Replacing rear speaker

Gee, I'm very surprised that a modern car stereo worth any salt uses 8 ohm speakers! Where exactly does it say that the speakers are 8 ohms?

There is an inherent power limitation when it comes to car radios, and the speaker impedance has a direct effect on this. Here's the deal: If you have a simple sine wave whose top peak reaches 12 volts, and bottom peak reaches zero volts, you effectively have the strongest wave you can get from a 12 volt power supply. Any stronger and it will distort. Do the math -- feed this into a 4 ohm speaker and you get about 9 watts RMS (6 watts practical). With an 8 ohm speaker you only get 3 watts! This is why car stereo typically goes the 4 ohm route -- and in some cases, 2 ohms.

There are a couple of methods of getting around this power limitation. First, if you bridge two amps together, you have an effective + 12 volts and = 12 volts, or a 24 volt spread. Now you can get up to 22 watts per 4 ohm speaker (25 if you fudge the numbers just a little -- and most modern day stereos are 25 watts per channel). Of course with stereo, it's easy to double the wattage to 50. With four channels you get up to 100 watts. With

Yes, you get do more than this, but only by using a high-power chopper power supply. These are the big amps that mount in the trunk of the car. From a dash mounted stereo however, the above applies to power. With this in mind, it's true that you can increase output power by using 2 ohm speakers, provided the amp is designed to handle it. However if you up the speakers to 8 ohms, you cut the available power considerably, no matter how the amp was designed. This is why I'm surprised to see 8 ohm speakers in your car.

It's possible you can get by with 4 ohm speakers, even if the originals are

1. Since the amps are designed for 8, your power output may be slightly less, and technically there is more load on the output, but it's doubtful this is enough to create a problem. A 2 or 1 ohm speaker load is a different matter. As for sound quality, the only difference is that of the speaker design -- it does not affect your audio signal in any way.

-- Gary E. Tayman/Tayman Electrical Sound Solutions For Classic Cars

Its says they are 8ohms on the speakers themselves. Prior to making my posting I read several posts/FAQs about OHMs and impedience(?) but didnt understand them. What did catch my eye was a few posts saying that going the 4 ohm route draws more power and could fry the radio. I just wanted to get a clarification before putting in new speakers. Thanks John

Well, here's the rundown, without getting really technical. Any audio amp is designed for a certain output impedance, and this is called maximum power transfer. The output transistors are biased in such a way that the maximum amount of power can be sent to the speakers if they have an impedance of 8 ohms. Most car stereos are designed for 4 ohms. If you read the spec sheet of a large home or pro amp, you may see something like "200 watts per channel into 8 ohms, 400 watts into 4 ohms," etc. What they're saying is that, while 4 ohms may give you the peak power output, 8 ohms will work but will give you less power.

More than likely, if your speakers are 8 ohms, the amp is designed to give you the highest amount of power at 8 ohms. 4 ohms will work, but will give you less power. Personally I think you'll be fine; if you install 4 ohm speakers and the amp blows, I'd be looking for a reason other than the speakers.

-- Gary E. Tayman/Tayman Electrical Sound Solutions For Classic Cars

where do you find 4ohm's? all i can find are 8ohms. use 8ohms or you will be sorry