Rick Baxendale of Universal Music needed a monitor controller, but one with specific features that would streamline his recording workflow. After looking at everything on the market, he found nothing that hit the spot for him. He discussed it with Gavin Miller at Kazbar Systems, who said “Let’s try Douglas at DACS; if anyone can do it he can.” Monitor Control Central is the result.
Rick wanted the standard features usually found on a monitor controller: multiple inputs, multiple speaker outputs, volume, dim, mute, mono, headphone, metering and talkback. He wanted a Desk Top Control Unit and a separate Audio Rack. He wanted everything on XLR, not D25 connectors. He wanted relay switched volume control. And he wanted a retro looking Record Light whose control switch is on the Desktop box along with the other controls.
All of this is fairly standard, but it’s the specific non-standard features that streamline and simplify his recording workflow, very quickly giving the artists what they need to hear, that really make it work; the Cue section. This is described in some detail below.
Why it sounds so very good
All the balanced inputs in the Monitor Control Central Audio Rack use a circuit developed over the years based on an instrumentation amplifier. Both legs of the source signal are presented with a perfectly matched impedance, creating an ideal load and maximising the rejection of any unwanted interference. If the source is unbalanced the circuit deals with it flawlessly too. All the subsequent signal path is DC coupled with the purity and absolutely critical phase coherence this provides, particularly at the low end. The result is a signal path with a frequency response from 5Hz to 40kHz that is within ±0.1dB
The main speaker volume control uses reed relays to switch 1% metal film resistor ladders giving channel matching to within .05dB. Adjustment on the silky-smooth volume pot is seamless; the control board’s response is tailored to be similarly seamless. All balanced outputs use a DACS circuit, each one individually trimmed for very low CMRRs. The hot and cold outputs have a servo-linked function that provides the same output level whether connected to a balanced or unbalanced destination (as long as the cold terminal is shorted to ground).
The eight THAT2181 VCAs in the Cue section are individually trimmed with multiturn presets achieving their very best performance, and gain is controlled via an 8 BIT DAC to better than .4dB resolution.
All digital control signals are sent via an MC2 buss, and of course the 5V system powering this and the control chips is isolated from the audio. Headphones vary widely in impedance, but broadly handle between 100mW and 500mW in power. This means for maximum headphone SPL an amplifier may need to provide the same power to loads that vary by a factor of 50, so between 2V into low impedance 100mW headphones (8 Ohms), and 14V into 500mW high impedance headphones (400 Ohms).
If you consider a standard audio amplifier, it will efficiently power loads that vary over a factor of 4 (8 Ohms to 2 Ohms); outside this range distortion increases and the amplifier’s performance drops significantly. Headphone loads vary by a factor of more than 50, and the amplifier needs to deliver pure sound into all loads, up to and past the distortion point of the headphones. This is not a straightforward task. The amplifier circuit we use in Monitor Control Central is that of our legendary and exemplary HeadLite, renowned for its clarity and accuracy. It’ll drive anything, even (small) loudspeakers!
Inputs 1-3 are set to unity gain, while inputs 3-6 are designed for -10dBV inputs from MP3 players, CD players and the like. All six inputs are balanced, but all operate flawlessly unbalanced.
The Cue Section
The unit has four balanced stereo ‘Cue’ inputs which are routed to the artists via Monitor Control Central. THAT 2181 series trimmable VCAs drive the stereo balanced outputs that feed the external headphone system. On the Desk Top Unit, four pots control the Cues’ volume, a toggle switch on each Cue pair allowing the engineer to route either the Cue input or the main stereo mix to that Cue output (pre VCA volume control). Finally there is a rotary switch allowing any one of the 4 Cue signals (post VCA volume control) to be routed to the main control room speakers, so the engineer can check the feed. Talkback, when engaged, is routed to the Cue and automatically dims, by an amount adjusted on the front of the rack unit, the Cue input/stereo mix signal going to the Cue outputs.