Real World Gear: Change For The Better (Part 1)
The emergence of multi-channel amplifiers

By Jeff Kuells and John Murray

For years, power amplifiers with multiple channels (generally defined as more than two) were for either multiple headphones in studios or low-power-per-channel commercial applications.

But are multi-channel amps viable for mainstream professional sound reinforcement? You bet. Times have changed, and once again with regard to equipment, the change is for the better.

TECHNOLOGICAL ADVANTAGE Units with three to six channels have actually been around since the early ‘80s. However, these either had a rather low power-per-channel rating or were fraught with design and reliability problems. As a result, relatively few touring or commercial professionals used them. With the advent of switching power supplies, PWM (Pulse-Width Modulation) class D output topologies, these design limitations are becoming a thing of the past. Most of the major players in the amplifier production game (as well as a host of lesser known ones) have recently shown an increased proliferation of models in the genre.

Professional user acceptance is also becoming more widespread as a result.

The advantages are obvious. New output topologies are more efficient and waste far less energy. The classic A/B class is perhaps 40 percent efficient, while some of the newer class D designs are up to 95 percent efficient. This means smaller, lighter power supplies and far less heat that needs to be dissipated by fans or large heat sinks.

Newer switching power supplies, already required to produce only half the power for class D versus class A/B output stages, also jettison the large, heavy and expensive power transformers required of yesterday’s power supplies.

Thus the size and dollar-per-watt of multi-channel amps has been dropping dramatically in recent years. Typical chassis sizes range from 2U to 3U rack spaces, with 4 to 8 channels common. Weight reduction is also substantial. Few multi-channel units exceed 40 pounds while producing as much or more power per channel than heavier two-channel units of decades past. Less weight isn’t as critical for fixed installations as for portable use, but less rack spaces and more channels benefits both markets greatly.

IT ADDS UP

Cost savings is another big advantage. Multi-channel amps cost less to build. Not only is there less metal per channel and no power transformer when using switching supplies, but fewer components, such as power cords, switches, fuse holders, circuit breakers, etc., adds up and the savings is passed on to the buyer. Further cost savings can result from less rack space used, thus less racks needed, and for mobile use, less truck space occupied, faster load-ins, load-outs, etc.

The main disadvantages of these newer multi-channel amps are a lack of power-supply redundancy and new-technology reliability. With most multi-channel models, there’s only one power supply for all the channels, so you can have up to eight channels on one supply, as compared to the traditional two channels per supply.

Amp-channel failure usually results in the power supply being turned off for all channels, although some models avoid that problem using intelligent circuitry or simply employ multiple power supplies.

However, before assuming that one power supply is a disadvantage, consider the application. For studio or home theater applications, it’s generally not all that critical. For live uses like stage monitoring, it can be quite the opposite. Your application will be the deciding factor here.

How reliable is this new technology? If you ask the manufacturers, it’s rock solid. If you ask the “bleeding” users on the cutting edge that have experienced problems, you’ll get a different answer.

The bottom line, as with all new technology, is that commensurate reliability will come with the passage of time. There was a day when those new-fangled transistor-based amplifiers would blow up if you looked at ‘em sideways, and only tube models were dependable.

The best hedge is to ask around and find out which brands have got the reliability angle figured out. One thing is for sure - this new approach will be a rock-solid reliable mainstream design in the near future.

WHERE THEY GO

Probably the most popular use multi-channel amps to this point has been for commercial installed systems, where more channels in the same package are great for multi-zoned background music and paging systems, such as those common to restaurants, clubs, churches and commercial buildings.

The power requirements for these installs are generally less demanding, while the number of channels required is higher than for mobile use. These applications lend themselves to the lower-powered, conventionally designed units that have been available for decades and have been accepted over the years. This has also spawned the 70-volt versions with limited low-frequency capability and optional isolations transformers available on the market today.

The big growth potential for multi-channel amps is in the pro realm. The newer technologies now allow full-range high-power channels in very small, light, and efficient packages. Where once only large, heavy, two-channel amps were the only choice, now four-plus channels of equal or more power can occupy the same rack real estate and weigh less.

Multi-channel amps are here to stay, and will likely see more proliferate use in larger, dynamic sound reinforcement systems as we move forward. More and more monitor mixes, as well as the multiple amp channels needed for line arrays are applications that cry out for multi-channel amps.

These units simply bring too much to the table to be ignored.

Jeff Kuells is an audio engineer and Live Sound’s amplifier editor. Reach him at proamps@foothill.net.

Live Sound Technical Editor John Murray is a 27-year industry veteran who has worked for EV, MediaMatrix, and TOA. He has presented two published AES papers, chaired four Syn-Aud-Con workshops, is a member of the TEF advisory committee and an ICIA Adjunct Faculty member. He can be reached at jmurray@-livesoundint.com

December 2003 Live Sound International