The Phil Jones Double Four BG-75 Bass Combo Amplifier is a single channel bass amp with an incredible compact design and remarkable tone. Dual 4 inch speakers pump out a sound so huge it's hard to believe it's coming from such a small amp. The Double Four amp also features an auxiliary input for ipad/ipod, mp3 or drum machine, 3 band EQ, headphone out jack and a line out for recording. Phil Jones designed the amp to be ideal for both 4 and 5 string basses. The cabinet of the Double Four BG-75 amp may be as small as a shoe box but it is built to the same standards of all PJB cabinets, heavily braced internally and damped with acoustic damping materials to prevent cabinet coloration that can kill the true sound of your instrument. What you hear from the Double Four is the closest you can get to how your instrument actually sounds. The EnclosureSo how do you get huge bass sound from a small speaker cabinet? Phil Jones applied their vast experience to an even smaller driver just 4 inches across. Using their extensive R&D facility which includes the largest anechoic chamber in the bass amp industry and the KLIPPEL transducer measurement system, they were able to develop the perfect 4 inch loudspeaker. They analyzed the neodymium magnetic system, the cone behavior at all frequencies in 3D representation, cone suspension, frame etc. until they had achieved a transducer beyond what anyone would have ever thought possible. Then Phil Jones looked at the acoustic loading using the acoustic simulation software and calculated exactly the full power cone displacement at every frequency. Phil Jones designed a custom Rectangular Auxiliary Low Frequency Radiator (RALFR). That is a pneumatically coupled radiator loaded to the rear of the two 4 inch speakers that operates at the lowest bass frequencies only. It augments the output of the loudspeakers at frequencies from 30 to 150 Hz. It also reduces the cone excursion of the speakers allowing greater power handling. The proprieta