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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