Joe D
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Joe D asked in Science & MathematicsEngineering · 10 years ago

Electronics ppl, Please help with this power/wattage question?

I have a car stereo amplifier that is rated at 200 watts RMS. When the amp is turned on, but there is no music signal from the stereo (i.e., amp is on, stereo is off) roughly how much power is the amp drawing from the cars electrical system? In other words, does it only draw 200 watts when there is a music signal and the volume is cranked up? Or does it draw 200 watts all the time no matter what? Sorry if dumb question :-/

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  • 10 years ago
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    No, it definitely doesn't draw 200 watts all the time.

    Modern MOSFET audio power amplifiers are operated in "Class D" mode which means they draw negligible standby (or "quiescent") current and the actual current drawn from the DC supply is always proportional to the RMS power being delivered to the speakers at any instant. And modern class D amps achieve 90%+ efficiency, so for example when delivering 180W of audio they will only be wasting about 20W as heat, and so drawing about 200W of power from the supply.

    So its only when you crank up the volume that the amp starts to draw the big Amps!!

    However, your modern car stereo is also microprocessor controlled, as is the amplifier, so the overall standby current could be 1 Amp or so. This is about 12W of power from the 12V system. At full audio power, the current would AVERAGE about 200/12 ~= 17 Amps, but the PEAK currents would be 50 Amps+.

    If you have a multimeter with a DC 10 Amp range, or similar, you could measure the standby current by removing the in-line fuse from the +12V supply lead, and then connect the multimeter leads in its place. You could do this for the stereo system as a whole, and for the amplifier only - if it has a separate fuse. Then you could start cranking up the volume and watch the current increase.

  • 10 years ago

    Can't be answered. The 200 watts is the maximum RMS power developed in the speakers. The typical speaker power is a lot less, perhaps 10-50 watts.

    But when there is no sound at all, then the only power is the quiescent power in the amplifier. And that depends on the design of the amplifier. It could be 1 watt, it could be 10 watts, it could be (unlikely) 50 watts.

    But it's not 200 watts.

    .

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