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Project by Brandon Speedie upgrade your instrument with these Electric & Bass Guitar Pickguards The control circuitry on electric basses has remained remarkably simple since Leo Fender first introduced the instrument over 70 years ago. These modern PCBs offer more advanced features and a cool aesthetic. They suit many popular models of electric bass as well as the Fender Telecaster electric guitar. Image source: https://unsplash.com/photos/teal-and-brown-electric-guitar-phS37wg8cQg M ost electric guitars and basses have circuitry built into the instrument’s body. Typically, it includes a passive network of potentiometers and capacitors to give the musician control of output volume and ‘tone’. The standard configuration is shown in Fig.1. The volume control is a potentiometer that divides the audio signal voltage from the pickup(s). The tone control is an adjustable low-pass filter to reduce the amount of treble and therefore change the instrument’s sound to suit different music and playing styles. Its simplicity has made it a popular circuit, remaining relatively unchanged since the early 20th century. However, it has some weaknesses. These updated circuits aim to correct some of those shortcomings and add some handy new features. Most electric instruments, including guitars and basses, use passive inductive pickups to sense the vibrations of the metal strings, converting them into electrical signals that can be amplified. Pickups There are predominantly two types of electromagnetic pickups used on guitars and basses. The most common is the ‘single coil’ type, so called because it is constructed of a single inductor wrapped around a set of permanent magnets. The magnets are made of an iron alloy known as alnico (aluminium, nickel & cobalt), which are positioned under the instrument strings to form ‘pole pieces’. Wrapped around these magnets are several thousand turns of enamelled copper wire (see Fig.3). The pole pieces magnetise the strings, Fig.1: a typical control circuit on an electric guitar or bass. The ‘tone’ control is an adjustable low pass filter, while the volume control is an adjustable voltage divider. 86 Silicon Chip Australia's electronics magazine producing a changing magnetic field for the copper coil when plucked. This movement induces a voltage in the coil, which is ultimately sent to an amplifier for playback or recording. The other type of pickup is called a “humbucker” because it can cancel interference and therefore reduce the hum induced by noisy sources such as nearby transformers and fluorescent lighting. The humbucker has two coils mounted next to each other in the Fig.2: the magnetic field lines around a typical humbucker pickup. Source: Lawing Musical Products – siliconchip.au/link/abw4 siliconchip.com.au bridge-mounted pickups will sound brighter with strong mid-range and treble sounds. The electronic control circuitry allows the musician to select these different pickups and control the final sound. I have designed four different circuits with custom-shaped PCBs to suit some of the more common or interesting electric basses and guitars. J&D Luthiers T-Style electric bass Photos 1 & 2: the outside surface of the T-style bass pickguard has a nice tinned pattern. The labels aren’t upside-down; at least, not from the player’s perspective! same package. One coil has its north magnets facing the strings, while the other has its south poles facing the strings (Figs.2 & 3). The coils are wired 180° out of phase, so any external interference that impinges on the pickup will induce an opposing voltage in each coil that is therefore cancelled out. Any (wanted) voltage induced by the strings will be out-of-phase due to the opposite orientation of the pickup magnets, and those signals will reinforce due to the out-of-phase wiring. Humbuckers are known for their stronger and fuller sound but tend to lack clarity and brightness compared to a single coil. These differing characteristics, as well as pickup placement, can be used by the instrument designer to influence its overall voicing. Pickups placed towards the neck of the instrument tend to have more bass and sound more mellow, while The inspiration for this project came during the restoration of an old bass guitar, known as a T-Style, from the Australian designer J&D Luthiers (see Photo 1). The existing circuitry was mounted directly to timber veneer, which was showing its age and needed replacement. I routed the veneer off, leaving a hole in the body to be covered by a new fascia. The obvious choice for a new material would be custom-machined sheet metal, but making that is time consuming and quite expensive. Most parts of this nature are also chrome-coated, which adds further expense. Instead, I decided to make a new fascia from a printed circuit board (PCB), which could act as both a visually appealing fascia and house the new circuitry (see Photo 2). The new circuit is shown in Fig.4 and Photo 3. The instrument features two pickups: a single coil near the neck and a humbucker near the bridge. These wire directly to CON1, a 7-way screw terminal. Both coils’ negative ends and the humbucker shield are grounded, along with the instrument drain wire. The drain is electrically connected to Fig.3: the internal construction of a single coil pickup, an early “PAF” Humbucker with bottom-mounted magnet and steel pole pieces, and a more modern form of humbucker with alnico magnet pole pieces. Source: https:// lawingmusicalproducts.com/dr-lawings-blog/the-wide-range-humbucker-and-the-genius-of-seth-lover siliconchip.com.au Australia's electronics magazine September 2024  87 Fig.4: my new circuit for the J&D Luthiers T-Style bass allows you to select which pickups are active and the configuration of the humbucker. It also provides tone, overdrive and volume controls and has compensation so that the frequency response doesn’t change too much with volume level. the bridge, to suppress interference induced on the strings and metal hardware. The ‘middle’ of the humbucker (coil one negative, coil two positive) connects to switch S1, a DP3T toggle switch with a slightly unusual on/ on/on switching pattern, as shown in Fig.5. This provides the option of series/split/parallel selection for the humbucker coils. When in the split position (centre), one of the coils is grounded, so the humbucker operates as a single coil, giving a clear and bright tone. When in the series position (down), the coils are in series. This gives the strongest output and a rich tone but less brightness than a single coil. When in the parallel position (up), the coil one negative is grounded and the coil two positive connects to the output, placing the coils in parallel. This gives a tone somewhere between the other two modes. The output of the humbucker switch leads to S2, the bridge/neck pickup selector switch. In the bottom position, the neck single coil will be active; in the top position, the bridge humbucker will be selected (in whatever mode S1 has it operating in); and, in the centre position, both pickups are active. Switches S1 & S2 provide a lot of flexibility for the musician, selecting between a total of seven different configurations for the two pickups. Photo 3: there’s a fair bit of room inside the T-Style bass guitar body for the components on the underside of the PCB. 88 Silicon Chip Australia's electronics magazine The signal is then fed to the traditional tone control, made from potentiometer VR1 (connected as a rheostat) and the four paralleled capacitors. This configuration forms a low-pass filter but in a slightly unusual way. Because the pot is in series with the capacitor(s), it effectively works as a magnitude control. With the pot all the way up, the signal sees a high impedance and very little of the high end is shunted. With the tone all the way down, the capacitor(s) are connected directly in parallel with the output, giving a strong high-frequency roll-off. The RC combination of the pickup source impedance and filter capacitance will loosely set the cutoff frequency. Still, with no buffering, there is a strong interaction with the other controls. Because we prefer to use plastic film dielectric capacitors for linearity, the range of values available in the size used (SMD M3216/1206) is only up to about 1μF. The four footprints therefore allow for a broader range of capacitances and for tuning the sound by connecting smaller capacitors in parallel with larger ones. A typical value is 47nF, but I prefer higher values to scoop out a bit more of the mid-range, so I use 200-220nF. In the prototype I built, I used two 100nF film capacitors in parallel, but I’ve specified a single 220nF in the parts list for simplicity. The next control in the signal path is potentiometer VR2, which is a new feature: a passive overdrive/distortion control. It works similarly to the tone control, except there are back-to-back (inverse parallel) schottky diodes (D1, D2) in series with the rheostat. When the pot is fully down, there siliconchip.com.au Silicon Chip kcaBBack Issues $10.00 + post January 1997 to October 2021 $11.50 + post November 2021 to September 2023 $12.50 + post October 2023 onwards All back issues after February 2015 are in stock, while most from January 1997 to December 2014 are available. For a full list of all available issues, visit: siliconchip.com. au/Shop/2 PDF versions are available for all issues at siliconchip.com.au/Shop/12 We also sell photocopies of individual articles for those who don’t have a computer Fig.5: the unusual switching patterns of the “on/on/on” and “Les Paul” DP3T switches used in these circuits allows the coils to be used together or individually. is a high resistance in series with the diodes, so they have little effect on the signal. When the pot is all the way up, the diodes are connected directly to the signal line. The voltage from the pickups is too low to fully forward bias these diodes, but even operating in their square law region, they introduce some nonlinearity to produce a subtle ‘overdrive’, a popular effect amongst guitarists. The seasoned musician will note that the overdrive effect is applied before the volume control, so its impact won’t be reduced if the volume is adjusted. This offers a useful contrast to other distortion sources, such as downstream foot pedals or amplifiers, which are mellowed by their input voltage level. By combining the onboard distortion with downstream effects, the musician has the flexibility to dial up or down distortion from a mixture of sources. Constructors might like to exper- iment with different combinations of diodes here; for instance, a single schottky diode would give asymmetric distortion, while back-to-back combinations of small signal diodes like the 1N4148WS would provide a more mellow effect. You could even have a combination, with one 1N4148WS and one schottky diode facing in either direction. The final potentiometer, VR3, is a traditional volume control with a twist. In a conventional circuit, the output signal is simply tapped off the pot’s wiper. But this arrangement has a drawback: as the volume is turned down, the pot resistance appears in series with the output. When connected to an amplifier via a coaxial cable, this resistance forms an unwanted low-pass filter (with the cable parasitic capacitance), reducing upper frequencies. In this updated circuit, a 1nF capacitor is placed in parallel with the volume control to ‘bleed’ additional treble into the output as the volume is turned down, compensating for the undesirable tone loss. Additional series and parallel footprints R5, C6 and C7 are provided for other combinations of capacitance or resistance to do this job. For instance, 100kW || 1nF may sound more linear as the control is turned down. The output signal appears at the output jack, CON, a ¼-inch (6.35mm) TS socket to suit a standard instrument cable. The PCB is secured to the front of the instrument using 3mm stainless steel self-tappers. They mount through 3mm plated through-holes and secure directly into the timber. I prefer plated holes for mechanical mounting, as they are a bit more hardy than bare fibreglass against the metal screw threads. A standard 1.6mm thickness PCB with black solder mask is best; any thinner would be too flimsy, any thicker would present too much of a lip. We will be supplying boards with a lead-free HASL finish (basically tin plating) as the solder will be on the outside of the guitar. If a gold finish would suit your guitar, you could go for an ENIG finish, although it will make the board considerably more expensive. The Fender Jazz Bass Many of the features of this T-style bass circuit can be applied to more Fig.6: my Jazz Bass circuit is similar to the one for the T-Style bass shown in Fig.4, except the pickup switching is simpler because both pickups are single-coil types. siliconchip.com.au Australia's electronics magazine September 2024  89 Photo 4: a Fender Jazz electric bass. Source: www. megamusiconline. com.au/product/ fender-americanperformer-jazzbass-guitarrosewoodfretboard-3colour-sunburst/ Photo 5: the unusual ‘Les Paul style’ DPDT switch closes all contacts in its central position, rather than opening them all, as in a normal DPDT centreoff switch. 90 Silicon Chip common instruments. One of the most popular bass guitars is the Fender Jazz Bass (Photo 4), which has two single-­ coil pickups. Jazz basses have been played extensively by legends like Jaco Pastorius, John Paul Jones of Led Zeppelin, Flea of Red Hot Chili Peppers, Adam Clayton of U2 and Geddy Lee of Rush. There are also clones of the Jazz Bass (and the other guitars listed below) that would likely fit my new pickguard designs, possibly with slight modifications to the inside of the body. Traditionally, the two pickups would be wired to individual volume control potentiometers with a shared common tone control. My new arrangement is shown in Fig.6. The negative of the second coil and the bridge shield are connected directly to circuit ground, similarly to the T-Style circuit from above. The first coil’s negative and the second coil’s positive are routed to S1, a DPDT toggle switch that provides series/individual switching for the two coils. With S1 in the position shown, S2 allows the player to select either coil or both in parallel. Parallel is the standard configuration for a Jazz bass, while series is a new mode that will give a stronger and fuller tone. Series switching with two single-coil pickups is a rare configuration but, in my opinion, heavily underrated. On a genuine Fender, it can give the player a beefier tone that is more akin to a humbucker. It can compensate for low-cost pickups, which tend to sound thin on a cheap imitation. Photo 5 shows how this special ‘Les Paul style’ switch works; with the toggle in the central position, all contacts are closed. Moving it to one side opens the contacts on the opposite side, while leaving the set on the same side closed. In its standard configuration, that lets you choose one pickup, the other or both in parallel. With S1 in the standard parallel mode, S2 can select between the neck pickup only, bridge pickup only, or both pickups in parallel. With S1 in the series position, S2 selects between both in series or mute. Mute can be helpful for live work, to avoid unwanted sounds when moving around on stage between songs, or it can be rapidly switched on and off to give a tremolo-style effect. The signal is then sent to the tone control potentiometer, VR1. A single capacitor is used here (220nF recommended), as there is no space for more footprints. Following this is pot VR2, the passive overdrive with dual schottky diodes. After that is pot VR3, the volume control with a single 1nF treble Photos 6 & 7: the outside of the Jazz Bass pickguard has a pleasing zigzag pattern in tin along with the necessary labels. The other side of the pickguard is where all the components are mounted. Australia's electronics magazine siliconchip.com.au Fig.7: the Music Man Stingray circuit is virtually identical to the one for the J&D Luthiers T-Style bass, except that there is no switch S2 as it doesn’t have a neck pickup. bleed capacitor. Again, there is no space for additional footprints or series/parallel combinations as there was on the T-Style circuit. The entire PCB fits in place of the existing chrome-plated controls (visible in Photo 4) and is screwed down to the body. If your instrument doesn’t quite fit the new controls (shown in Photos 6 & 7), additional space can be made by filing, rasping, or routing out a larger body cavity. Music Man Stingray Another very popular bass guitar, the Stingray (Photo 8), was designed by Leo Fender after he sold his interest in his founding business, the Fender music instrument company. Unusually for a Fender design, it features a humbucker pickup. Legends who have used this bass include Cliff Williams of AC/DC and John Deacon of Queen on tracks like Another One Bites the Dust. The standard Stingray was the first electric instrument to feature active electronics, with a volume control and a two-band EQ powered by a 9V battery. Some models add a knob to provide a three-band EQ. Cheaper copies of the Stingray will come with a similar circuit to the Jazz Bass: two volume control potentiometers for each of the humbucker coils and a common tone control. My replacement circuit is shown in Fig.7 and Photos 9 & 10. The bridge shield and humbucker coil two negative are connected to circuit ground. Like the humbucker wiring in the T-Style circuit, humbucker coil two positive and humbucker coil one negative connect to S1, a DP3T toggle switch with the special on/on/on switching pattern. This provides the same series/split/ parallel switching for the humbucker as the T-Style circuit, with the same tonal flexibility. When in split mode, only a single coil is active. Depending on which coil the user prefers, this can be the one closest to the bridge or nearest to the neck. It can be changed by simply swapping the wiring of coil 1 with coil 2 in the screw terminal. As the two pickups are mounted so close to each other, there is only a very minor difference in sound between the two, but the bridge coil will be marginally ‘brighter’, so I prefer to use it. The signal from S1 connects with the humbucker coil one positive and is sent to the tone control formed by potentiometer VR1 and up to four paralleled capacitors. In this example, a single 220nF film Photo 8: a Music Man Stingray bass with my new pickguard in place. This guitar has seen plenty of use! ► Photos 9 & 10: a close-up of the controls on the Stingray, showing how the orientation of the labels makes sense for the guitar player. Generally, only capacitors C1 and C2 are required, as shown here, but the extra pads give you more options. siliconchip.com.au Australia's electronics magazine September 2024  91 Fig.8: while the other three circuits were for bass guitars, this one is for a Fender Telecaster standard electric guitar. It’s the simplest of the four due to the minimal space available on the guitar, with pickup switching, tone and volume controls and a simple on/off overdrive option. cap is loaded. After this is the overdrive control, formed by potentiometer VR2 and inverse-­parallel schottky diodes D1& D2. The final control is the volume control, VR3, with the treble bleed circuitry already described. A single 1nF capacitor (C2) is used in my prototype, but footprints R5, C6, and C7 are also provided to give flexibility to the builder. Fender Telecaster The previous circuits have all been designed for electric bass guitars but can also be applied to electric guitars. The Fender Telecaster (Photo 11) is one of the oldest and most popular electric guitars, and its two-pickup combination can work with a circuit similar to that of the Jazz Bass. There are too many famous Telecaster players to list, including Jimmy Page, Keith Richards, Bruce Springsteen, George Harrison and Muddy Waters. The standard controls on a telecaster are a three-way pickup selector switch (neck/bridge/both) and the typical volume and tone controls. These can be replaced with the circuit shown in Fig.8, Photo 12 & Photo 13. The output jack negative, the second single coil negative and the bridge shield connect directly to circuit ground. The negative of the first single coil and the positive of the second single coil are routed to S1, a DPDT toggle switch providing series/parallel switching for the two coils. The output from S1 is sent to S2, a three-way bridge/neck selector switch. With S1 in the standard parallel mode, S2 can select between the neck pickup only, bridge pickup only, or both pickups. With S1 in the series position, S2 selects between on and mute. The signal is then sent to the tone control potentiometer, VR1. Two footprints are provided for capacitors. After this is the passive overdrive, which differs from other circuits in its use of a switch rather than a potentiometer. Due to space constraints, the dual schottky diodes are simply switched in or out of circuit. An SPST or SPDT toggle switch can be used here. After that is pot VR3, the volume control, with a single 1nF treble bleed capacitor. This circuit does not include a ¼-inch output jack, as the Telecaster mounts this off-board on the bottom edge of the body. The output signal instead connects back to the 7-way Photos 12 & 13: once your Telecaster pickguard has been assembled and the wires added, it should look like this, ready to install in the guitar. The pickguard is packed with controls and has a zigzag pattern to add a bit of interest. Australia's electronics magazine siliconchip.com.au Photo 11: a Fender Telecaster electric guitar. Source: www. keyboardcorner.com.au/fender-player-telecaster-pau-ferrofingerboard-3-color-sunburst/ Parts List – Electric & Bass Guitar Pickguards screw terminal, where it can connect to flying leads that wire down to the output jack. Construction All versions can be approached similarly, referring to the photos presented so far and the relevant PCB overlay diagram (one of Figs.912). Begin by mounting the toggle switch(es) and securing them to the PCB with the supplied washer and nut. I prefer a flush mount; if the toggle stands too proud for your liking, an additional nut can be placed on the back of the switch to adjust the length of shaft that protrudes through the PCB. Tin the SMD pads and toggle switch terminals with solder, then run short lengths of fine-gauge wire from the pads to the terminals. The DPDT and DP3T switches require all six connections, while the SPST/SPDT on the Telecaster circuit only needs the two connections as marked. Continue by mounting the ¼-inch jack (excluding the Telecaster). Secure it to the circuit board using the supplied washer and nut, like the toggle switches. Tin the two terminals and the pads and make connections with the fine gauge wire. The hot pad connects to the tip of the instrument cable, the longest conductor on the jack. The ground connection is to the jack’s sleeve. Now mount the potentiometers. The Volume and Tone controls are nominally 500kW audio taper, but other values can be substituted if the builder prefers. siliconchip.com.au Parts common to all versions 2 500kW logarithmic taper single-gang 16mm spline shaft potentiometers (VR1, VR3) [Altronics R2237] 1 6.35mm switched SPST mono jack socket (CON2) [Altronics P0062] * 3 16mm aluminium ¼-inch shaft grub screw knobs [Altronics H6331] 2 BAT43W 30V 200mA SMD schottky diodes, SOD-123 (D1, D2) [DigiKey, Mouser etc] 1 220nF 16V metallised plastic film or NP0/C0G ceramic capacitor, M3216/1206 size [DigiKey ECP-U1C224MA5] 1 1nF 100V metallised plastic film or NP0/C0G ceramic capacitor, M3216/1206 size [DigiKey ECW-U1102JX5] 1 1m length of black light-duty hookup wire * omit jack socket and one knob for Telecaster Fender Jazz Bass specific parts 1 double-sided PCB with black solder mask coded 23109241, 127 × 105.5mm 1 5-way SMD screw terminal, 3.5mm pitch (CON1) [DigiKey 2383942-5] 1 DPDT solder tail mini toggle switch (S1) [Altronics S1345] 1 "Les Paul 3 Way Selector" DP3T switch (S2) [AliExpress 1005001900886767] 1 100kW linear taper single-gang 16mm spline shaft potentiometer (VR2) [Altronics R2228] J&D Luthiers T-Style specific parts 1 double-sided PCB with black solder mask coded 23109242, 200.5 × 87.5mm 1 7-way SMD screw terminal, 3.5mm pitch (CON1) [DigiKey 2383942-7] 1 ‘on-on-on’ DPDT miniature toggle switch with solder tags (S1) [Pedal Parts Australia SWTS0008] 1 "Les Paul 3 Way Selector" DP3T switch (S2) [AliExpress 1005001900886767] 1 100kW linear taper single-gang 16mm spline shaft potentiometer (VR2) [Altronics R2228] Music Man Stingray specific parts 1 double-sided PCB with black solder mask coded 23109243, 190 × 71mm 1 5-way SMD screw terminal, 3.5mm pitch (CON1) [DigiKey 2383942-5] 1 ‘on-on-on’ DPDT miniature toggle switch with solder tags (S1) [Pedal Parts Australia SWTS0008] 1 100kW linear taper single-gang 16mm spline shaft potentiometer (VR2) [Altronics R2228] Fender Telecaster specific parts 1 double-sided PCB with black solder mask coded 23109244, 28.5 × 148mm 1 7-way SMD screw terminal, 3.5mm pitch (CON1) [DigiKey 2383942-7] 1 DPDT solder tail mini toggle switch (S1) [Altronics S1345] 1 "Les Paul 3 Way Selector" DP3T switch (S2) [AliExpress 1005001900886767] 1 SPDT solder tail mini toggle switch (S3) [Altronics S1310] Australia's electronics magazine September 2024  93 Fig.9: the replacement pickguard for the T-Style bass is relatively large and easy to assemble. Connections to the guitar are made via a 7-way SMD screw terminal, as through-holes would mar the appearance of the outer (visible) side of the PCB. Fig.10: the Jazz Bass pickguard is quite a bit smaller than the T-Style bass but all the controls still fit neatly. A 5-way terminal is used this time because the bass doesn’t have a humbucker pickup with its two extra terminals. For instance, 250kW and 1MW pots are also commonly used on many instruments. Generally speaking, higher values will give a brighter voicing to the instrument but will also increase the circuit’s output impedance. The potentiometers are mounted flush against the PCB, with the provided keyway holding the control in place to prevent unwanted body rotation. The terminals can be bent down towards the pads on the PCB and soldered with a blob. Begin by applying 94 Silicon Chip solder to the legs of the pot and wait for the solder to reflow down onto the pads for a good connection. Finish the potentiometers by mounting the brushed aluminium knobs to the shaft. If the pot shaft is too long, it can be cut with a hacksaw and filing to give a flush mount. The knobs secure to the shaft by tightening the grub screw. Next, mount the SMD screw terminal. Solder each leg individually in a similar way to the pot legs; flow solder onto the legs and wait for it to reflow onto the pads of the PCB. Australia's electronics magazine Finally, the passives can be soldered to the pads on the circuit board. I used plastic film caps throughout, as plastic dielectrics are highly linear. Finish by soldering the schottky diodes with opposite orientations. Editor’s note: C0G ceramics are more linear than many common film caps, such as polyester types, so they might give a more neutral sound. The pickups can now be wired to the screw terminal, and the entire assembly mounted to the body of the instrument with 3mm wood screws. Happy playing! SC siliconchip.com.au ALL DIAGRAMS ARE SHOWN AT 100% FULL SIZE N/I = NOT INSTALLED FOR STANDARD BUILD Fig.11: the Music Man Stingray pickguard can double as a boomerang! It’s pretty large, so all the controls are nicely spaced out. The single humbucker pickup means that only a five-way terminal is required for this one. Fig.12: the Telecaster pickguard is the most compact of all, so there’s only room for the necessary components. To save space, the overdrive pot is replaced with a switch. Raspberry Pi Pico W BackPack The new Raspberry Pi Pico W provides WiFi functionality, adding to the long list of features. This easy-to-build device includes a 3.5-inch touchscreen LCD and is programmable in BASIC, C or MicroPython, making it a good general-purpose controller. This kit comes with everything needed to build a Pico W BackPack module, including components for the optional microSD card, IR receiver and stereo audio output. $85 + Postage ∎ Complete Kit (SC6625) siliconchip.com.au/Shop/20/6625 The circuit and assembly instructions were published in the January 2023 issue: siliconchip.au/Article/15616 siliconchip.com.au Australia's electronics magazine September 2024  95