Help with amp design

billybob said:

My friend is wanting to build his own car sub setup around 1500Watt max.
Im by far no expert and was hoping I could get some knowledge on this subject. Would the wattage be too much to effect car function? What are some good schematics I can study? Class A, AB, or maybe D? He is a beginner, but is willing to learn. Would giant caps be necessary to filter the power consumption?
Thank you,
Ben

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JimB said:

Assuming that the car has a nominal 12v electrical system, 1500W will require a current of 1500/21 = 125 Amps.
That is assuming that the amplifier is 100% efficient, which it will not be, lets be generous and say it is 75% efficient, the current drain on the battery will be about 167 Amps.
Giant caps may just help with the peak currents.

But the real problem that I envisage is:
What will be the sound pressure level inside the car when this is running?
I would advise the use of giant ear defenders, as used by airport ground crew while working near a jet engine which is running.
I see your friends future, it involves severe hearing loss.

JimB

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MacIntoshCZ said:

I though watts on sounds is something else. Its not exactly power.

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rjenkinsgb said:

Watts RMS out of an amplifier are no different to watts running a heater, it's an exact calculable and measurable figure.

It's the "peak" or "PMP" and such figures that many advertisers use which are totally imaginary.

The big old PC speakers I have on this machine are a good example - they were listed as 100W peak power, but only take 4W input from the mains.
Probably more like 1 - 2W RMS total so 0.5 - 1W each, in reality.

That's how ludicrous and meaningless "peak" figures can be.

(Sound pressure is very different; that's a combination of power, speaker efficiency and distance from the speaker or source).

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rjenkinsgb said:

Class AB is analog, the power transistors dissipate a good proportion of the total input power; eg a 15W Class AB guitar amp may take 40W input power.

Class D amps are digital, effectively a PWM signal modulated with audio. They are more complex and need filtering on the output to remove the PWM carrier waveform, but can be incredibly efficient, 90% or more even.

With integrated ones most of the complexity is hidden so they can be quite easy to use, if there is a suitable power IC available.

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JimB said:

But the real problem that I envisage is:
What will be the sound pressure level inside the car when this is running?

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Les Jones said:

It is probably true but it does not matter as it's only purpose is to bring one early deafness and annoy other people. Speakers in the car driven with that power level will probably generate more distortion than the amplifier.

Les.

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billybob said:

Ok good to know. So AB is not so great because it uses most of the energy as heat while class D does not. Sound quality I heard that AB is better is this true?

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audioguru said:

Here is a large sub-woofer for a car. It is so heavy that it takes a few guys to carry it. It is rated for 4000W and can break all the car's windows and the driver's hearing.

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schmitt trigger said:

You can always face the speakers rearward, and obtain additional thrust.

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