The product is only 50cm x 50cm x 9cm.
I cant see there being two 2kW class D amplifiers and a 4 x 1000W = 4kW offline SMPS inside that case?
...I mean , you are plucking a bass guitar at a festival........theres no way you are continuously plucking......the maximum average power must be very low?.....i reckon youre talking a 500W offline SMPS inside that FFA4004?
Amplitude of a weak signal goes only slightly above and below centerline. PWM translates this into very narrow pulses either positive or negative. Only slight power is drawn.
Even if the power supply is single polarity The case is the same. Any output stage must provide 0 V and 0 A to the speaker when volume is zero. Therefore a DC blocking capacitor or smoothing capacitor is in the path, minimizing current when volume is zero.
No problem to pack 4 kW continuous power into 2U 19-inch case. Beyond the question of used average power for the instrument amplifier, I presume the 4004 can supply 4x1kW continuously and is tested for.
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We see e.g. 30 kW in 3U in Keysight Regenerative Power System, even with bidirectional PFC.