The Bass Station

Powering all these fancy LEDs is not a realistic proposition from battery power. The maximum dissipation from the neck is around 100W, but a more realistic value is around 30W as there is no intention of turning all 400 LEDs on at full power. Even so, I do not want the weight of a battery capable of providing this level of power round my neck for a few hours, so the power needs to be sent through the guitar cable.

12V is going to be sent to the guitar which is then locally converted to 5V for the LEDs. Distributing power at 12V instead of 5V reduces I2R loss and if the voltage drops by 10% due to ageing connectors etc, the local 5V is unaffected. The same trick is used to power the processors on many PC motherboards - bulk supply 12V and locally convert to 1V.

A one-cable-only solution to the guitar is important as it reduces tangling, tripping and connection errors. The downside is that changing LED patterns can cause high current swings of several amps which could contaminate the pickup signal easily.

Prevention is always better than cure, so the effect of noise sources are minimised where possible by the usual techniques of segregation, bandwidth limiting and keeping loop inductance low. In other words keep the noise-makers and noise receivers as far away from each other as is practical, reasonable filtering / decoupling and use twisted pair wiring for power connections.

The Bass Station will contain a differential receiver, power source, filtering, switching and the ability to sense the guitar being connected. Front panel LEDs show mains power, connection, guitar power and signal mute. When the guitar is plugged in, the power is applied after a delay, followed by the bass-station output signal being un-muted after a further delay.

The Bass Station is housed in a 1U high 19" rack case so there is plenty of space for the 12V 80W PSU with two 25mm cooling fans and some ducting. The circuitry is built on veroboard and controls the power to the guitar and converts the differential signal back to a standard single ended 1/4" jack socket.


An Inside Job


The space available inside the guitar is quite limited. A Jazz Bass body was purchased online and a router used to enlarge the cavity under the normal JB scratchplate. To keep noise under control the circuitry is split into two parts - pickup controls & amplifier and the LED controller section.

A typical JB has three controls and a 1/4" socket on the control plate. The plan is to keep the guitar looking quite normal, so the controls remain in the same places, but I want them to do different things. I'm not a big fan of volume - why would I ever want anything less than full beans? - so there is going to be a pickup pan control in the middle and two separate tone tilt controls. I can then dial in either pickup singly or a blend of the two. The individual tilt tone controls give warm and boomy at one end to twangy and edgy at the other. I can then have the top end from the bridge pickup blended with the thump from the neck pickup. That should give me enough variation...

The two boards were designed in one go and built together. The pickup amps are on the left hand side. On the slightly larger LED controller side, the power converter section is on the right hand side, with the SO-28 outline visible on the left ready for a PIC24EP256GP202 which will be doing all the clever stuff. This is one of the most powerful microcontrollers that is still able to be soldered in by hand (well, by me anyway). Plenty of higher spec devices are about but they are in TQFP or worse still BGA packages. I don't want to solder those!


The Missing Link


The cable linking the bass to the bass station has also undergone some careful consideration. In the end Van Damme XLR 4-way 'snake' cable was used - this is four twin screen pairs that will *just* fit into a 5-way XLR connector and provide a single screened twisted pair for the signal, leaving the other three pairs to carry the power between them. 5-way XLR connectors are rated a 7.5A. The cable is superbly flexible and not much thicker than a normal guitar lead.

In the middle of a gig, if the LEDs stop is a bit of a shame, but if the bass cannot be heard at all then it's goodnight and so long. In this sort of catastrophe, the 1/4" socket on the guitar is routed directly to the bridge pickup for a sort of limp-home mode. It will allow a normal guitar lead to be used and will carry the passive signal in case all the electrickery goes up the wall!


Powder And Paint

The usual sanding through the grades and covering in primer was carried out. With 400 RGB LEDs, a neutral colour seemed to be a good idea. I like the white JB look, but I wanted a pearlescent finish. This tends to make the paint very exclusive / expensive but also difficult to know exactly what you're getting as the light falling on the surface at different angles will give a different look.

From different websites it seems that pearlescent paint is (in simple terms) ordinary paint with sparkly flakes of mica added. Further investigation revealed that cosmetic grade mica is widely available sold in powdered form.

I hatched a crazy plan. I painted several white colour coats and let them dry for two weeks before a very light sanding. In between application of the top lacquer coats, I puffed a small amount of the mica dust when the surface was still tacky. Subsequent lacquer coats seal in the mica. This was repeated four of five times, finishing off with a couple of un-puffed lacquer coats. The progressive application of the mica in this way has resulted in a white guitar that, upon closer inspection, has a bit more going on than first met the eye.

After waiting another few weeks, the body and headstock was lightly sanded and polished to bring out the full depth of the colour. The pearlescent effect is not up to custom car standards, but does give subtley different hues dependent on lighting / viewing angle. The mica chosen was a cream / blue combination which gives the guitar a pleasing vintage white colour with blue sparkles where the light hits the contours.

Don't you just love it when a plan comes together...

The additional angled board seen by the lower horn is a bluetooth to serial link module. This will allow text entry to the microcontroller from a phone app :-)