Bass Wiring and Electronics

An electric bass has one or two pickups and wiring and electronics that you do not see unless there is a problem. The pickups are discussed in a previous section. The pickups sense the motion of the strings and produce a voltage across the terminals that is related to that motion. The voltage signals are carried to an electronics compartment that is either under a pickguard if there is one, or in the body at the back. Basses often do not bother with pick guards because there just isn't typically the hard driving strumming that is often done on a 6-string, especially for an acoustic. The volumes of these signals get blended for relative amplitude and filtered at least by a tone knob. If there is only a tone knob, it rolls off the high frequencies above some set frequency. The volume may be set individually for the pickups or as a total. The electronics may be passive, or active. If active, there may be an equalizer that adjusts amplitude of low frequency, mid-range, and high frequency output. I had a figure showing the filtering done by the equalizer, but I have not figured out how to add figures to the text in LinkedIn (ah, well). Knobs at the front of the body allow adjustment of the signal before it is connected to the output jack. All the groundings are connected to the jack for grounding through the amp ground. If the electronics are active, there is a 9-volt battery in the circuit and there may be two 9-volt batteries. When the jack is engaged, the battery powers the active circuitry. The jack must be pulled when the guitar is not to be played for a time or the battery will drain.

The strings are moving in the magnetic field of the pickups and are connected to the bridge and the signals are quite small. The wiring in the electronics is very fine. The bridge is grounded with the electronics to the output jack. Electromagnetic interference (EMI) may cause fields in the vicinity of the guitar that may introduce inadvertent signals in the electronics. Essentially, the electronics act as an antenna. This is often caused by transformers in the room and is picked up most prevalently by single coil pickups as discussed in a previous section. The noise is typically a 60 Hz tone or buzz with harmonics. This is commonly mitigated by shielding of the electronics compartment by aluminum or copper foil lining (a Faraday cage). Some guitars also have a shield over the strings to minimize the impact of EMI. Also, double coils in the pickups can be used to cancel this noise, but may reduce the high frequencies.

The electronics compartment is quite small. If there is only a tone filter and volume controls, this is probably fine. When you add in all the controls and knobs for the active components, it gets cramped. Given the fine gage of the wiring, care must be taken when repairing or replacing parts. At one time, I had purchased and received a new replacement of an active electronics set. The pickups I had ordered were not available for several months. By the time I was ready to install the wiring harness, there was this one little wire that clearly had broken off a soldered tab. I had to pull up several diagrams from online sites to figure out just where to solder it. There are a lot of soldered joints and wires. I put check marks everywhere on the diagrams. It was the last joint I checked…it turned out to be the last joint left to check.