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The Tone-ScratcherPage | 32

The Tone-Scratcher: A guitar distortion boxDave PhilpottDr. Ali SetoodehniaECPI University8 December 2013I pledge to support the Honor System of ECPI. I will refrain from any form of academic dishonesty or deception, such as cheating or plagiarism. I am aware that as a member of the academic community, it is my responsibility to turn in all suspected violators of the honor code. I understand that any failure on my part to support the Honor System will be turned over to a Judicial Review Board for determination. I will report to the Judicial Review Board hearing if summoned.

AbstractIn many instances, guitars and other instruments require a change in tone to provide sonic interest. An effects pedal can be used to provide such a tonal change. Distortion can be achieved through the use of signal clipping. Transistors provide amplification and can be used to create signal distortion by overdriving the transistor. Resistors and capacitors can be configured to provide tonal control. Diodes can be arranged to further clip the signal. Once constructed, the circuit can be customized through the use of potentiometers. Care must be taken when assembling the circuit to prevent shorts or other electrical errors. A sturdy housing, insulated from the electrical circuit, is required for long-term usage

The Tone-Scratcher A guitar distortion boxA musician is an artist who expresses his or her statement through sound. Often, that statement is an expression of creativity, reflecting an original point of view with regard to musical structure. Chord progressions, melodies, harmonies, and phrasing make up the mechanics of a musicians expression, but only represent part of the artists creative intention. Beyond the concrete construction of music lies the sonic perception of the musicians statement.Tonal variation and color are as important to music as a painters color palette is to a portrait. The audience hears the musicians expression not only in the choice of notes but in the way the notes are played. Sonically, variation is achieved in several ways, starting with the instrument the musician uses to convey his or her art. Variation can be further achieved by processing the voice of the instrument. In modern times, such processing is made possible through the use of amplification and distortion of the instruments pure sound by electronic means.As a musician and a student of electronics, I have a vested interest in the hardware used to process guitar tones. From an artistic standpoint, I try to find new and interesting sounds through which to express musical creativity. From an academic standpoint, I am curious as to how the processing equipment operates. Further, I have a desire to construct my own processing gear. Not only are the effects processors expensive, the manufacturers do not always design equipment to replicate the sounds I wish to express.My challenge for this project is to design and build a circuit capable of providing tonal variation to an instruments voice, such as what is found in commercial processing equipment. Through the use of components such as transistors for amplification, resistors and capacitors for filtering, and diodes for clipping, I intend to construct a basic distortion circuit and house it in a form known as an effects pedal. The purpose of this pedal will be to add an aggressive signature to the tone, often described as fuzz or crunch. Once completed, I will compare the tone with my expectations of sound, and I will compare the cost of construction to the price of commercial equipment to see if original construction proves financially feasible.MethodsSystem DescriptionThe goal of the Tone-Scratcher is to apply a moderate amount of distortion to the output of the instruments tone prior to the main amplification equipment. The basic requirements of a typical distortion pedals use are: The pedal needs jacks for input and output cables. The input signal will most likely come from a passive (non-energized) pick-up and will need pre-amplification. The pedal should be able to provide tonal balance, known as equalization, between high frequencies and low frequencies. The pedal should provide an adjustable, variable amount of distortion to the signal. The pedal should be sturdy enough for practical use. The pedal should be light enough to be portable. The pedal should have a switch to toggle between a clean (unprocessed) tone and a distorted (processed) tone.These requirements will be addressed in this section and the Hardware Description.The input signal is a frequency or combination of frequencies within the range of human hearing, nominally 20 Hz to 20 kHz. For demonstration purposes, all diagrams accompanying this report show the input frequency as 440 Hz, also known as Concert A. The input signal is first applied through a capacitor that acts as a gate, allowing ac signal flow, but blocking dc flow from the pedals power source (nine-volt battery). The signal then flows through two stages of processing; a pre-amplification stage and a filtering/distortion stage. A final capacitor at the end of the circuit ensures the dc power source is blocked from the signal path as the processed output heads to the main amplification circuit.Hardware DescriptionThe pre-amplification stage consists of a single transistor configured with emitter-feedback bias (Diagram 1). Any increase or decrease in the voltage at the collector is met with an opposing change in the base voltage. This helps the stability of the transistor operation; as collector current increases or decreases, emitter voltage increases or decreases proportionally, creating a similar increase or decrease proportionally with base voltage (VB = VE + VBE) (Floyd, 2008, p. 234).Bias for the pre-amplification stage is set such that any change to the gain potentiometer results in some distortion of the signal waveform. This means any signal input to the Tone-Scratcher will have a distorted tone at the output.Diagram 2 shows the gain potentiometer at 0%. The collector voltage is 7.5 V, the base voltage is 635 mV, and the emitter voltage is 310 mV. The output waveform is inverted from the input wave and shows how the input waves negative iteration is clipped.Diagram 3 shows the gain potentiometer set at 10%. The collector voltage is now 6.4 V, the base voltage is 203 mV, and the emitter voltage is 130 mV. As the collector voltage decreased, so did the base and emitter voltages. The output waveform is inverted from the input wave and shows the input waves negative iteration is clipped; the amplitude of the output wave is higher. The output waveform is starting to show signs of further distortion.Diagram 4 shows the gain potentiometer at 50%. The collector voltage is now 5.526 V, the base voltage is -109 mV, and the emitter voltage is 44 mV. As the collector voltage decreased, so did the base and emitter voltages. The output waveform is inverted from the input wave and shows the input waves negative iteration is clipped; the amplitude of the output wave is higher. The output waveform is starting to show signs of further distortion, approaching the appearance of a saw-tooth waveform.Diagram 5 shows the gain potentiometer at 75%. The collector voltage is now 4.7 V, the base voltage is -170 mV, and the emitter voltage is 32 mV. As the collector voltage decreased, so did the base and emitter voltages. The output waveform is inverted from the input wave and shows the input waves negative iteration is clipped; the amplitude of the output wave is higher. The output waveform is starting to show signs of further distortion, even further approaching the appearance of a saw-tooth waveform.The filter stage consists of two separate filter circuits. A filter circuit functions as a gateway for specific frequencies, allowing some frequencies to pass through, but blocking all others. The term critical frequency is used to identify the point at which the signal will attenuate considerably (Floyd, 2008, p. 756). One of the filters used in the Tone-Scratcher is a low-pass filter circuit. The low-pass filter allows frequencies below the critical frequency to pass, while attenuating higher frequencies. Diagram 6 shows the low-pass filter component arrangement, including an LED circuit and potentiometer to be discussed later in the Conclusion section. Diagram 7 shows the frequency response of a low-pass filter; note how the signal attenuates as the frequency increases beyond the critical frequency. The formula for finding the critical frequency for the low-pass filter is:fc = 1 / (2RC), or1 / (6.283 * 33 k * 0.01 F) = 482 Hz (Floyd, 2008, p. 757).Diagram 8 shows the high-pass filter circuit. This circuit allows frequencies above the critical frequency, while attenuating lower frequencies. Diagram 9 shows the frequency response for a high-pass filter; note how the frequency attenuates below the critical frequency. The critical frequency for the high-pass filter is:fc = 1 / (2RC), or1 / (6.283 * 22 k * 0.01 F) = 723 HzA comparison of input-to-output waveforms illustrates how the two filter circuits compare. Diagram 10 shows a waveform comparison for the low-pass filter at 200 Hz; Diagram 11 shows a waveform comparison for the high-pass filter, also at 200 Hz. Note the low-pass waveform is nearly identical input-to-output, while the high-pass comparison shows attenuation at the output signal.Diagram 12 shows the low-pass filter at 440 Hz. Again, note the similarity of input-to-output for the low-pass filter, while the high-pass filter shows attenuation in Diagram 13. However, at 10 kHz, the comparison changes. The low-pass filter, shown in Diagram 14, now shows attenuation in the output signal, while the high-pass filter in Diagram 15 shows no such attenuation, despite the differing waveform.Diagram 16 shows the operation of the volume potentiometer at 0%. Being an audio-taper pot, the output signal at this level is 0. Diagram 17 shows the output signal at 50%, showing some attenuation of the signal. Diagram 18 shows the output signal at 100%, showing full signal to the output amplifier.The overall operation of the Tone-Scratcher is shown in Diagrams 19 through 21. Diagram 19 shows the gain adjustment at 0; the output waveform is clipped. Diagram 20 shows the gain adjustment at 50%; the output waveform is similarly clipped and reflecting an increase in gain. Diagram 21 shows the gain adjustment at 100%. The waveform has attenuated slightly and shows a pronounced distortion in shape.User InterfaceThe Tone-Scratcher is designed to be used either on the floor or on top of the performers amplification gear. Diagram 22 shows the interface. Stereo input jacks (1/4) are used for input and output of signal, carried via instrument cables. A single pole, double throw switch toggles between the clean (unprocessed) tone and the distorted (processed) tone. Diagram 23 shows the potentiometers capped with knobs to show the adjustment level.When the switch is set in the clean position, the instrument tone is unaffected from input to output. When the switch is set in the distorted position, the output tone is affected based on the settings of the potentiometers.

Detailed CostI purchased most of the parts at Radio Shack, either in-store or online. I had solder and wire, but I purchased all of the components necessary for the build. Following are the components and costs:(x1) 2N3904 NPN transistor$1.34$1.34

(x2) 1N4148 diodes$0.20$0.40

(x2) 5mm LEDs$1.75$3.50

(x1) 0.1uF capacitor$1.12$1.12

(x1) 0.047uF capacitor$1.12$1.12

(x2) 0.01uF capacitor$0.67$1.34

(x1) 100K logarithmic potentiometer$3.14$3.14

(x1) 100K linear potentiometer$3.14$3.14

(x1) 10K linear potentiometer$3.14$3.14

(x1) 2.2M ohm resistor$0.27$0.27

(x1) 33K ohm resistor$0.27$0.27

(x1) 22 ohm resistor$0.27$0.27

(x1) 3.3K ohm resistor$0.27$0.27

(x1) 680 ohm resistor$0.27$0.27

(x1) PC Board$3.14$3.14

(x1) 9V battery snap$0.57$0.57

(x3) Knobs$1.05$3.15

(x2) Stereo audio jacks$2.02$4.04

(x1) Enclosure$3.49$3.49

(x1)SPDT Switch$3.29$3.29

Total Cost$37.27

Completed Project: PhotographsDiagram 24: Interior view of the Tone-Scratcher, showing components and wiring.

Diagram 25: Component soldering

Diagram 26: Completed, assembled project, top view.

Diagram 27: Completed, assembled project, top and side view.DiscussionConclusionI set out with the intention of designing and building a distortion pedal having a unique, customizable tone and costing less than most professional gear. As a musician, sound is important to me; having a personal tone singles a performer from other musicians. It is a source of pride to be asked, Where did you get that? Where can I get that sound? Seeing a design come to life and be practically used is another source of pride. In many ways, this project has given me the first step toward the goal I had when I started school, which was to design and make my own processing and amplification gear. The build was not a complete success, however, and I need to address a design flaw in my project. When I first researched the idea of building a distortion pedal, I began by studying other distortion circuits used by professional-grade equipment. Most of those circuits were more complex than I felt I was able to tackle in the timeframe allowed for this project, so I narrowed my focus down to a basic design. Using Electronic Devices (Floyd) as my guide, as well as notes from previous classes on electronic components, I came up with the design of my circuit. I tested various configurations in Multisim until I came up with something that seemed to work.One inclusion I made to the circuit was a configuration of diodes designed to add further distortion to the signal due to clipping. I searched an online forum on building distortion pedals, seeking advice from some of the long-timers there, and was advised to include the diode array to add depth to the tone. One particular arrangement seemed popular among the forum members, so I included it in my build (Diagram 28).After testing the circuit in Multisim, constructing the hardcopy, and testing the finished product, I figured out the diode circuit was not contributing distortion to the output signal. Further, the potentiometer used to sweep between the high- and low-pass filters seemed have little effect on the output. I attributed this to poor circuit design on my part, thinking I had left something out of the equation. I dug into Electronic Devices once again and came to the conclusion that I needed to provide bias voltage for the diode arrangement.Diagram 29 shows the circuit as I constructed it, showing the input and output waveforms. Diagram 30 shows the same circuit with the diodes bypassed. Other than a slight change in amplitude of the output signal (attributed to the potentiometer setting), the signal is unchanged, showing the diode configuration not adding much to the design. Diagram 31 shows a revision in the circuit, adding bias voltage to the diodes. I used a potentiometer to vary the amount of bias. In this configuration, I only have bias applied to the lower diode circuit. Note the immediate difference in the output waveform, showing a noticeable amount of distortion to the output signal. Diagram 32 shows how the signal is clipped by adjusting the sweep potentiometer. Diagram 33 shows even further distortion of the output signal when bias is applied to both diode arrangements.I did not have enough time to purchase the components needed to make the changes necessary to the build to improve it. I believe the addition of two SPST switches connected to the potentiometers of the corrected diode arrangements (Diagram 34) would allow the user several options for customizing the level of distortion. I believe this to be a superior circuit to the product I completed and demonstrated.Applying practical experience with the technical knowledge I learned makes a difference in my understanding of the material. This project forced me to revisit previously learned (and forgotten) material, and I feel I understand the material far more now than when I first encountered it. Although my project had a design error, I was able to find that error and learn from it, even finding a solution to correct the error and improve the design.I set out to design and build my own effects pedal in an effort to use what Ive learned while working on my degree. I intended to make something unique and adjustable, and save money along the way. I met that goal because I understand my circuit technically; I made something unique and adjustable, and at a cost of under forty dollars, Ive saved anywhere from thirty to a hundred dollars or more. I feel Ive accomplished what I set out to do with this project.

ReferencesFloyd, T. (2008). Electronic devices, conventional current version (8th ed.). Upper Saddle River, NJ: Pearson.Diagram 7: Retrieved 1 December 2013 from http://www.freesoftwaremagazine.com/articles/ sound_filtering_with_the_gimp.Diagram 8: Retrieved 1 December 2013 from http://www.angelfire.com/planet/ funwithtransistors/Book_CHAP-2.html.

AppendixDiagrams

Diagram 1: Amplification stage with emitter-feedback bias.

Diagram 2: Waveform analysis, gain at 0%

Diagram 3: Waveform analysis, gain at 10%

Diagram 4: Waveform analysis, gain at 50%

Diagram 5: Waveform analysis, gain at 75%

Diagram 6: Low-pass filter arrangement

Diagram 7: Analysis of low-pass frequency response (http://www.freesoftwaremagazine.com/articles/sound_filtering_with_the_gimp)

Diagram 8: High-pass filter arrangement

Diagram 9: Analysis of high-pass frequency response (http://www.angelfire.com/planet/funwithtransistors/Book_CHAP-2.html)

Diagram 10: Waveform comparison at 200 Hz, low-pass filter

Diagram 11: Waveform comparison at 200 Hz, high-pass filter

Diagram 12: Waveform comparison at 440 Hz, low-pass filter

Diagram 13: Waveform comparison at 440 Hz, high-pass filter

Diagram 14: Waveform comparison at 10 kHz, low-pass filter

Diagram 15: Waveform comparison at 10 kHz, high-pass filter

Diagram 16: Output signal, volume potentiometer at 0%

Diagram 17: Output signal, volume potentiometer at 50%

Diagram 18: Output signal, volume potentiometer at 100%

Diagram 19: Waveform comparison, gain at 0%

Diagram 20: Waveform comparison, gain at 50%

Diagram 21: Waveform comparison, gain at 100%

Diagram 22: User interface, top and side view

Diagram 23: User interface, top view with knobsDiagrams 24 27 included in Completed Project: Photographs section of report

Diagram 28: Diode arrangement for tone clipping, no bias voltage applied

Diagram 29: Waveform comparison, diode arrangement in the circuit

Diagram 30: Waveform comparison, diode arrangement out of circuit.

Diagram 31: Analysis of revised circuit, lower diode arrangement in circuit

Diagram 32: Analysis of revised circuit, lower diode arrangement in circuit, sweep potentiometer at 0%

Diagram 33: Analysis of revised circuit, both diode arrangements in circuit

Diagram 34: The Tone-Scratcher, revised