What the PJB website says:
The DOUBLE FOUR is the pinnacle of micro bass amps. With new driver design, acoustic loading and the latest digital amplifier technology, this tiny bass combo may be designed as a practice amp but continues to amaze everyone with the great PJB signature bass tone. From a cabinet with size of a jewel box, this seems to defy the laws of physics.
Driver and Amplifier Technologies
Driver and Amplifier Technologies Pushing our expertise of using smaller drivers to get the ultimate bass sound, we applied our vast experience to an even smaller, “Neo Power”4 inch driver built with precision of a Swiss watch. This 4” baby is able to cope with the extreme signals coming in from an amplified bass guitar. By using our extensive R&D facility which includes the largest anechoic chamber in the bass guitar amplifier industry, and the KLIPPEL transducer measurement system, we were able to develop the perfect 4 inch transducer for our purpose. We analyzed the neodymium magnetic system, the cone behaviors at all frequencies in 3D representation, cone suspension and frame, until we achieved our goal. We then looked at the acoustic loading using the acoustic simulation software and calculated exactly the full power cone displacement at every frequency.
Each one of these 4” speaker is fed with its own Pulse Width Modulation PWM amplifier. PWM amplifiers are extremely efficient compared to the traditional analog solid state amplifiers, converting electrical energy to power using on the speakers instead of wasting it on hot. Since the amp is running at lower working temperature, the DOUBLE FOUR will give years of service long after other amps have faded away. By using dual amplifiers one for each speaker, the bass control is greatly improved since the damping factor ( the amplifier’s ability to put on the “brakes” to stop the speaker cone from overshoot ) is doubled. The end result? A bass tone that is well controlled, punchy, tight and never muddy or bloomy.
To maximize its tiny cabinet enclosure, we designed a custom Rectangular Auxiliary Low Frequency Radiator (RALFR®). RALFR® 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, so power handling is greatly improved. The cabinet is also heavily braced internally and lined with acoustic damping materials. This damping prevents cabinet coloration that can interfere with the true sound of your instrument. Now, what you hear from the DOUBLE FOUR is the most realistic amplification of your instrument.
What we say:
Wow! If you want a lightweight and powerful single channel bass combo, which you can fit in the boot of even the slinkiest of sports cars, and which does not compromise on sound quality, look no further. All PJB bass combos focus more on quality of sound as opposed to volume, so do not expect a brutally loud amp; but, it balances perfectly well against a ukulele group, or small acoustic group really nicely.
However, if you want a mic channel as well, you would have to look at the BG-100, which has two channels.
Power Output: 70 Watt (RMS)
Speaker: 2 x PJB NeoPower, 4 inch speakers. Rectangular Auxiliary Low Frequency Radiator (RALFR®) Switch able high gain input OR: low gain stage with mute function
EQ: 3-Band EQ (Treble, Mid, Bass)
Input: AUX-Input Jack, DC-Power Input Socket
Output: Line-Out, Headphone-Out
Other: Power Switch
Power: 100/240V AC
Dimension (W x D x H): 11 x 8 x 6.8 inch / 280 x 203 x 172 mm
Weight: 8.8 lbs / 4 Kg