Hertz SP 4.900 AMPLIFIERSP 4. 900 D CLASS FOUR CHANNEL AMPLIFIER Hertz ADC (Advanced D Class) The Herz SP compact amplifier range consists of three high performance models. There are two four channel models, the flagship SP4. 900 with 160 x 4 W RMS at 4 and the SP4. 500 with 90 x 4 W RMS at 4, and a mono model SP1. 900 boasting 1000 W RMS at 1. Hertz SP amplifier are the first Hertz amplifier featuring the renewed Hertz ADC (Advanced D Class) output stage, reaching up to 87%