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| By: swez on Thu, 2 Oct 15:38:00 -0500
| | Someone requested an article on RMS power ratings of a typical car audio amplifier. This is a bit more complex subject than it seems at first blush. But let's see if we can spell out the basics.
Class D amplifiers are generally 80-90% efficiency
Class A/B amplifiers are 50-60% efficiency
Class T or Digital amps are close to 90% efficiency
As mentioned earlier, to find the true output of an amplifier, we need to know the actual DC input voltage, current draw and efficiency rating. We can roughly determine the peak output power of an amp, then with a little math... (Ohm's Law) we can estimate the RMS power of same.
Let's run some numbers on a popular Class D amp: JBL BP600.1
Voltage Source: 12.9 volts/specs (assumed)
Peak current draw: 57 amperes/specs (2 x 30A. fuses)
Class D Circuitry (eff % > 0.816)
RMS Ratings: 300 RMS @ 4 ohms, 600 RMS @ 1-2 ohms (specs)
The Math: (Ohm's Law)
Volts x Amps = Watts (this is peak or max rating #)
12.9 x 57 = 735.3 watts (peak draw) DC input
RMS/735.3 = .816 eff. rating
RMS power = peak watts x eff. %
735.3 x .82 = 600 RMS @ 1-2 ohms (300 RMS @ 4 ohms) RMS AC output
OK, that the scoop. I did fudge a bit to determine the efficiency rating number. But we do know that the minimum output specs for this amp are 600 RMS into a 1-2 ohm load. We also know that this Class D amp is above 80% eff.
To really dial this in perfectly, we would need a regulated DC power supply source, a fixed load resistor, a calibrated input signal (1-4 volts AC) source, an AC/DC voltmeter and an ammeter to measure voltage and amps at the load resistor. Having all this test gear, we can then make a precise output measurment of a given amp.
Hope that helps,
Swez
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