This document describes the design of a guitar multi-effect stompbox pedal. The pedal will contain four effects - distortion, phaser, delay, and reverb - as well as a graphic equalizer and switching system. The distortion effect will be achieved using op-amps, the phaser will use a low frequency oscillator to shift frequencies, delay will be done using bucket brigade devices, and reverb will excite guitar springs. A microcontroller will control series switching of effects using CMOS switches. Key challenges will be minimizing noise, floating the reverb tank, and designing circuits to run on a 9V supply.

1. Guitar Multi-Effect
Stompbox (P154.2)
Sponser: Isaac Cohen
John Lynn
Lucas Schulte
Spring 2016
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2. What is a Stompbox?
• Most commonly referred to as guitar pedal or
effects pedal.
• Changes the sound of the guitar signal based
on the type of circuit.
• Operated by the guitarist by using
footswitches for bypass and control knobs to
adjust parameters.
• Can be a single sound, or several effects in the
same box.
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3. Typical Operating Scenario
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15.2 x 5.1 x 7.6 cm
(6 x 2 x 3”)

4. Why Have a Multi-Effect Pedal?
• Allows the guitarist to access multiple effect
options in one package which cuts down on
cable lengths, size profile, and travel weight.
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5. Design Constraints
• Device must contain 4 different effects and signal filtering.
– Distortion of choice (overdrive/fuzz)
– Frequency modulation
– Delay
– Reverb
– Signal filtering
• Configurable series switching system.
• Individually bypassed effects (via footswitch).
• All analog effects with exception of the Delay Effect.
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6. Design Restrictions
• Interface with electric guitar (passive and
active pickups) and guitar amplifier or other
effects.
• Utilize standard 1/4” connectors for
input/output and DC barrel plug for power.
• Control interface that is standard for electric
guitarists to use (footswitches, knobs, slide
potentiometers, LED bypass indicators).
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7. Design Specifications
• Input Signal Voltage: 0 – 2V
• Output Signal Voltage: 0 – 2V
• Input Impedance: 1MΩ
• Output Impedance: 10kΩ
• Power Supply: 9V (2.1mm center negative DC barrel
plug)
• Input/Output: 1/4” TRS Stereo Neutrik jacks
• Final Package: 9 x 15 x 3.75” or smaller Anodized
Aluminum enclosure
• 1/4 watt, 1% resistors and 16V+, 5%-10% capacitors
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8. Enclosure Concept
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15.24 x 25.4 x 6.35 cm
(6 x 10 x 2.5”)

9. 9

10. System Schematic
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11. Operational Scenario
• Electric guitar is plugged into the stompbox input,
the stompbox output is plugged into a guitar
amplifier, another effect, or a direct box.
• The user programs the series configuration and
adjusts effect parameters to achieve the desired
sound.
• Effects are bypassed (activated or deactivated)
individually via a footswitch while the guitarist is
playing.
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12. Overdrive
• Chosen for the distortion effect.
• Originally designed to mimic the sound of overdriven tubes in
amplifiers.
• Uses op-amps to boost the signal and creates distortion by
saturating the op-amp and using the non-linear properties of
feedback diodes to clip the signal.
• A potentiometer (variable resistor configuration) in the
feedback loop controls the amount of distortion by increasing
or decreasing the gain of the op-amp.
• A potentiometer on the output controls the effect volume.
– Can be used to boost the effect output for solos.
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13. Distortion Block Diagram
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14. Distortion Schematic
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15. Distortion Simulation
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Volts
Input (Green) vs Output (Red) Voltage

16. Phaser
• Produces a warbly, sweeping effect.
• Shifts the input signal 180˚ and mixes it with
the un-altered signal.
• Creates notch cancellations at frequencies
determined from component values.
• Notches are moved up and down the
frequency band by an LFO (low frequency
oscillator).
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17. Phaser Block Diagram
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18. Phaser Schematic
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19. Phaser Simulation
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Input (Green) vs Output (Red) Voltage
Frequency Response
Frequency
59Hz
340Hz

20. Delay
• Repeats copies of the original signal after playing,
similar to an “echo” sound.
• Volume, rate, and duration of the repeats can be
controlled by the user.
• Analog delay can be accomplished using Bucket
Brigade Devices (BBD).
– Each chip contains a long line of capacitors that are
charged and discharged in order by the input signal.
– The number of capacitor stages determine the rate of
the delay effect.
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21. Delay Block Diagram
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22. Delay Schematic
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23. Reverb
• Mimics the sound of playing in a large room or
enclosed space.
• Analog reverb can be accomplished by driving the
signal to a spring tank.
– A transducer is used to excite the springs, the reverb
sound is mixed back with the original signal by a recovery
amplifier.
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24. Reverb Block Diagram
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25. Reverb Driver Schematic
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26. Graphic Equalizer
• Allows the user to target specific frequencies
and boost or attenuate them to achieve the
desired sound.
• Controlled by slide potentiometers: center
position is unity.
• Target frequencies are set by component
values in passive or active filters.
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27. Equalizer Block Diagram
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28. Equalizer Schematic
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29. Equalizer Simulation
Sliders at 0%
Sliders at 50%
Sliders at 100%
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30. Switching System
• Series switching mode allows the user to
configure the order of the effects.
• A microcontroller and shift register combo drives
CMOS switches to achieve the desired order.
• The user will press a program button and select
the configuration by pressing each button
representing an effect in the desired order.
• The active effect order is displayed on the 7-
segment display.
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31. Switching Block Diagram
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32. Switching Schematic
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33. Code Flowchart
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34. Design Challenges
• CMOS switches that operate on TTL level control.
• Completing the component layout on 4 layer PCBs.
• Aligning measurements of the PCBs with the enclosure
for attaching board-mounted components.
• Efficient grounding and shielding to minimize noise.
• Floating the reverb tank so that it isn’t disturbed by the
footswitches.
• Creating Reverb circuit to use a unipolar 9V supply
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