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Depends on the design - from 12V you can only get about 4W in a 4 ohm speaker, bridging the amplifier effectively doubles the supply voltage, allowing 16W. Any more than that uses an inverter to generate a higher supply rail, perhaps as much as 100V (or more) so you can get some real power out of it.
A regulated inverter PSU will use pulse width modulation to control and regulate the output voltage, and recent amplifiers are probably class D as well, which is a form of pulse width modulation.
A car stereo that is advertised as having 200W produces only 16W RMS continuously into each of its 4 channels when the car battery is being over-charged at 14.4V. That is a total of only 64W, not anywhere near 200W.
The peak power of 16W RMS is 32W. Each amplifier can produce 32W when its volume is turned way too high so it produces terrible distortion. If the load is almost a short then each channel might produce 50W for a moment.
The Sears catalog had fine print that said, "Our 200W stereo console produces 4W RMS per channel continuously with both channels driven into 8 ohms from 20Hz to 20kHz at no more than 0.1% harmonic distortion".
Car stereos don't say any of the details. Their Watts are actually Whats.
Look at the little car amps that have big letters, "100W". They are powered by 13.2V and have a 1A fuse. Figure 60% efficiency then the fuse will blow if the output power into two 4 ohm speakers is only 4W per channel.
Most of the car amplifiers and speakers sold in the UK also have similar wildly exaggerated ratings - it's not a question of a down right 'lie', it's a question of misleading the public as much as possible.
For example, a car amplifier might be advertised as 200W.
1) - it's four channel, so it's only 50W per channel.
2) - this isn't continuous RMS, it's music power, and the absolutely shortest highest possible peak they could measure, and only on one channel using a higher supply voltage than it will ever get in a car - so probably only 12.5W RMS.
So which sounds better? - a 200W amplifer, or a 12.5W one!.
This isn't a joke, these are the sort of figures than car amps (and speakers) are overrated by - connect a real 50W amp to a 200W car speaker and it's only likely to last a few seconds!.
Always look for RMS power output per channel, continuous, over a specified frequency range and with a specified distortion - for example:
65W RMS per channel, continuous sinewave, all channels driven, from 20Hz to 20KHz, at less than 0.05% distortion.
They do not seem to apply to car stereos. It is unheard of for the stereos to not advertise absolute peak power.
One thing they might do is to put a bulk cap in to hold up the rail for a bit... I did design an audio amp that required 8 amps peak to run off a 2A supply with about...I think it was 10,000uF.
IC manufacturers also exaggerate the power ratings of their IC power amplifiers:
1) They use a 14.4V supply. Your car battery might over-charge at 14.4V but the voltage loss in the fuse and in the wiring to the amplifier reduces it to about 13.2V, then the power rating is 30% higher.
2) They turn up the volume way too high so the sine-wave input produces a square-wave output with 10% distortion, then the power output is 42% higher.
so this is why car speakers use 4 ohms instead of 8. to get more current because of the lower voltage? but im sure there are amps out there that step up the voltage aren't there?
My car was built with pretty good 2 ohm speakers installed! It also has a high power amplifier and a 10" sub-woofer that shakes my bones and ears.
All high power car amplifiers have a DC to DC stepup power supply. Here is a project that produces positive 35VDC and negative 35VDC. The total is 70V at 6A. It produces a momentary power of 420W. The continuous power is about 300W. These are real RMS Watts at low distortion. **broken link removed**
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