Two of the most practically important amplifier specifications — power bandwidth and dynamic headroom — are among the least commonly listed on manufacturer data sheets. Their absence is not accidental. These measurements reveal how an amplifier actually behaves under real listening conditions, and for many designs they tell a less flattering story than the headline wattage and distortion figures.
Power bandwidth describes the frequency range over which an amplifier can deliver its rated output. A well-designed amplifier maintains its rated power from below 20 Hz to above 20 kHz. Many amplifiers — particularly those using heavy global negative feedback to achieve low distortion figures — begin to compress and clip at frequencies below 50 Hz or above 15 kHz before reaching their rated output. Music contains significant energy at frequency extremes, particularly in the bass region. An amplifier that clips at 40 Hz before reaching its rated power is a meaningfully less capable device than one that maintains full power to 20 Hz — but this distinction does not appear on most specification sheets.
Dynamic headroom describes the difference between an amplifier’s sustained power output and its short-term peak capability. Music is rich in transients — brief, high-energy events that demand instantaneous power far in excess of the average program level. An amplifier’s ability to meet these demands without compression or clipping depends on the design of its power supply: the transformer rating, capacitor bank size, and rectifier capability. A generously specified power supply can store and release energy quickly, producing dynamic headroom of 3 dB or more above the rated continuous output. An amplifier with a modest power supply may clip on peaks at power levels well below its rated figure.
Dynamic headroom is where the gap between specification and reality is often most significant. It is also where musical performance is most directly affected. An amplifier that measures 200 watts in a steady-state test but compresses on transients at 80 watts is a 200-watt amplifier in specification only.
Ask for power bandwidth measurements. Ask about power supply design. These questions reveal more about real-world capability than any headline wattage figure.