Provoke: Digital Sound Studies

Finding Ibrida

Guiding Tenets

A few guiding principles informed the design and prototyping of Ibrida. The tenets that served as a set of mission objectives imposed creative limitations that gave me guidance in the face of uncertainty and during moments of difficulty.

  • Designing the guitar from scratch is ideal, however, it is not essential.
  • If the guitar is to be modified rather than built, everything should fit in the existing pickup cavity with no extreme modification to the body.
  • The effects and components should be conceptually unified and ergonomically incorporated.
  • The face of the guitar should be as clutter free as possible with no extraneous switches, knobs, plates or screws.
  • If an Arduino microcontroller is to be used, it must interact with the guitar’s effect signal directly, specifically by means of adjusting an effect’s parameters.
  • Alternative sensor and interface technologies should be preferred over conventional solutions like switches, knobs, pots, etc.
  • The final instrument will only have one channel of sound output. It will be from a single jack.
  • The sound of the guitar should be maximized with only a single transducer or pickup.
  • A vacuum tube must be inside the guitar.

Once I began to build Ibrida my concern for lofty goals slowly morphed into addressing real world problems. Though I attempted to adhere to these tenets, solutions usually came in the form of compromise. When confronted with difficulty or tricky issues in the process, these tenets served as a starting point in my search for realistic solutions.

Failure is ALWAYS an Option

It is no secret that failure is a vital and unavoidable part of the design process. The work for Ibrida involved much trial and error through testing and listening to circuits. Three compelling sonic artifacts resulted from these tests.

The first of the two sonic failures (as noted above) prevented incorporating this effect into the design because the noise in the signal was of considerable volume.

This recording is the idle noise-floor of the circuit, void of any guitar signal. Since tracking down this noise’s cause would have been too time consuming, I decided to remove the effect from Ibrida. Unlike the first failure, I solved the second problem to choosing a different vacuum tube, which resulted in a positive contribution to the project.

The high pitch ringing is the dominant sound of this example. This sound artifact is specific to this particular model of tube. In an attempt to solve the problem, I purchased two separate 1V6 tubes and tested both in circuit with no eradication of the ringing. I concluded that the long tube leads and the poor circuit board layout I used during testing of this particular circuit created a 60hz hum that could not be removed. When used as a high frequency oscillator—like the ones found in antique AM radios—this ringing is most likely not a problem with this tube. Yet my setup created an unmanageable disruption.

To solve the problem, I decided to use a 1AD5 tube, which has nearly the same characteristics as the 1V6. I also decided to move away from printed circuit board and, instead, use point-to-point wiring on a turret-board. This reduced the stray capacitances in the circuit and made the amplifier operate much quieter. These changes minimized the noise, rendering the hum nearly silent and the completely eradicating the high pitched ringing.

The thumping on this sound file is a phenomenon known as microphonics, where mechanical vibrations inside the vacuum tube result in a situation where outside jostling and mechanical movement create audible artifacts. Slight microphonics are a reality of nearly every vacuum tube ever made, meaning that these particular microphonics are common in other subminiature tubes as well. This is perhaps yet another reason why I welcomed the move away from tubes and to transistors with open arms.

Today, the real-world characteristics and application potential of these exotic vacuum tubes remain largely unknown. Since they are mostly purchased online, arriving untested and unheard, such tubes have the potential of sounding beautiful. The joy of using such tubes comes in exploring what they sound like by building and prototyping custom circuits like these.

Design Phases

The prototyping of Ibrida’s electronics occurred in three phrases. First, I constructed a dry-run assembly and overhauled the instrument’s setup. Second, I separated the effects from the instrument for detailed experimentation and a thorough redesign. During the third phase, I used the knowledge gained from a crude layout-centric assembly and an electronic-centric protoboard to create the final prototype—Ibrida in its fully integrated form. I continue to refer to this form as a prototype since the conceptual motivations behind the guitar continue to suggest further development and additional exploration.

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I had already chosen most of the effects…

In the podcasts, I discuss the decision making process that went into choosing Ibrida’s particular effects. Because I worked on many aspects of the design before purchasing the pawnshop guitar, I kept each circuit board as small as possible so that it would eventually fit inside an existing pickup cavity. This meant that I could incorporate several effects right away after purchasing the guitar. I chose the fuzzface to be my first effect since it was already quite small and I was familiar with the schematic.

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The day I picked up the guitar and emptied it of its electronics…

After purchasing the pawnshop guitar, I immediately took photographs of the instrument in its original condition and removed all the original electronics and parts. It was a complete disassembly. Since I knew I would replace everything with my own custom electronics, I never even plugged the original instrument into an amplifier. Most of these original parts were dirty and full of rust; the inside of the pickup cavity smelled terrible.

The existing setup required major adjustments, since the action of the strings over the frets was too high and the setup of the bridge was too low. In guitar-speak this means the guitar was barely playable. So after my disassembly and cleaning, I shimmed the neck to increase the angle, raised the bridge to increase the downward string tension, replaced the tuning machines, and drilled a new hole for a side-facing output jack in order to maximize internal cavity space.

I needed a dry run assembly…

I was then ready to do a first run assembly of the guitar, which included the Fuzzface as well as the reverb and the voltage regulator. At this point, there was no control of the effects whatsoever; this was essentially a dry run. For this first prototype, I needed hands on knowledge of how much space I really had to work with and wanted to experiment with different mounting techniques for the circuit boards. So I made a crude covering plate with clear acrylic for the instrument and mounted the electronics to that.

Ultimately I realized that mounting everything to a cover was a bad idea. With everything covered up and sealed in the instrument, it becomes difficult to make small adjustments or tweaks as needed. In short, it slows down the design flow. I also broke one of my creative tenets by cluttering up the top of the instrument with screwheads and washers from the effect mounting. Since top plate was located underneath the strings, removing it to make adjustments required careful finagling. Anyone who has ever removed the pickguard and pickup assembly from a Fender Stratocaster will understand how utterly painful this can be. At this point, I knew that I would need a different way of mounting the effects and a better method of working on the electronics.

Building My All-in-one Electronic Protoboard Station

This phase took place outside the context of a guitar body and focused my attention directly upon the electronics.

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My workflow wasn’t working…

I was building each particular effect from a schematic, requiring me to solder and install piece by piece. This process worked alright when I was building a tried and true schematic like the Fuzzface since this effect has been around since the late 1960s and its popularity meant that there were many web resources in the DIY effect community devoted to it. However, this workflow ultimately prevented me from being able to prototype and design the other electronics to work in tandem with each other as well as fit in the physical constraints of the guitar. Designing circuits from scratch and then incorporating everything into a larger electronic circuit is an entirely different process altogether.

I realized that designing and testing would be simpler if I did not have to worry about other details, such as keeping the guitar in a playable state while I worked. My current workflow required me to secure the circuit boards to the guitar so they did not fall out while I attempted to test the sound and functionality of the circuits. There was a more efficient way to do these tests.

I needed to concentrate my efforts on the electronics…

My solution was to separate the effects from the guitar in order to work on them without worry or concern for where they would go. I used a piece of hardboard that included legs for each individual electronic effect—like the tube preamp or the fuzzface—securely installed to the underside. Jumper wires ran through the board to the top so that the circuits could be breadboarded and connected to one another with relative ease.

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I would describe this as an effect prototyping station. On top, surrounding the breadboard, I mounted several components including two quarter-inch jacks for input and output, a jack for a power supply, a power switch and indicator LED, and the pots for adjusting the volume or fuzz. The end result was something that looked like one of those kits at RadioShack that teach children the basic principles of electronics.

For the guitar, I installed the pickup as it would be in final assembly and directly wired it to the output jack. This created a streamlined and simulated guitar that I could use to test the sound and effects while I worked. This enabled me to focus my attention on specific parts of the design without worrying about various infrastructural concerns.

The Final Assembly, The Main Circuit Board and the Importance of Space

Once I knew how everything fit together I could start the reassembly…

After hammering out the final design and integration of components on the jig, I set about the task of transferring everything into the actual guitar. The knowledge I had gained from the previous attempts was invaluable as I carefully installed each finished component while leaving some room for experimentation with the main circuit board, one of the most essential elements of the final prototype. This circuit required several components, including a voltage regulator, a charge pump IC, two digital potentiometers, a timer IC for the digipots, and the hex inverter IC used for the signal selection.

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I knew this main board would need as much room as possible. I carefully built the main circuit board part by part, testing soldered connections as I went and always making sure my work area was clear of anything that could cause a short circuit accidentally. Building this circuit board this way forced me to work in a deliberate, diagnostic and thorough way.

Space, the final frontier…

During assembly, saving and conserving space was my primary focus. I needed to make two major modifications to the body to free up room, which required me to go against one of my creative tenants. I installed a self-contained battery compartment routed into the back to of the guitar, which freed up space in the front and allowed me to keep the power in one place.

To further maximize the space for the main circuit board inside the front cavity, I also decided to install the Arduino on the back of the guitar. This solution had the additional benefit of enabling the wires to run directly from the Arduino’s board into the cavity underneath the main circuit board. These two modifications, I felt, made the layout clean and efficient.

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The Arduino’s Role

In order to understand the entire custom electronics, it may help to interpret each constituent electronic subsystem as a different organ in the human body. In this analogy, the power system functions like the heart pumping blood (or electrons) through Ibrida. The individual effects are like specific internal organs. The inductive pickup acts like the senses, as a gateway to the “external world” of the magnetic strings. In Ibrida’s anatomy, the Arduino serves as the brain since it is responsible for operation of the body’s many subsystems. Like the brain, the Arduino microcontroller enables higher-level control functions, with the pins acting like the nervous system as they relay information to the appropriate destination.

The Arduino is responsible for three individual tasks. First, it is the subsystem of signal routing and effect selection; virtual switches. Second, it controls the effect’s parameters; virtual knobs. Finally, it regulates the human interface electronics (i.e., the things we see or touch directly). In each case, the Arduino's pins toggle between a high or low state depending on the required configuration for the desired effect. Either directly or in tandem with the specific integrated circuit (ICs), these pins are able to automatically act in the circuit to toggle between or combine effects.


Arduino as Signal Router and Effect Selector

An LED light is what enables the Arduino to control the signal flow without the need of physical switches. I built my circuit upon ‘Blink’, the circuit in the first LED light tutorial that beginning hackers and makers typically use to learn Arduino on day one. The familiar ‘Hello World’ of physical computing became a simple yet powerful paradigm, allowing for signal interactions through easily implementable means, without specialized parts.


A component called a light-dependent resistor (LDR) makes this easy signal switching possible. The LDR is a resistor that lowers its resistance value in proportion to the level of light, meaning that it operates by proximity without being directly connected to the microcontroller. It keeps each system isolated from one another.

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Arduino as Effect Parameter Controller

Ibrida uses specialized ICs to accomplish a more nuanced control in the signal side of the circuit. Chips called digital potentiometers (digipots) allow the microcontroller to vary a signal, akin to knob on a volume control but without something physically moving.

For the uninitiated, selecting and utilizing digipots can be a dark art because there are countless types of digipots with many varying technical specifications. These specifications—typically presented in a table or spreadsheet—contain parameters like number of positions, communication protocols, voltage requirements, current limitations and so on. In addition, these parameters often vary wildly from manufacturer to manufacturer. This, of course, is a complexity not typically found in the selection of conventional potentiometers.

As was often the case, the choices of available values of these conventional pots became the firm starting point for the design of the original and now antique (or boutique) circuits. Thus making it even more difficult to recreate and retrofit these circuits with digital potentiometers, since the choice of resistance values with digipots is even more limited, often restricted to the decades of 10 beginning with one thousand Ohms.

The large number of parameters became a limiting factor when choosing the specific circuits and parts for Ibrida’s design. Of the two digital potentiometers in the guitar, one digipot creates a variable feedback loop effect in the reverb by routing the input directly to the output; the other adjusts the amount of fuzz in the Fuzzface distortion.


Arduino as Physical Interface

The Arduino’s most conventional subsystem is made of blinking lights and touch sensors. The indicator LED lights are fairly straightforward. A single three-color LED glows red, blue, or green and each color represents the signal state in which the guitar is currently configured. For Ibrida, the red is the reverb on and blue is the reverb off. The original design was more complex with various effect states and color configurations. Because the delay circuit had to be removed, Ibrida’s final design used only two colors and states.

Arduino’s touch censor pulses a voltage connected to a piece of metal through a resistor and then sensing how long that metal takes to charge to that emitted voltage. It’s capacitive since when the touch of the finger is AC coupled to the metal, it drains some residual current to ground and thus delays the charging time thereby allowing the microcontroller to time this event and ‘know’ its sensor pin is being touched. Upon which this sensed value is then able to toggle the various states inside the Arduino without the need for physical switches or buttons.

The Spirit of Ibrida

If one envisions a new guitar designed with an embodied historical narrative at its core, what might it look like? What if this guitar took conceptual impetus from Ibrida’s electronics combined with the divergent histories of stringed instrument body design? How would designing and building this guitar from scratch result in practical advantages? Would it solve some of the challenges of the retrofitted prototype?

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The inspiration for the new guitar shape is the Hofner Violin Bass (link to external page) played by rock and roll luminary Paul McCartney. Historically speaking, most electric guitars and basses do not borrow their design shape the bowed string families. However, the Hofner Violin Bass is unusual for its violin-influenced shape.

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Louis Guersan, France. Pardassus de Viole, 1759 - This viol was made nearly 200 years prior to the manufacture of the embedded vacuum tube. “Like the hurdy-gurdy, the pardessus was in vogue for a short period in the eighteenth century, when, irrespective of its musical value, it was thought to be a suitably bucolic accessory to the pastoral pose of fashionable ladies…[n]ot surprisingly, it was of no further use after the French revolution.”

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Ibanez Guitars, Japan, Model 882, 1963 – Ibanez, a Japanese guitar company, mostly sold cheaply made copies of classic guitars in the US. This is a pre-copy era Ibanez, not unlike the “orphaned” pawnshop guitar used throughout this project.

My design is an unusual hybridization. On one hand, it draws from an instrument popularized and played among the French aristocracy in the late eighteenth century. On the other hand, my design looks forward to the popular Ibanez guitar, which at the time of this writing, can be found on ebay for $55 plus shipping).This outline of the guitar body aims to reconcile two fundamentally different genealogies, a marriage across centuries, cultures, and continents.

My serendipitous discovery of the shape came one day when I found the same size photos of the French viol and Ibanez guitar. I placed a tracing paper outline of the viol over the Ibanez image and used the differences and overlaps of the two shapes to create a hybrid design.


Though difficult in its own right, constructing a guitar from scratch can solve many problems and pitfalls of Ibrida’s prototype and layout. This new guitar would not have to be retrofitted with custom electronics inside an existing pickup cavity. Instead, the design would include specific cavities to go hand in hand with the embedded effects and the accompanying signal flow. Special plates, covers, circuit boards and mountings could be manufactured to fit the exact necessities of the design.

Practically speaking, it would be ideal to integrate a more robust power system for the electronics. Borrowing from smartphones and laptop computers, rechargeable lithium-ion batteries could provide greater power output without the need for costly battery replacement or a wall plug. Audio signal integrity could be further insured by allowing physical separation of the vital signal electronics from the power system, sensors, and a microcontroller.

The streamlined design would include more variability, less noise, and a greater customizability. By further integrating the electronics, it would become possible for more expressive deployment of these technologies. This would enable unusual electronic sensors (pressure, ribbon and/or light sensors, etc.) and human interface technologies (capacitive touch sensors or touchscreens) to exist alongside additional signal or effect indicators (LEDs, signal meters, etc.). By artfully embedding these sensors into the surface of the guitar, thereby fusing electronic design with lutherie, this new Ibrida would further satisfies my aims of meaningful hybridization.


At this time, this guitar is still in its design phase. The prototypes of Ibrida necessitated fighting with the hacked and retrofitted effects, circuit boards, and microcontroller. This new design, when completed, would elevate Ibrida its fully realized state. The blank body represents the possibilities of what might come.

About the Contributor

Kenneth David Stewart is a bit of a Renaissance man. As a solo performer, he creates work by weaving together live instruments, voice, physical interfaces and sound processing into songs with beautiful textures and a wide range of styles. As a composer, his compositions range from symphonic music to multimedia work and everything in between, including vocal music, solo instrumental pieces and music for film. Kenneth is currently developing a stage-deployed performance interface for music and dance in collaboration with Natalia Duong, a choreographer and scholar, and Jonathan Cain, a cellist and baroque instrumentalist. His electro-acoustic work, 'Thetastate' was given a studio recording by the jazz trio The Bad Plus and was released online by Duke University in conjunction with Duke Performances. Kenneth is presently pursuing a Ph.D. in Music Composition at Duke University and holds degrees from Duke, Rice University, the University of Arizona, and Pima Community College.