Power Amplifiers & Signal Distribution
Realizing that size and weight are important factors when choosing power amps for a large portable system, Clair conducted extensive research into the power amplifier field before finally designing special units. Currently, most Clair systems are powered by amplifiers based on the Carver PM2.0 series chassis. Typically, one dozen of these modified units will be housed in a hinged road case complete with custom input/output panels and ac power hookups. One side of an amp rack case powers up to four S-4 Series II enclosures; thus, a single amp rack that can be picked up by two persons will drive eight speaker enclosures.
“Ultimately, the size and weight of the amplifiers determine the amount of headroom that a portable system will have,” Borthwick said. “There is a limit to how much you can haul around. By working with a number of manufacturers, we were able to almost double the ‘watts per truck’ without sacrificing real-world audio quality.”
Large sound reinforcement systems will typically require a tremendous amount of heavy-gauge speaker cable; for indoor hanging arena shows, speaker cable runs can be 200 feet in length.
“The majority of our shows are done with speakers rigged in the air to provide better audio coverage and to block less sight lines,” Borthwick said. “To maintain service access to the power amps, we like to locate them on the ground. The long cable runs needed can become a source of significant power and damping factor loss. With arrays of up to 1,000 separate loudspeaker units, one can quickly hear the effects of too much impedance in speaker wiring.”
Clair’s new special “High-D” speaker multi-core cable is a non-traditional approach to the amplifier/speaker interface. Because of the large number of speakers employed, the company was able to interest vendors in fabricating custom wire and connectors to its specifications. This has allowed Clair to design the amplifier, cabling and speakers as a dedicated, integrated system. According to Borthwick, the company was able to put the copper where it was most needed for power transfer. It also used a technique commonly employed in another industry, but not normally applied to audio power systems, to increase the effective damping factor.
The result is that power transmission more than 200 feet of the “High-D” speaker system cable is said to suffer only as much signal degradation as that of 25 feet of conventional 14 -gauge speaker wire used in the classic amp-to-speaker wiring.
One method used to create the most efficient speaker wiring harnesses to match the S-4’s needs is to use custom-made cable.The firm needed to find wire that had enough internal slippage to prevent copper wire-strand breakage, yet was heavy enough to take years of road abuse. The heavy-duty cabling includes 12-, 10- and 8-gauge wiring in one rugged, specially lubricated, insulated package.
The Clair crew carries its own 480V electrical transformers, so that high -powered ac is brought from the hookup panel or generators right to the sound wing. The power line is then dropped down to 208V 3-phase, with the ground-to-neutral bond happening within a few feet of the amp racks. Two 150kVA transformers are available, one at each sound wing; a 45kVA transformer offers electrical power for band gear, the monitor system and front-of-house equipment.
House Mixing Setup
In the driver’s seat was Clive Franks, perhaps best-known for his many years of mixing house sound for Elton John. Having worked with a variety of artists, including Hall & Oates, Peter Gabriel, Toto and Robert Plant, Franks was particularly interested when the call came in for touring with the Who.
“I’d done Elton’s live sound since 1972, and produced three of his albums,” Franks said. “I was first introduced to Clair when Elton toured the United States for the first time. I’d never been on the road before. Gene Clair was out with us for a 3-1/2 week tour, and that gave me some good training.”
Franks mixed the Who on a pair of Yamaha PM-3000-40 consoles. Signal processing available at the house position included two AMS S-DMX dual-channel digital delays, three Yamaha REV-5s and two Eventide H949 Harmonizers. Brooke-Siren DPR-402 noise gate/de-essers were channel-inserted for each vocal mic input.
“I was first introduced to the Yamaha console in Australia, when Elton did a symphony project with 102 people on stage,” Franks said. “Gus Dudgeon was there to do the orchestra submix, and we came to like the console with its mute groups and VCAs. Here, we are using audio subgroups 7 and 8 on the drum sub -mixer console, going right into a pair of input channels on the main desk.”
A pair of dbx 900 racks loaded with 903, 904 and 905 modules were channel-inserted on instruments such as the electric bass, guitar, keyboards, and Leslie mics. Dbx 160x compressor -limiters were available for use on the vocals. An AMS rmx -16 digital reverb system was used for the drum submix, and Drawmer noisegates were supplied for the drum kit.
Drive racks for the system included Clair’s new crossover, and modified dbx 160s were used for signal monitoring, level adjustment and bandpass protection. (See Photo 3)
More than just a traditional crossover, Clair’s new system drive unit has been named the Coherent Transfer System. A study in classical frequency- dividing network analysis and synthesis as applied to a specific loudspeaker system, the Coherent Transfer System was designed and pro-totyped by Clair to complement the S-4 Series II family of 4-way speaker systems.
The new electronic system has been designed to do the following:
• Provide active correction networks for broadband on-axis amplitude, phase and group delay errors inherent in the response characteristics of the raw drivers.
• Provide active correction networks for broadband on-axis amplitude, phase and group delay errors in the response of the speaker enclosures caused by closely -spaced drivers in the boxes (i.e., multiple 18- and 10 -inch speakers).
• Provide active signal delay networks for signal convergence of the low, mid and high ranges of the enclosures. (The signal delay is an analog realization with 137dB of dynamic range, 41dB better than 16 -bit digital systems.)
• Provide active crossover networks that result in a “flat” on-axis amplitude vs. frequency response, such that the flatness is limited by the narrowband amplitude vs. frequency response anomalies of the drivers in the enclosures.
• Provide active crossover networks that result in the main lobe of the polar response always being on-axis.
• Provide active crossover networks that result in a smooth off-axis amplitude versus frequency response over the useful coverage angle of the enclosures.
In conjunction with the new Coherent Transfer System units, Klark-Teknik DN27A graphic equalizers were available for system control. A White Series 4000 filter set with 1/6-octave precision adjustments was available for use on the subwoofer send.