here you have three choices of the sound of the two Rupert Neve Designs own custom inter-stage transformers. Red Silk saturates in the high mid-range and high frequencies, Blue Silk warms up the low frequencies or choose the Off position for the transformers only.
Texture is a rotary control that sets the amount of Silk when activated—the Silk effect is added to both Channels A and B together and is highly dependent on hot, incoming analog levels.
The MBC has an internal steel-framed chassis with nine pots mounted so that their shafts freely pass through holes made in a printed circuit board mounted on the back of a thick aluminum front panel. The front panel’s circuit board holds all the LED indicators and push-buttons and is connected to the main board using ribbon cables.
The main circuit board uses surface mount technology and a small switch mode power supply for +3.3-VDC and +5-VDC, and a Motien DC-to-DC converter provides ±15VDC for the analog class-A circuits. The AKM AK5397EQ A/D converter chip is companioned with an AKM AK4115VQ transceiver chip to manage internal and external clock signals.
I certainly admire the engineering and design here; the unit is over-built inside and much better than I usually see inside of a lot of pro audio gear. The more expensive details add to the MBC’s longevity, as well as the solid feel and confidence when operating it.
MBC as a Master Clock/ADC
Using the MBC as a master word clock, I connected its Word Out BNC to the Word Clock In BNC on the first Avid I/O in my rack, which acts as Loop Sync Master in my Pro Tools HDX system. Multiple I/O boxes can be clocked in a “star configuration” if you use a word clock distributor. Because I didn’t have one, I went with Loop Sync.
I routed the SSL Sigma‘s stereo analog summing mix bus output to the L/R analog inputs of the MBC and set the ADC Calibration to -18 dBFS like all the other analog I/O boxes. The MBC’s AES/EBU XLR digital audio output connects to the I/O’s XLR input Enclosure connector. The MBC also outputs coaxial and optical S/PDIF digital audio simultaneously.
I record all mixes in Pro Tools onto a new stereo audio track and monitor the mix audio with that track locked to input. Audio playback comes out of the companion Enclosure AES/EBU output XLR connector to the DAC input in my Avocet II monitor controller.
I periodically check all I/O boxes with a 1kHz, -18dBFS tone (from Pro Tools) through the Sigma’s 16 stereo stems to verify calibration, plus the Sigma’s stereo bus output to the MBC, and then digitally through the AES/EBU XLR Enclosure connectors. It was -18 dBFS everywhere—within the resolution of the meters. It is such a pleasure to work with precision audio gear!
With the limiter switched out, I started to mix a dance pop mix with wide dynamics, lots of bass and loud vocals, all stemmed out to 16 stereo stems on the Sigma.
I used the MBC’s variable highpass filters for this bass-heavy song, and I counted clicks to match both left (channels A and B) controls—no problem, but (especially for recalls) I could use pointers on all the knobs and finer graduated marks on the front panel for Gain, Threshold, and Release parameters.
The high-pass filters worked great, and you can see their effect instantly on the kick and bass synths peaking on the main meters and watching the GR meters dip less. I had the HPFs set past straight-up, and I liked that the big bass synth and kick drum hits were no longer punching holes (level dips) in the overall mix.
There is a lot of gain available for quieter mixes, but with full mix levels coming in from the Sigma, the Gain control ended up being above 0 dB while Threshold was at the center position of -6 dBFS. Each click equals 0.75 dBFS. The Release time knob was midway in its range—not too fast so as to distort the low frequencies! I got around 4 to 7 dB of peak gain reductions and a good bump in clean loudness!
There is no overall bypass button on the MBC, but you can switch the limiter and the transformer/Silk feature in/out to compare. I didn’t use the transformer and Silk feature for this modern New Disco song, as the producer likes the mix to stay clean yet loud as possible.
MBC as Outboard Processor
I connected a stereo line-level output feed from my Pro Tools analog I/O #3 to the MBC’s XLR Line inputs. In the Hardware page, I set the output to analog to feed the MBC but changed the paired input to AES/EBU using the Enclosure connector—analog out and digital back in. This same setup is also useful for recording live audio, except your mic preamp’s line outputs would connect to the MBC’s XLR line inputs.
Next, I connected a BNC clock output cable from the same #3 Avid I/O box to the MBC’s Word Clock In, and I was in business! The MBC’s Sync LED lit up immediately, showing the 48kHz sample rate; I kept the same -18dBFS ADC calibration. At 48 kHz, the round-trip latency was 63 samples—no problem at all for Pro Tools’ delay compensation.
I tried the MBC as a stereo limiter and gave those interstage transformers and the two Silk modes a good workout. As Gain is only active when the limiter is engaged, I used the ADC Calibration to boost the input level easily. If I change (from -18dBFS) to -14dBFS on the MBC, I’ve got 4 dB more for free! This “stupid pet trick” also works when using the limiter.
I like using the Red Silk mode to brighten up individual tracking elements—backing vocals, (real) grand pianos, synth pads, dull acoustics all sounded great. I tended to leave the transformer always in circuit and maybe switch in Red or Blue after hearing (whatever the MBC was on) with the rest of the track. I ran the transformer/Silk Texture control on or near “Max.”
If you have put off adding a proper 2-bus processing chain and high-quality modern ADC, I can highly recommend the Rupert Neve Designs Master Buss Converter. It delivers the crucial, final step in delivering a finished, high-quality master recording all in a single unit. It is well-designed and well-engineered, and it is a solid piece of professional kit you can rely on for years to come for great sound.
Company: Rupert Neve Designs
Product: Master Buss Converter
Price: $3,999 MSRP
Pros: Precise control over A/D conversion
Cons: Recalling settings might be tricky