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I'm on the market for a car subwoofer system, and it occurred to me that there were many amplifiers capable of producing 1000W+ of output. However, even with a 1-ohm speaker, the power equation \$P=V^2/R\$ seems to dictate that even with a car battery voltage of 16V, the amplifier would not be able to drive more than 250W into the load.

So how do they do it? Is some kind of voltage conversion done inside the amplifier?

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    \$\begingroup\$ The voltage rails have to separated by more than \$2\sqrt{2\cdot P\cdot R}\$ volts. With \$4\:\Omega\$, this means about \$200\:\textrm{V}\$, or rails of \$\pm 100\:\textrm{V}\$. You can pretty much bank on the fact that they use a switcher to develop the needed voltages. The same thing had to be done in the 1950's for HAM radio operators with transceivers in their cars. But they used mechanical vibrator switches and heavy transformers for that job. (You could still use those, if you wanted. It would just cost more and take up more room and added weight.) \$\endgroup\$
    – jonk
    May 23, 2017 at 20:23
  • \$\begingroup\$ The output stage is most defiantly class-d or class-d inspired \$\endgroup\$
    – sstobbe
    May 24, 2017 at 2:15

2 Answers 2

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There are a few things to keep in mind.

First, the 1 kW figure is almost certainly "peak power", or instantaneous peak power, not rms.

Second, the amplifier is certainly configured as a bridged amplifier, along the lines of

schematic

simulate this circuit – Schematic created using CircuitLab

with R3 equalling R4 to give a second stage gain of -1. Then the peak-to-peak voltage will be double the voltage from a single amp and a grounded (or, in this case, pseudo-grounded) load.

So, for your nominal 16 volts and 1 ohm, the maximum power will be 1024 watts.

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The answer is actually impedance conversion. The amplifier is designed for a spot on the load line that develops the required output power. The load resistance that developes this power is then transformed to the actual load (speaker in this case) impedance.

The impedance transformation can be facilitated with a transformer, an L network, a Pi network, or other means - not all of which are practical for a broad frequency range. The impedance transformation can result in a load voltage higher than the source voltage.

The amplifier design may also include a method to step up the supply voltage to the final amplifier in order to minimize the impedance conversion requirements.

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    \$\begingroup\$ Remember for audio we are looking for 3 DECADES of bandwidth, not going to do that with an L or Pi network... What is actually done is that a switch mode supply drives a ferrite transformer to generate the +-70V or so required (The loads are generally bridged). \$\endgroup\$
    – Dan Mills
    May 23, 2017 at 23:24
  • \$\begingroup\$ Good point Dan. I will edit my answer. \$\endgroup\$
    – Glenn W9IQ
    May 24, 2017 at 0:05

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