Rant: 2 Ohm speaker loads on power amplifiers
When power amplifier manufacturers came out with amplifiers "capable of driving 2 ohm loads" it was greeted with much satisfaction by quite a number of people. This meant they were given a theoretical "green light" to cut down on their amplifier requirements, using fewer more powerful ones, and drive more speakers from the same speaker cable runs. The problem is that many involved in the decisions to implement this, did so in spite of some important principles of audio.
The problem lies in an amplifier specification known as "damping factor" (DF). The best way to view the importance of damping factor is to see it as the ability of an amplifier to effectively control the pistonic motion of a loudspeaker - to stop and start its motion. The load on a power amplifier, as represented by the series load comprising the impedance of the loudspeaker and the impedance of speaker cable, has a direct effect on the quality of this motional control. An acceptable load on an amplifier resulting in a high DF factor connection to the amplifier produces a "tight" sound, particularly noticeable at lower frequencies where the large mass cones of woofers and subwoofers have considerable inertia and are difficult for an amplifier to maintain tight control over.
A quick look at a Crown pro amplifier manual shows a rated DF of 1000 for their pro models. Using the DF nomograph in the Crown manual (download from site), you can clearly see that just by going to a 2 ohm load even under ideal cable conditions drops the DF to 100. In order to maintain that DF factor of 100, one would have to use no more than ten feet of #8 gauge speaker cable. Typical flown loudspeaker systems as fielded by sound reinforcement companies use as much as 75' to 100' speaker cable runs from the amplifiers on the floor to arena speaker arrays rigged to the roof and lifted with chain motors. Most of the sound systems I've encountered use #12 gauge speaker wire at best - many only use #14 gauge. At those lengths and wire gauges, a 2 ohms speaker load drops the DF to way below 10. The amplifier has trouble maintaining motional control over a loudspeaker at very low damping factors resulting in lower quality sound described by most as "loose" or "not tight". Arenas are loose sounding enough as it is.
Not only does it sound bad, it wastes amplifier power. According to Crest Audio's pro power amp manual (download from site), running 80' of 14 gauge speaker wire into a 2 ohm speaker load reduces the amplifier voltage delivered to the loudspeaker by more than 17%. This is due to sheer series voltage divider loss from cable resistance. With this load and cable, an amplifier would have a DF of around 5, but they fail to mention that. Going to a 4 ohm load and using 12 gauge speaker wire would triple the DF. Even then, at 80', the DF is still only be around 15. Some listening tests have put the lowest acceptable DF limit between 10 and 15 and professional conventional wisdom wants to see at least 20.
Meyer Sound, a loudspeaker manufacturer, gets around this problem in some of their loudspeaker models by installing a power amplifier within the loudspeaker cabinet. With such a short run of speaker cable within the cabinet, the amplifier can be more ideally matched to the speaker/cable load. This type of design requires signal and AC power to be run to every speaker in the array.
In my experience, 2 ohm amplifier loads only work with marginal acceptability in PA's when the amp racks are close to the array and have cable runs of about of 20 feet or less of 12 gauge wire to the speakers.
SG
