One of my rules I have held fast on is the “no 2-ohm” loading of power amplifier outputs. But back when I was buying my first real power amplifiers, the thought of 2-ohm loading capability was a godsend. This was because I had limited funds for power amplifiers, and the two Peavey M-3000 power amplifiers I bought for around $300 each was carefully thought through.
Yeah, they gave 300 watts of mono-channel power, but they could do a righteous 210 watts each into 4-ohms, and that was not even close to half of 300 watts. So I knew something was up, and using my electrical engineering skills I figured out that current limits and voltage limits did not both meet at nice ohmic values like two ohms.
But at the time, it was the mid-1980s and my nice Peavey MD-16 console had just a single monitor mix auxiliary send, and no one in the semi-pro world thought to have more mixes. So why not have one power amplifier drive four 8-ohm wedges, especially if the amplifier was capable of driving the impedance? And, yeah, each wedge got its 300 ÷ 4 = 75 watts of nice monitor power. And my home built, Radio Shack bought, 16-gauge speaker zip-cord with phone plugs on each was just fine patching all those speakers together.
So now fast forward to today when 300-watt amplifiers are a “you kidding me?” and stereo and quad channel amplifiers have long replaced single channel amplifiers. And still today, most amplifier companies still put some kind of numeric meaning to all the watts they can dump into 2-ohm loading per channel. It is one thing to pass 75 to 225 watts down 16-gauge wiring, but do not consider such a thing when the watts are 500 and above.
Limitations
The first limitation on 2-ohm is the type of cabling and interconnects that can be used. If the amplifier boasts it can take 2-ohm stereo continuously at full power, then fine with me, let the amp have it. But if the cabling losses start eating up 10 percent or more of the power delivered, it is time to rethink the process. The classic example that really irks me is the guy with the 2500-watt-per-channel tour-grade amplifier who runs 50 feet of 12-2 speaker cable to a 4-ohm sub-woofer and then jumps in another 10 feet of 12-2 speaker cable into another identical subwoofer. They were just perfect at 4-ohm loading, but now, at 2-ohm loading, that 50-foot cable is just too light for the task.
Using the typical math rules and typical wire gauge resistivity, that 50-foot cable is really 100 feet of round trip wire with minor connector losses if using Speakon connectors. At 1.65 milli-ohms per foot, 100 feet of 12-gauge is 165 milli-ohms. Looking at this from a damping factor perspective, we want a damping factor of 10 or greater, which is the load impedance divided by wiring losses. At four ohms, the damping factor is 24, but at two-ohms the damping factor is a barely passable 12. And some engineers can hear the difference between 10 and 20 values in damping factor. Add some phone plug connectors for interconnects and things become sad.
Two alternatives exist, and both are shown in Figure 1. The first is the “Y-split” right at the power amplifier, and each speaker gets an individual 50 12-2 speaker cable run to keep the damping factor high. The second, and ever more popular, is to run a pair of 10-gauge speaker cabling to the first speaker, and then lighten up with a 12-2 patch to the second speaker. This way, the 10-gauge has about 1 milli-ohm per foot of resistance, or 0.1 ohms, of round-trip resistance. This way, the two-ohm loading has less than 5 percent of its power lost in the beefy cabling. And 5 percent of 2500 watts is 125 watts of heat, and not over 200 watts with 12-2 wiring.
Back to Amplifiers
If you remember my comment on 300 watts into 2-ohms and 210 watts into 4-ohms, then you are getting a clue that amplifier designers really did not want to design amplifiers that were optimally efficient at 2-ohms. To go from 4-ohms down to 2-ohms means that double the amperes of current for a given voltage output must be designed for this lower impedance. This usually means double the number power transistors to support this capability. Until recent times, most designers either limited the power supply or designed for non-continuous 2-ohm capability to keep the amplifier circuitry costs under control.
This practice still lives on today, but the modern tour-grade amplifiers really do regard two-ohm loading as a respectable practice. The big difference is that line array type speaker systems truly do need the two-ohm capability to minimize the amount of speaker wiring climbing up the array of speakers. Your typical 3-way line array cabinet will likely use a Speakon NL8 connection system with the first two pairs handling separate 8-ohm low frequency drivers in the cabinet. This is to permit daisy-chaining four cabinets on the same NL8 cabling system back down to the amplifier rack. Since most line arrays have 4, 8, 12 or 16 cabinets vertically aligned, this all works conveniently with identical amp racks set up for two low frequency, one mid frequency and one high frequency drive circuits. Note that many amp racks of this type have 50 ampere, 120/240VAC power connections or mini-cam-loks.
