Daniel Pico & Salomé Ramirez. Senior Project. Electrical Engineering Department. California Polytechnic State University, San Luis Obispo - PDF

3-INput Pre-Amp By Daniel Pico & Salomé Ramirez Senior Project Electrical Engineering Department California Polytechnic State University, San Luis Obispo June Daniel Pico & Salomé Ramirez Table

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3-INput Pre-Amp By Daniel Pico & Salomé Ramirez Senior Project Electrical Engineering Department California Polytechnic State University, San Luis Obispo June Daniel Pico & Salomé Ramirez Table Of Contents Section Page Acknowledgements...5 Abstract...6 I. Introduction...7 II. III. IV. Requirements and Specifications...8 Functional Decomposition INput Pre-Amplifier Design, Testing and Implementation 11 i. Input Signal Ranges.12 ii. Power..17 Power Source Switched and Voltage Regulator Switching Capacitor iii. Logic Line Decoder and OR Gate Programmable Gain iv. Audio Signals.. 26 Summing Amplifier Stage Power Boost Headphone Option v. 3-INput Pre-Amplifier Overview 32 V. Conclusion...35 VI. Future Improvements Appendices A. Senior Project Analysis...37 B. List of Materials...43 C. PCB Layout...44 D. Piezo Pick-Up Construction List of Tables & Figures Table Page 1. Requirements and Specifications Level One Block Diagram Description Peak-Peak Voltages For Piezo, Coil, and Microphone Pick-Ups Test Cases for Power Source Switching and Voltage Regulator Stage Truth Table for Inputs and Outputs of the 3-8 Line Decoder Inputs and Outputs of the 3-8 Decoder and OR Gate according to # of inputs ON Programmable Gain Values MDAC Simulation Test Cases MDAC Experimental Test Cases...30 Figure Page 1. 3-Input Pre-Amp: Level Zero Block Diagram AAA Battery Holder, Coil Transducer, Piezo Transducer and Electret Microphone Input Pre-Amp: Level One Block Diagram INput Pre-Amplifier Schematic Inputs Signals Schematic for Power Source Switching and Current Charge Pump Power Controller and Regulator Voltage Output Simulation Power Controller and Current Charge Pump Regulator Switching Capacitor Configuration Switched Capacitor Rail Noise Switched Capacitor biasing Circuitry Input and Output Signals with Switched Capacitor Filter Voltage Rails for Final Prototype MDAC Programmable Gain Schematic Programmable Gain of Programmable Gain of Programmable Gain of Pre-Amp Design Schematic Prototype I Minimum Output Voltage (All Signals ON and all Potentiometers at 100kΩ) Maximum Output Voltage (All Signals ON and all Potentiometers at 0kΩ) Coil Signal in with 1, 2 or 3 Switches ON Coil Input Only with 1 Switch ON Coil Input Only with 2 Switches ON Coil Input Only with 3 Switches ON Power Amplifier Circuitry for Headphone Option INput Pre-Amplifier Schematic Implementation of the 3-INput Pre-Amplifier Prototype Box Design for 3-INput Pre-Amplifier [11] D Box Design for the 3-INput Pre-Amplifier [11] Acknowledgements We would like to thank everyone that supported our ideas and help facilitate a greater understanding of electrical engineering and life. To our professors at Cal Poly San Luis Obispo, families, friends and distinctively our senior project advisor Vladimir Prodanov, thank you. There s no free lunch, we now know. 5 Abstract There is no commercial available pre-amplifier that takes 3 individual input signals and synthesizes them for an acoustic guitar. The acoustic pre-amplifier takes three separate small signal inputs and combines or isolates to endure amplification depending with user setting. The user deciphers which signal or signal combination is desired. The device features coil pick-up, microphone pick-up, and piezo disc pick-up for variety of tone quality and control. 6 I. Introduction In the world of music, a musician explores his instrument contained by creative bounds, grasping for refreshing new perspectives. The 3-INput Pre-Amp meets costumer needs by offering a creative door to be opened by the musician. This door allows the musician to embrace the full embodiment of the acoustic guitar. With renewed perspectives come innovative styles, new cultural uprisings, which all hold constructive value in a progressive society. The 3-INput Pre-Amp provides variation of tones with synthesis adding a further extension and control of the natural acoustic sound. 7 II. Requirements and Specifications Table 1: Requirements and Specifications TABLE I 3-INPUT PRE-AMP REQUIREMENTS AND SPECIFICATIONS Marketing Engineering Requirements Specifications 1 Signal Distortion: 1% of input/output signal Justification Based on similar commercial available products. Also test equipment limitations. Human audible sound range. 1 Frequency Response: 20Hz to 20KHz(-3dB) 1 SNR: Greater than -70dB Based on similar commercial available products. Also test equipment limitations. 1,5 Voltage Gain ~ 16 db (Per Individual Input) Based on similar commercial available products. 1,2,3 Input Impedance ~ 50Mohms Prevent loading transducers. 1,2,3 Output Impedance ~ 3kohms Prevent loading pre-amp. 2,3,4,5 Power Supply : 4.5V (3 AAA Battery), Less than 10mA consumption Commercially available power source, Extend battery life and efficiency. 2,3,4,5 Weight: 5 Lbs Easy to use, install and maneuver. 2,3,4,5 Dimensions: 5 L x 3 W x 2 H Comparable to guitar pedals. Marketing Requirements 1. The system should have great sound quality. 2. The system should be easy to install. 3. The system should have low cost. 4. The system should have long battery life. 5. The system should have switches for choosing input arrangement. For competitive purposes, product suitability and commercial standards, the goal is to meet specifications and compete commercially. Our amplifier should have great tone quality with low sound distortion, be low cost in comparison to market value products, have just as long battery life, portable or easy to fit next to other guitar pedals, durable and have the ability to give the user the choice of synthesis for their specific choice of tone. 8 III. Functional Decomposition (Level 0 and Level 1) Figure 1: 3-INput PRE-AMP Level Zero Block Diagram. Figure 1 above shows the main inputs and outputs of the 3-Input Pre-Amp. The system allows three different pick-up inputs: coil, microphone and piezo-disc, which can be selected individually or whatever combination choice by the user. Figure 2: AAA Battery Holder, Coil Transducer, Piezo Transducer and Electret Microphone 9 Figure 3: 3-INput PRE-AMP Level 1 Block Diagram. Figure 3 elaborates on the main internal functions of the pre-amplifier which require the Voltage regulation and Management, Signal Logic and Programmable Gain, Audio Signal Amplification and Output Control, and Headphone Power Amplifier Option. TABLE II LEVEL ONE BLOCK DIAGRAM DESCRIPTION Table 2: Level One Block Diagram Description Modules Inputs Outputs Functionality 3-Input Signal Amplifier 3 Small Signals: Magnetic Coil pick-up Signal, Microphone Transducer pick-up, and Piezo disc vibration pick-up. Power Source consist of two inputs one is optional. Primary Source is Battery with auto switch Auxiliary (USB) capability available. Amplified, synthesized output signal to either ¼ mono jack or 3.5mm Stereo headphone option. The system takes in three small input signals. Conditions and Synthesizes accordingly to create a large clean output signal. 10 IV. 3-INput Pre-Amplifier Design, Test and Implementation Figure 4: 3-INput Pre-Amplifier Schematic The 3-INput Pre-Amplifier can be divided into three different stages: the power stage, the logic stage and the audio signals stage. The power stage contains a power source controller, voltage regulator and a switch capacitor which sets the appropriate rails for the operation of the system and allows the user to power the pre-amp with either a battery or auxiliary source. The logic stage is composed of a 3-8 line decoder, OR Gate and MDAC. This stage controls the gains of the input signals according to the number of pick-ups that are set on manually by the switches. The audio signals stage equalizes all inputs signals so that the output of the pre-amp is approximately 1V-2V, which is standard for the input of power amps. The system also contains a headphone boost option which amplifies the signal to 2-4 V with internal adjustment for headphone use. 11 i. Input Signal Ranges The three different pick-ups were tested in order to obtain the voltage ranges of the input signals to the pre-amplifier system. Each pick-up was observed with different styles of playing to gather the setting values for the designing of the pre-amp. The average or common values were then considered for the selection of the components in the design to avoid distortion of the signal by clipping. The following figures and tables summarize the data gathered: Piezo Pick-Up E6 String (82Hz): Figure 5: Pluck Style Test Figure 6: Manual Pulling Test Figure 7: Guitar Pick Test 12 E1 String (330Hz): Figure 8: Pluck Style Test Figure 9: Manual Pulling Test Figure 10: Guitar Pick Test Other Testing Styles: Figure 11: Moderate Banging Test Figure 12: Extreme Banging Test 13 Coil Pick-Up E6 String (82Hz): Figure 13: Pluck Style Test Figure 14: Manual Pulling Test Figure 18: Guitar Pick Test E1 String (330Hz): Figure 21: Pluck Style Test Figure 22: Guitar Pick Test 14 Other Testing Styles: Figure 16: Nothing Test Figure 17: Open Strum Test Electret Microphone Pick-Up E6 String (82Hz): Figure 19: Pluck Test Figure 20: Guitar Pick Test E1 String (330Hz): Figure 23: Pluck Test 15 Other Testing Styles: Figure24: Open Strum Test Figure 25: Moderate Banging Test Figure 26: Nothing Test Guitar Test Style Table 3: Peak-Peak Voltages for Piezo, Coil, and Microphone Pick-Ups Nothing Manual Pulling Open Strum Extreme Slap Moderate Slap Guitar Pick(E6/1) Hard Finger Pluck(E6) Hard Finger Pluck(E1) Approx. Coil Pick-Up N/A N/A ~mvpp Piezo Pick-Up N/A V ~mvpp Electret Mic 47.5 N/A V 3.06V V 980 ~mvpp The data obtained from the waveforms in the figures above is summarized in Table 1. The highlighted values are the common values for each input signal. These values are the ones considered for setting appropriate component values for the design of the pre-amp gain stages in order to maintain an output signal free of distortion. 16 ii. Power Power Source Switching and Voltage Regulator The low-loss power path controller with current charge pump regulator provides automatic switching between two power sources, auxiliary and battery. The first stage of the low-loss power path controller controls a P-channel MOSFET which acts as a power switch. The sense pin of the LTC4412 outputs the highest voltage between the two power sources. Therefore, when the aux input is ON, the battery input would be disconnected saving power dissipation from it unless required. The LTC1754 current pump regulates output voltage of the controller and sets it at a steady 5V which is the operating voltage for the pre-amplifier. The regulator allows an input voltage in the range of 2.7 to 5.5 V in order to allow a 5V output. This particular stage enhances battery life. Figure 27: Schematic for Power Source Switching and Current Charge Pump 17 Below are the results for various tests done to verify that the output power to the system holds at 5V regardless of being powered by a battery or USB source and is able to output 5V with even a low input voltage of 2.7V. As seen from Figure 28, the output voltage contains some ripple but through testing, the operation of the pre-amp didn t seem to be affected by it. Table 4: Test Cases for Power Source Switching and Voltage Regulator Stage V BATTERY (V) V AUX (V) V OUT (V) V RIPPLE (mv) Figure28: Power Controller and Regulator Voltage Output Simulation 18 The Figure below shows the implementation of the power controller and regulator. It has two inputs, one for an auxiliary source and another for a battery source. The 5V output is then connected directly to the input of the switch capacitor. Figure 29: Power Controller and Current Charge Pump Regulator Switched Capacitor In order to simplify the design of the pre-amplifier and provide headroom (voltage range) to prevent distortion, the system is sustained using a dual-supply of 5V to 5V making the reference voltage at GND level. The power stage will produce a 5V output and therefore, a switched capacitor is used to produce the negative voltage of 5V. The figure below shows the configuration of the MAX1044 Switch Capacitor. 19 Figure 30: Switching Capacitor Configuration The Switched Capacitor effectively produced the -5V for the dual supply of the op-amps but introduced noise to the system. As seen in the waveform below, the positive rail or channel 2 waveform produced a noise voltage of approximately 560mVpp and the negative rail or channel 1 waveform produced 890mVpp noise. Figure 31: Switched Capacitor Rail Noise The noise was also present in the output of the pre-amplifier, for which filtering was required in order to reduce the corruption in the signal. The values for the filter design were found according to the calculations shown below considering that the total system pulls 20 7mA. The cutoff frequency was chosen at 5.3kHz to eliminate all high frequency noise while retaining the bandwidth required for the range of frequencies of the input signals of a guitar (~ 20 Hz 1.5 khz). The calculations and values chosen for the filter components were made for an initial 9V to -9V design but still applied to the final 5V to -5V design, therefore were kept unchanged. Figure 32: Switched Capacitor biasing Circuitry After filtering, the noise at the output reduced by a factor of 5 yielding peaks of 162.5Vpp at the same frequency of 27kHz as shown in the figure below. 21 Figure 33: Input and Output Signals with Switch Capacitor Filter The final design is operated with a 5V dual supply. The rails of the system are shown in the picture below for the final prototype. Figure 32: Voltage Rails for Final Prototype 22 iii. Logic 3-8 Decoder and OR Gate The switches required for turning ON/OFF the input signals and setting the appropriate values for the programmable gain are going to be controlled by a CD74HC line decoder and a CD74HC4075 OR Gate. Three external switches will allow the user to control the inputs signals to be used (coil, piezo, mic or a combination of these). The state of the switches will also dictate the appropriate digital values set for the MDAC in order to provide the corresponding gain for the various cases. The programmable gain is set to amplify the signal depending on the number of inputs that are going into the system. The gain is set differently when only one input is on, two inputs are on or all input signals are going through. The 3-8 decoder determines how many switches are on and the OR gate combines the different possibilities for a single signals or for two signals being on. The case were the three switches are on does not require any ORing. As seen from the tables below, Y1, Y2 and Y4 are inputted into one OR gate and Y3, Y5 and Y6 are inputs to a second OR Gate. Table 5: Truth Table for Inputs and Outputs of the 3-8 Line Decoder 23 Table 6: Inputs and Outputs of the 3-8 Decoder and OR Gate according to number of inputs ON # of Inputs Gain 3-8 Decoder Outputs (OR Gate Inputs) 1 6 Y1,Y2,Y4 2 3 Y3,Y5,Y6 3 2 Y7 Programmable Gain Figure 35: MDAC Programmable Gain Schematic The TLC7524 MDAC was set at three different programmable gains depending on how many pick-up signals are inputted. The minimum programmable gain is 2, therefore when the three pick-up signals are simultaneously ON, the MDAC was set to have a gain of 2. The table below summarizes the other cases together with the formula used for the calculations made. 24 Table 7: Programmable Gain Values Programmable Gain # Inputs Gain (1/D) D7-D0 D 3-input input input The following figures show the output of the MDAC driving by the switched capacitor dual supply. The waveforms were observed before filtering was applied to the switched capacitor stage and therefore noise in the signals is very noticeable. Figure 36: Programmable Gain of 2 Figure 37: Programmable Gain of 3 Figure 38: Programmable Gain of 6 25 iv. Audio Signals Summing Amplifier Stage Figure 39: Pre-Amp Design Schematic Prototype I Figure 39 shows the schematic for the audio signal stage of the 3-Input pre-amplifier system. The first amplifier stage is a LM741 summing amplifier that inputs all three signals with their respective gains. The 1µF capacitors are dc coupling capacitors to eliminate any offset present in the incoming signals. The resistor values were chosen considering the peakto-peak voltages that each pick-up produced in order to equalize the three signals and achieve the desired 1V-2V output signal. The three 100k potentiometers were chosen equally for the three inputs to allow approximately the same range of volume control. The final portion of the schematic represents the programmable gain stage of the system, the MDAC, which is simply represented by an op-amp with the varying gains of 2, 3 and 6. The simulations below were done with 200 mvpp input signals for the Coil and Piezo pickups and 400mVpp for the Mic with all inputs set at 100Hz. 26 Figure 40 shows the case when all switches are ON, allowing all three signals to go through. It demonstrates the minimum output voltage since the potentiometers were all set at 100kΩ. The minimum output signal is ~ 800mV. Figure 40: Minimum Output Voltage (All Signals ON and all Potentiometers at 100kΩ) Figure 41shows the same configuration than above but demonstrating the maximum output voltage when all potentiometers are set at 0kΩ. The maximum output signal is ~ 2.1V. Figure 41: Maximum Output Voltage (All Signals ON and all Potentiometers at 0kΩ) The simulation below was done in order to verify the operation of the MDAC programmable gain. All the waveforms demonstrate when only one input signal is coming in but what is varying is the number of switches that are ON or OFF. The MDAC sets a gain 27 of 2, 3 or 6 depending on the number of switches that are On or OFF regardless of whether the input signals are coming through or not. Figure 42: Coil Signal in with 1, 2 or 3 Switches ON The table below summarizes the results of the three cases to test the MDAC functionality. Table 8: MDAC Simulation Test Cases ONLY Coil Signal ON # of Switches Output Signal (mv) The same test was done to the actual circuit to verify the results obtain in the simulation. The waveforms below are all for when only the coil signal is being inputted. The number of switches is then changed to see the MDAC programmable gain changing. Table summarizes the results obtained. 28 Figure 43: Coil Input Only with 1 Switch ON Figure 44: Coil Input Only with 2 Switches ON 29 Figure 45: Coil Input Only with 3 Switches ON Table 9: MDAC Experimental Test Cases ONLY Coil Signal ON # of Switches Output Signal (mv) Power Boost HeadPhone Option In addition to the 1V-2V mono output to a power amplifier, the design allows a headphone option. The user can switch ON the boost selection with a double pull double throw switch which enables the signal to be heard through a 3.5mm stereo output. The LM386 low voltage power amplifier IC was used for this application. This stage takes the input signal and amplifies it to ~2V 4V output and the internal 10k potentiometer allows for volume regulation. When the DPDT switch is turned on, the LM386 gets powered and the current 30 through the system increases from 7 ma to 12mA. The DPDT switch efficiently prevents the circuit from drawing extra current without being needed. Only when the headphone option switch is enabled, the system draws the extra 5mA needed for this selection. Figure 46: Power Amplifier Circuitry for Headphone Option The schematic in Figure 46 provided by TI fails to illustrate the need of a bypass capacitor from the voltage source to GND. Without this capacitor the lower frequency bass sounds would sound static like and drastically decreased the quality of sound. 31 v. 3-Input Pre-Amplifier Overview The Figure below shows the final schematic for the 3-INput Pre-Amplifier. The pickup signals are inputted into three corresponding double pull double throw switches. One side of each switch is inputted to the logic stage of the system. This sets highs or lows (Vcc or GND) to the 3-8 decoder to determine
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