PJB Double Four, BG75 Bass Amplification at Bassdirect :: Phil Jones Bass Combos ::, Lightweight 70w combo, Neo Speakers:: For sale, UK, on offer, EU, London, Manchester, Birmingham, USA



Briefcase | Session 77 | Micro 7 | Compact suitcase

Cub | >Double 4 | Nanobass

The Double Four, BG75  (List price: £455) In stock

PJB Double 4 combo Black £395

PJB Double 4 combo Red  £395

PJB Double 4 combo White £395

Add Carry Case to order + £49

PJB External power supply (for older units) £28 + £5 UK shipping

PJB CARRY CASE £49 + £5 UK shipping

(1) Input Jack  (2) Clip Indicator  (3) Mute/ Passive/ Active Switch  (4) Input Level  (5) Bass-EQ (6) Mid-EQ (7) Treble-EQ (8) Master Vol. Control (9) AUX Level Control  (10) Input AUX Jack (11) Line-Out Jack (12) Head-Phone Jack (13) Power On/Off Switch  (14) Power LED (15) DC Power Input - no longer available - now replaced with IEC power lead

The DOUBLE FOUR is a single channel bass amplifier that has an auxiliary input for ipad/ipod, mp3 or drum machine. It features 3 band EQ that is perfectly tailored for bass guitar, headphone out jack and a line out for recording. It is ideal for all kinds of basses and can handle the open B string on a 5 string bass with ease.

The Enclosure

It is well known that big speakers are capable of reproducing true bass fundamental frequencies but as you decrease the physical size of a speaker, the bass response will fall away. Same reason why Violins are small and Double Basses are big!

So how do you get huge bass sound from a small speaker cabinet? Well with our know how of using smaller drivers to get the ultimate bass sound, we applied our vast experience to an even smaller driver just 4 inches across. It is built with Swiss watch precision; it has to be to cope with the extreme signals coming in from an amplified bass guitar. Using our extensive R&D facility which includes the largest anechoic chamber in the bass guitar amplifier industry and the KLIPPEL transducer measurement system (it uses lasers!), we were able to develop the perfect 4 inch loudspeaker. We analyzed the neodymium magnetic system, the cone behavior at all frequencies in 3D representation, cone suspension, frame etc. until we had achieved a transducer beyond what anyone would have ever though possible. Then we looked at the acoustic loading using the acoustic simulation software and calculated exactly the full power cone displacement at every frequency.

We 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 proprietary PJB NeoPower 4-inch speakers are full frequency range to the highest audio frequencies. Not only does this give greater clarity and transparency to your instrument but also gives a high fidelity sound to any backing track fed into the auxiliary input.

Each speaker is fed with its own Pulse Width Modulation PWM amplifier. PWM amplifiers are extremely efficient compared to more traditional analog solid state amplifiers converting more electrical energy to power the speakers instead of getting hot. Heat in electrical circuits is the enemy to reliability and 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 to each speaker, the bass control is greatly improved since the damping factor is doubled. Damping factor is the amplifiers ability to put on the “brakes” stopping the speaker cone from overshoot and tightening up the bass sound, so the sound is punchy, tight but never muddy or bloomy out of control bass.

The cabinet 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 DOUBLE FOUR may just be a practice amp but it does not mean that it has to sound like a practice amp. The sound is huge for its size and you may find it hard to believe that all the sound from your bass is just coming out of such a tiny jewel.

Specifications for this product:


Double Four BG-75


Micro bass combo

70W output


3 band EQ

Bass and AUX input

headphone out

pre-amp out


8.2 x 8 x 6.8 inch(W x D x H)


2 x PJB NeoPower Type C

Net Weight

8.8 lbs / 4 kg


New 2020 units now use a IEC lead and NOT an external power supply.

Older units use a100 to 240V AC - external power supply

Will run off optional Laptop style re-chargeable battery

Optional padded cary case;

As used by Dave Swift

From Phil Jones;

The ambiguous Double Four battery.

Originally when the PJB Double Four was created, it was not actually conceived as a battery powered amplifier but as it is running on the same DC voltage as most lap top computers ( after all the Double Four is actually a computer that processes the bass signal ), I discovered that an external laptop battery would be able to power it perfectly.

Technology and regulations change at an ever increasing pace and in light of the infamous exploding Samsung Note phone, FAA Regulations have tightened up as to what kind of battery can be carried on or check in on a flight. One of the strictest countries on these restrictions is China where nearly all these batteries are produced and in fact I had a battery that could power a Double Four confiscated at Chinese airport security a few years ago.

My preferred battery for the PJB Double Four BG75 is the Lenmar PowerPortPPU916RS as it has good energy capacity of 5500mAH and can run on 19Volts which is enough for the amp to play up to 4 hours. Plus it was one of the better prices and can slip between the rubber carrying handle and the cabinet.

It seems now that this battery is being discontinued however on Amazon.com (sorry not UK Amazon), you can still find sellers of this. If you cant find this battery then you are not out of luck but be prepared to pay a bit more. This battery will also run the Double Four.


The key parameters you need to look for in a battery are:

1. Its voltage: you need at least 18 volts minimum and 21 volts max.

2. Energy capacity. The more MAH the longer it will last but many items on Amazon have typos. The main one is there are at least too many zeros on the mAH specs. So if you see something like 27000mAH, unless it is over $200 it is more than likely 1/10 of that capacity. 

3. Peak current capacity: many other batteries have a short circuit protection to stop them blowing up or catching on fire so the current capacity is limited to 2.5 amps. These batteries will still rum a Double Four but only at lower volumes. As soon as you start to play louder the battery just shuts down and you think something is wrong with your amp. It is the battery unable to supply the current to you amps demands. 

If you are flying with your Double Four and a battery for it, you should read this article to check that you are in compliance with US airline restrictions on batteries,.


Other countries have slightly different restrictions but all of them will block you taking on a battery that has massive energy reserve: example 300 watt hours. The energy in these high capacity batteries if release at once like a short circuit in it would be like a small bomb going off and that is definitely a big NO NO!


Tel: 01926 886433