in November 2008
By Christos Desalernos
Beyond the distribution panel.
How vulnerable are your installations to whatever Mother Nature might offer?
You’re assembling the AC infrastructure for either a new or rejuvenated arena or large concert venue. Of course, you’ll stage massive high-voltage supplies from your utility, possibly back-up generators for critical loads, and break these multi-phase services out via distribution: breaker panels, ideally with surge suppression, earth grounding, lightning arresting, and possibly even some basic voltage regulation and filtering.
Everything is top drawer, code compliant and professionally installed. The 208VAC or 120VAC services provided to your audio/video and automation equipment racks have everything an electronic component could ask for...if you’ve built the facility circa 1978.
Today’s technology is going to require a bit more....We need to look seriously at the AC power management that’s placed in our component racks, beyond the distribution panel.
Why? Simply put: microprocessors. These circuits are incredibly vulnerable to damage, lockups, increased distortion and lost resolution. You can’t manufacture a circuit that would have been the size of your mixing console 30 years ago (if you’d built it with discrete transistors or vacuum tubes), then miniaturize it and place it inside a surface-mount chip the size of your fingernail and expect durability. In fact, three to 10 volts arced across the wrong junction could destroy many of these integrated circuit chips permanently!
Where do these vulnerable chips reside? Your computers, audio servers, laptops, HD video monitors and flat-panel televisions, A/D and D/A converters, digital time clocks, digital mixing consoles, digital processors, microprocessor automation and sequencing controls. The list grows larger every year.
Of course, with the superb AC infrastructure detailed in the opening paragraph, any concerns with damage, lost resolution or intermittent behavior are a moot point...right?
The problem is that, even though controlling and conditioning the AC power service back at the panel is prudent, it’s not enough. Transient voltage spikes, AC noise and ground loops are just a few of the problems that can, and will, be generated post the AC distribution panels. This is due to the fact that all of your audio, video, computer, lighting and motorized equipment will backwash these problems via their AC cords!
Path Of Least Resistance
Because electrical problems tend to look for the path of least resistance, and because everything is common via the neutral and ground wiring, adjacent lighting switches can send voltage transients into your digital mixing console. Your high current amplifier racks can modulate your computers and audio servers and the switching power supplies in your flat-screen monitors can mask the resolution of your microphone preamps (even if they’re assigned to different phases from the distribution panel). Virtually every critical component is affected by this.
Though some components will be relatively immune to AC-induced problems, others will not fare so well. If we were mixing sound for an arena 35 years ago, we would utilize robust transistor technology, transformer-based linear power supplies exclusively, and the closest thing to a signal “lockup” would be a sticky mechanical switch! That’s not today’s reality. Today’s arena is more likely to incorporate a PM5D, VENUE, Vi6 or a programmable DSP engine, for example.
Even if we have a generator on standby, will it be able to switch over during a power blackout before your processors lock up or lose their presets? In theory, if manufacturers produced perfect power supplies, this might not be an issue.
However, I haven’t found a “perfect power supply” in any component I’ve ever tested. Perfection is simply not possible and, because we are all fighting weight, size and cost in the products we design for professional audio/video applications, we tend to incorporate “switching power supplies” that are more susceptible to lockups and lost presets than ever.
With microprocessors playing a role in most of today’s technology, care must be taken.
To alleviate this, a UPS (uninterruptible power supply) may be required. Though high capacity UPS units could be placed at the panel, they have their limits. Raised AC impedance (which is far from ideal for power amplifiers), wiring faults and accidental disconnections post the distribution panel are just some of reasons to have rackmount UPS (battery backup) at the appropriate component racks.
Another consideration is control. It is desirable to sequence the power amplifiers to eliminate the massive current in-rush that could occur at the ¼ wave peak at turn-on. Further, because there may be several racks spaced hundreds of yards away from each other, it may be necessary to switch or sequence these from a single location (or multiple locations). Components that are more advanced may be controlled via microprocessing controller system with RS232, CobraNet, EtherSound or, perhaps, IP control. However, for most, the tried and true dry contact closure with twisted pair or quad wiring is the favored approach. Again, this must be done at the component racks, not at the panel.
What’s the appropriate combination of AC power management technologies and components? What’s your application? No two systems will be the same and, even if the rack components were cloned in multiple arenas, the technicians would have different requirements based on their needs. AC power management products are valuable tools. Your application determines the appropriate combination of tools.
It’s important to understand that a rackmount AC power device should offer more than a convenient set of outlets. Some of the technologies available include:
• Advanced Transient Voltage Surge Protection and Extreme Voltage Shutdown: Because any equipment downtime is costly, nonsacrificial surge suppression is an advantage in that lockup and circuit damage are greatly minimized. Further, these more advanced surge suppressors protect your delicate equipment and themselves: no servicing or downtime.
Extreme voltage shutdown is critical for professional AC power management. Fighting a lightning-fast voltage or current transient is a must, but what if a failed generator or intermittent neutral wire allowed 208+VAC where your rack expected 120VAC? With a less sophisticated surge suppressor, after about ¼ second, the dangerously high voltage will pass through the AC conditioner and take out virtually every power supply! That can’t happen with extreme voltage shutdown circuits. They monitor the input voltage, and automatically disconnect the input voltage from the outlets if they detect excessively high voltage in excess of ¼ second.
• AC Noise Filtering and AC Isolation Transformers: Granted, nobody’s concerned about squeezing an additional 15 decibels of signal to noise out of your audio system when your mix is competing with 50,000 screaming fans. On the other hand, will your facility ever be called upon to broadcast the performance, or to make a professional recording from your mixing console? Even if a location recording truck is hired, it will still require signal feeds from you. In today’s digital age, even minute audio buzz from ground loops, or excessive electronic self-noise, is no longer acceptable.
A high bandwidth linearized AC filter is the minimum requirement to ensure your components are not hampered by AC-induced noise. A rackmount AC isolation transformer, such as balanced toroid design with the capability to ground isolate any “single-ended” components, will be indispensable, along with good wiring and grounding practices to eliminate any chance of ground loop-induced audio buzzing.
• UPS (Uninterruptible Power Supply) Battery Backup: As previously stated, a must for digital devices and server-based products that will not tolerate even a ½-second power disconnect or “black-out.”
• AC Sequencing and Switching: This is obvious; we do not want to get our day’s workout running across a field or from room to room to energize our remote equipment. Control must be possible from a centralized location of your choice.
These are just some of the tools an AV integrator must consider when assembling the AC power management and equipment racks for today’s arena or stadium.
Christos Desalernos manages pro sales, system design and noise elimination as domestic sales manager for Furman, Petaluma CA. He holds an electronic engineering degree from ITT Technical Institute.