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Guitar Multi-Effect Stompbox (P154.2)
Sponser: Isaac Cohen
John LynnLucas SchulteSpring 2016
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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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Typical Operating Scenario
15.2 x 5.1 x 7.6 cm (6 x 2 x 3”)
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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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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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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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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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Enclosure Concept
15.24 x 25.4 x 6.35 cm
(6 x 10 x 2.5”)
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System Schematic
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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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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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Distortion Block Diagram
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Distortion Schematic
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Distortion SimulationVolts
Input (Green) vs Output (Red) Voltage
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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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Phaser Block Diagram
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Phaser Schematic
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Phaser SimulationInput (Green) vs Output (Red) Voltage
Frequency Response
Frequency
59Hz340Hz
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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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Delay Block Diagram
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Delay Schematic
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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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Reverb Block Diagram
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Reverb Driver Schematic
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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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Equalizer Block Diagram
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Equalizer Schematic
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Equalizer Simulation
Sliders at 0%
Sliders at 50%
Sliders at 100%
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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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Switching Block Diagram
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Switching Schematic
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Code Flowchart
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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