Recently, we had a customer ask us about installing a high power car audio system that could “drop down to 1/2 ohm”.  We have in fact heard something like that many times in the past, and the premise seems to be that using speakers that add up to a 1/2 ohm load, and an amp that is rated for 1/2 ohm operation, can put out the most wallop.  In addition, some customers have the notion that making the load switchable, ie., it operates at perhaps 2 ohms most of the time, and 1/2 ohm when doing SPL or similar duty, is the best bet.  (Paul supposes the reason for this would be to only “stress” the amp on the rare occaisions when the extra power is really needed.  There is no doubt that a 1/2 ohm load makes an amp run hotter, the vehicle’s electrical system that supplies the amp works harder, etc.*

However, is 1/2 ohm operation really the best way to go, to get the most wallop for your money?  Well, not really:

Take a look at almost any “high current” car amplifier’s ratings, and, if the manufacturer is being honest with their ratings, you will see something like this (courtesy Sundown Audio / copied from their forum):

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1 amplifier, single 1000ca everstart maxx battery, factory alternator:

4 ohm nominal = 518 watts @ 88% efficiency (voltage fell from 14.3v, to 14.0v)
2 ohm nominal = 948 watts @ 85% efficiency (voltage fell from 14.3v, to 13.6v)
1 ohm nominal = 1510 watts @ 78% efficiency (voltage fell from 14.3v, to 12.4 volts
.5 ohm nominal = 1615 watts @ 72% efficiency (voltage fell from 14.3v, to 11.8v)
.35 ohm nominal = 1748 watts @ 67% efficiency (voltage fell from 14.3v, to 11.4v)

amplifier was protection happy @ .35 ohm with tones, and some music.

IMO.. one 1500d amp should be limited to 1 ohm nominal use in factory electrical systems.. for the sake of the car.. not the amp

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now… to have fun with some juice..

1 amplifier, single 1000ca everstart maxx battery, 1 Eagle Picher 12a5000 battery, factory alternator:

4 ohm nominal = 520 watts @ 88% efficiency (voltage fell from 14.3v, to 14.1v)
2 ohm nominal = 960 watts @ 84% efficiency (voltage fell from 14.3v, to 13.7v)
1 ohm nominal = 1635 watts @ 75% efficiency (voltage fell from 14.3v, to 12.8 volts
.5 ohm nominal = 1890 watts @ 71% efficiency (voltage fell from 14.3v, to 12.8v)
.35 ohm nominal = 2245 watts @ 65% efficiency (voltage fell from 14.3v, to 12.8v)

amplifier did fine @ .35 with tones and music with the added battery

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(The above are actual test results on a Sundown Audio SAZ-1500D, posted on their forum @  http://www.soundsolutionsaudio.com/forum/index.php?showtopic=7116 .  We have edited the text, but not the numbers, slightly, for easier reading.   For Sundown’s published ratings, which are more conservative, see http://www.sundownaudio.com/1500d.html .  The test results, BTW, are very impressive!)

As can be seen, with use of a beefed up car electrical system (high $$ alternator and 2nd battery), dropping from 4 ohms to 2 ohms, power increased by a factor of 1.85 (85% increase), and amplifier efficiency only lost 3% — pretty doggone good.  Dropping from 2 ohms to 1 ohm, power increases another 70%, and efficiency loses 9% — not bad, but a trend is beginning.  At 0.5 ohm, power increases only 15.6 percent further (with efficiency still dropping), and so on…

So, half ohm operation does not get you that much power, but it DOES mean:

The amp runs much hotter.

Heavier wire must be used from battery to amp, and from amp to speakers, in order to not lose any power increase in the wires.

Distortion will be higher (trust us on this – it’s normal / typical of virtually any amplifier.)

Either special speakers must be used, or, more speakers must be used / paralleled.

Due to the efficiency loss, even more capacity from the car’s electrical system is needed, than otherwise would be, for a given amount of power actually delivered to the speakers.  (The wasted power is showing up as / in the hotter amplifier.)  Granted, you probably need to really beef up the car’s electrical system, anyway, for 1/2 ohm operation.

We would suggest that you are better off sticking with a 2 ohm or one ohm setup, with more amplifiers, if you need really high levels.  Linking amps or simply running two of them, each into a 1 ohm load, may make more sense, than 1/2 ohm operation.  Two linked SAZ-1500D’s, assuming a beefed up car electrical system, can put out 3150 watts (see above link to the power testing thread), which pretty much blows away the single SAZ-1500D’s 1890 watts into 0.5 ohm shown above.  Two of the SAZ-1500D’s, each running a 1 ohm load would add up to a total of 3270 watts.  Whew!  (Two of these are more money, but, you save some back elsewhere, and it’s a lot better setup / system.)  Another possibility would be to run a couple SAZ-1000D’s into 1 ohm loads, which comes out to be 1800-2000 watts (depending on configuration).  This too, is a significantly better setup, than a 1/2 ohm setup, at only a bit more money.

*So, how did this 1/2 ohm stuff get started?  Well, some years ago, in SPL competitions, some categories were created for people with lower power amps.  The challenge was to see what was possible without putting in a huge system, ie., how “efficient” could a system be in producing high sound levels without having huge amps.  However, competitors found a “loophole”.  The power classes were determined using an amplifier’s 4 ohm power rating.  So, by using the amp at, say, 1/2 ohm, it was possible to actually “stealth” deliver a lot more power to the speakers than the competition category limit intended to allow.  Initially, people just found / used really robust amplifiers that were not necessarily rated to 1/2 ohm loads, but could withstand them.  Next, a few companies started making amplifiers specifically for this type of usage, but not necessarily publishing the high power (at low load impedance) capability.  It was almost a sort of deliberate underrating.  Eventually, the competition organizations caught on, and redefined the categories, thus eliminating the “stealth” advantage.  But, by then, the “urban legend” of 1/2 ohm operation for maximum SPL’s had been established.

More information on the details of  various impedances’ operation, how power is defined, etc., are available here.