As we head into the outdoor gig season, there's always a lot of concern about the subject of scrounging for power for the show. Now, the big shows can demand genny trailers and venue/city electricians for hookups, but many of the smaller performances are left to people who think any electrical access is enough to get the job done. In the past, I have written about power distribution and load balancing; but let's think about not having enough power and what to do about it. When thinking about live sound and power consumption, audio power amplifiers come to mind. In reality, power amps reproducing live music do not consume much average power, but peak power demands are the basis of how most electrical power is allocated. This is easy to see, with today's power amplifiers capable of 2,000 watts per channel of audio power output, yet staying well within the 20 ampere, 120VAC circuit capacity of 2,400 watts. But if you have tough power gigs ahead of you, how do you choose power amplifiers that can be efficient?
Power amplifiers typically break down into two sections and a couple of variants per section. Today we have the amplifier section and the power supply unit (PSU) section. In the amplifier section, two major categories are linear vs. switcher amplifier designs. In the linear category, the popular high power amplifiers break into classes of AB, G and H. In the switcher category, the classes are D, I and TD. In class AB, the most efficiency you can get is about 66% at full output with the remaining 33% or more lost as heat. In classes G and H, the AB circuit design is extended with two or more steps of increasing power supply voltages that engage as the input signal increases. The result is amplifier efficiencies up to about 80%, not bad when you consider heat management to circuit complexity.
Digital power amps in the D, I and TD classes typically can render efficiencies up to 90% or 95%, permitting huge output powers in very small packaging. So if you have efficiency on the priority list, the class G and H linear amplifiers plus all the switcher amplifiers are candidates for
sipping power.
On the PSU section front, conventional power supply units use traditional 60 Hertz transformers to match the input 120VAC to the required DC voltage rails of the amplifiers. Of course, high power capable transformers have a lot in common with bowling balls, especially weight and size. Switcher PSUs chop the incoming 60 Hertz 120VAC at high frequencies. And at these 80kHz to 500kHz switching frequencies, the transformers shrink in size and weight to small palm-sized devices while moving the same amount of power as conventional 60Hz transformers. Think of your amplifier circuits being re-supplied with energy at 250 thousand times a second, instead of 60 or 120 times a second. The re-supply amounts can be smaller and more responsive to varying energy demands.
But switcher PSUs have more complexity compared to the brute force of conventional transformers. So issues of size, reliability, weight and cost have to be compared for selection of a power amplifier. Both conventional and switcher PSUs have about that same efficiency, so that does not help or hurt the inadequate power problem. However, for those of us humping amp racks, the light weight and smaller size of switcher PSUs win out over the cheaper, heavier, bigger conventional power supplies.
Think Stacks
So now that you are biased towards efficient amplifiers, do not forget that fewer speakers require fewer amplifiers and less power to drive them. Most low frequency drivers convert nearly 80% of the power at the voice coils to heat. So make sure you have more efficient speakers and use less of them to make the most of the remaining 20% that can be used for acoustic output.
Things like horn-loaded mids and lows are not just remnants of the Woodstock generation, and many modern touring speaker stacks still retain the horn-load design for efficiency. (See the March Theory and Practice column for brain fodder and some review on efficiency.)
Other Loads
Backline power for musos is something that cannot be overlooked when conserving electrical power draw. While many groups can get by with a single circuit, you may be fighting a lost cause when the Marshall stacks and Ampeg SVTs invade the stage. If possible, communicate with the talent ahead of time about the power shortage, and persuade them that smaller backline gear will make the show a go instead of a breaker-popping event. Also, let them know that the 700- to 1,200-watt fog/haze machines will not be permitted for the gig.
Thankfully, the consoles and processing at Front of House and Monitor Beach tend to sip power, and in some cases these loads can live paired up with other audio loads. Also ripe for pruning are the number of monitor mixes and the redundancy of wedges per performer. Each wedge sucks juice, and that can add up fast. Also, note that larger digital consoles are trending into power hogs as well. That's something to consider when comparing a 300-watt analog console with 200 watts of outboard signal processing vs. a 1,000-watt digital console and nil outboard processing.
One tip I wish to share is placing power conditioner/monitor devices in your racks to keep a visual indication of current draw. I prefer the older Furman PM-PRO and PL-PRO power conditioners on racks, with the AC draw represented in a LED bargraph, so that I can monitor from a distance the average and dynamic power consumption of each circuit. I cannot do that with subwoofer amplifiers, but it helps to know your limits on other circuits monitored.
Sniffing Out Power
When you are giving up on using your power distro, and are down to the detective work of finding separate 20-ampere venue circuits, you have a few tools to use. The classic tool is a neon bulb circuit tester or a plain old night-light, with you snapping off selected breakers and tracking down dead receptacles that correspond to each branch circuit. Of course, this takes precious time.
Another method involves an electrician's tool called a circuit sniffer or breaker finder. Knowing where the venue's breaker panel is, and with a little electrician intuition, you can make pretty good guesses on the circuit paths on strings of receptacles using the sniffer. Traditionally, a circuit sniffer works by sticking a transmitter device in an energized receptacle, and placing the sniffer device on the breaker that receives the transmitter tones the loudest. But sniffers can also be used to sense the receptacles between the transmit receptacle and the breaker. So the clever choice of the transmitter location can buzz out common circuit receptacles without having to make treks back and forth to the breaker panel.
Another note is that you really want to ensure all your circuits run to the same panel, at least on the audio loads. I do not recommend a different panel feed for stage lighting loads, but you may be okay if both panels join to a master panel. When in doubt, meter your grounds and neutrals between differing panel circuits. In most newer commercial buildings, you may have good success; but watch out for old venues that could have multiple master power feeds due to decades of remodeling. Also consider having the new non-contact 120VAC finders that use your hand as the safety ground reference. These finders are typically the Fluke Volt-Alert or Triplett Cricket, which light up or chirp when they are in proximity of energized wires. Ideally, all grounds and neutrals will not indicate, and only hot wires will indicate. Volt-Alerts and Crickets are also great for finding disconnected cords when you expect them to be energized.
Low Power Operation
Firing up a live sound system under constrained power conditions requires an easy-does-it approach. First on the checklist is turning gear on separately instead of hastily running up the amp rack toggling power switches. During the actual gig, look for ways to get the loudness good enough, but quieter than usual operation. The same goes with stage light operation, as it can also topple audio circuits through heating of nearby circuit breakers. Keep the energized fixture-count on scenes low and attempt to switch scenes incrementally through fades instead of just turning fixtures on and off. All those sudden stage light power-ons have nasty peak current draws that can push a breaker into the trip threshold.
The science of household and commercial circuit breakers bears repeating from my previous columns on power distribution. These breakers trip through a thermal means where a small (shunt) resistance is monitored, and if hot enough, will release the branch circuit from the main buss of the panel. Minor overloads will take minutes to trip, but major overloads will trip in seconds. The important fact is that a warm to hot breaker trips quicker if the load is not lightened. A hot breaker also warms adjacent breakers, further lowering their trip thresholds.
