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THE BLUE BOX - CDR
Miles Blair
Cody Dinges
Greg Entzel
Derek Glass
PROJECT UPDATE
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Class A Amplifier – Analog Board Update:
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Thorough understanding of tube theory and single-ended design
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30W design not economically feasible with single ended design
• Currently driving 10W using 250VDC
• Plan to drive closer to 15W signal power.
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Foot Pedal & Effects Circuitry – Digital & Analog Board I/O
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Larger SPI potentiometers set with good accuracy at 8-bit precision
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Plan to implement effects/preset command signals using digital logic
MSP430 – Digital Board Update:
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SPI drivers set potentiometers with ease
• Plan to transition to I 2C with surface mount potentiometers
• Plan to focus on interfaces, buffers, and database before UI
FUNCTIONAL DECOMPOSITION – LEVEL 0
FUNCTIONAL DECOMPOSITION – LEVEL 1
FOOT PEDAL – LEVEL 2
Foot Button
Inputs: User button press
Outputs: Logic message to the processor
Implementation: Digital Button integrated circuit triggers message
Test Plan: Power the integrated circuit and press the button while using an oscilloscope to check that an I2C
message was generated.
Preset number display
Inputs: I2C with new preset number sent from the processor.
Outputs: 7 segment display to user
Implementation: Premade integrated circuit with display
Test Plan: Send a I2C and observe a display change.
SMARTPHONE – LEVEL 2
Phone computer system
Inputs: User interface input
Outputs: Wireless messages to the phone Bluetooth modem
Implementation: Android operating system with application software
Test Plan: Unit tests for individual software modules.
Phone Bluetooth modem
Inputs: Inbound wireless messages from the device Bluetooth modem
Outputs: Outbound wireless messages to the device Bluetooth modem
Implementation: Android device and operating system
Test Plan: It is built into the phone, hope it works.
DIGITAL BOARD – LEVEL 2
Device Bluetooth modem
Inputs: Inbound messages
Outputs: Outbound messages
Implementation: Premade device with software
Test Plan: Send a message and read the TX pin with an oscilloscope.
Processor
Inputs: Button hit messages and inbound Bluetooth messages
Outputs: Foot pedal display changes, outbound Bluetooth messages, and I2C messages for effects changes,
EQ changes, volume changes, and effect bypass switch changes
Implementation: Premade hardware and custom software
Test Plan: Use the phone and foot pedal to change the preset or change an effect setting and observe outgoing
I2C messages to the digital potentiometers and seven segment display.
MSP430G2553
MICROCONTROLLER
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Inexpensive
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UART connection to Bluetooth modem
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I2C drivers for digital potentiometers and effect bypass switches
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20 pin designs offer enough general purpose pins for the foot-pedal buttons
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16 kB non-volatile flash memory to store presets and performance sets
DIGITAL POTENTIOMETERS AND SWITCHES
• Digital potentiometers have 256 resistance settings
• Control Analog effects circuits and amplifier
• Digital Switches bypass effects that are not used in active preset
• Can all be connected to the same I2C serial bus
Bluesmirf RN-42 Bluetooth Modem
Class 2 Bluetooth Device
18 meter range makes it perfect for a stage
DIGITAL BOARD SYSTEM CONTEXT DIAGRAM
ANALOG BOARD – LEVEL 2
ANALOG BOARD – LEVEL 2
Analog amplifier
Inputs: The altered analog signal from the equalizer.
Outputs: Amplified analog signal to drive the speaker.
EQ filter
Inputs: Analog signal from the effects circuits and I2C messages with
potentiometer changes for the filters in the equalizer.
Implementation: High power tubes to amplify the signal in 2 stages and
an audio transformer steps down the voltage to a level where the current
is high enough to drive the speaker.
Test Plan: Input a 100 mV signal from a waveform generator and observe
15W signal from the power amplification stage. Then connect the speaker
and check that it sounds correct.
Outputs: Analog signal to the power amplifier.
Implementation: Series of bandpass/highpass/lowpass filters that are
adjusted with digital with digital potentiometers.
Speaker
Analog effects circuits
Inputs: Guitar signals from electric guitar and I2C messages containing digital
potentiometer changes and relay changes to bypass/connect effect circuits
Inputs: Amplified analog signal from the amplifier
Outputs: Sound
Implementation: Premade inductive driver
Test Plan: Connect to amplifier and hear if it sounds correct.
Test Plan: Input a sinusoidal waveform signal 200 mV peak to peak and use an
oscilloscope to observe an alteration in the input signal’s amplitude for
different frequencies.
Outputs: Analog signal to the equalizer circuit.
Implementation: These will be a series of filters and analog effects circuits.
The effects can be adjusted by digital potentiometers, and they are selected
on/bypassed by a digital relay integrated circuit that is controlled by the
processor using I2C messages.
Test Plan: Input a signal generator signal 100 mV peak to peak and use an
oscilloscope to observe an alteration in the input signal’s wavform.
SINGLE-ENDED AMPLIFIER SCHEMATIC
Pre-Amplifier (Twin Triode 12AX7): 45 X 45 = 2025 Gain
Power Amplifier (Power Beam Pentode 6L6GC): 10W Signal Output
PRE-AMP DESIGN
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Triode (12AX7):
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Anode/Plate:
• Determines operating point of
tube.
• Delivers output signal of gain
stage
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Cathode:
• Determines sensitivity to input.
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Grid:
• Input signal.
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Heater:
• Improve cathode conductivity.
• Minimize effects of any gas.
PRE-AMP DATA SHEET CONSIDERATION
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VSUPPLY=250V
VPLATE=175V
VIN,MAX=100mV
VCATHODE=1V
ITRIODE=1.65mA
VR_P=75V
• RP=VR_P / ITRIODE
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46 Kohm
Plate Resistor
• RK=VCAT. / ITRIODE
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610 Ohm
Cathode Resistor
Mutual Conductance: gm = 1.950 mA/V
Plate Resistance: rp = 53 KOhm
PREAMP EQUIVALENT CIRCUIT
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rP = 53 Kohm
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RP = 51 Kohm
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RLOAD = 200 Kohm
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RK = 680 Ohm
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RTOT = rp || RP || RLOAD = 23 KOhm
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gm = 1.950 mA / V
VIN,MAX = 100 mV
∆iP = gm * VIN,MAX = .195 mA
VOUT = RTOT * ∆iP
AV = VOUT / VIN,MAX = 45
PRE-AMP SIMULATION RESULTS
• VPP = 7.6V
• VP = 3.8V
• AV = 38
PRE-AMP TEST RESULTS
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VPP = 8.6V
• VP = 4.3V
• AV = 43
PRE-AMP GAIN / DISTORTION RESULTS
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First gain stage outputs 4.5V peak.
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Second gain stage biased to 1.5V
• Input signals near and above
1.5VP cause distortion
• Begins at 90VPP
• Set currently by a 500 Kohm
logarithmic potentiometer
POWER OUTPUT TUBE
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Power Beam Tetrode: (6L6GC)
• Anode, Cathode, Heater, Control
Grid:
• Same roles as triode, except
plate drives an inductive load.
• Suppressor Grid:
• Help increase output current.
• Reduce effect of oscillations
• Tied to ground to reduce control
grid-ground capacitance
internally
• Screen Grid:
• Similar function to suppressor,
close to high voltage.
OUTPUT DESIGN STARTING PLACE
• Output transformer (125ESE) rated for a bias of 80mA before saturation and
frequency attenuation
• Some saturation emulates compression and works well with high tube gain
• Bias for 75 mA
CRITICAL OUTPUT DESIGN
POWER OUTPUT
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Transformer Input Waveform
• 388 VP!
• A lot of energy
stored in the
output
transformer
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Speaker Waveform
• 17.5VPP indicates near
5W output
• VRMS = 6.189
• P = VRMS2 / R
• P = 4.8W
NOTE ON POWER OUTPUT
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There is no standard for determining ratings for amplifiers
• 5W was only obtained at 100Hz and 100mV p
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Used a guitar with humbucker pickups (generate a 200mV P signal)
• Drove the speaker to V PP = 23 V (a 10W output) without noticeable distortion
• Drove the speaker to V PP,MAX = 35 V (near 20W output) with noticeable distortion
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Safe to rate the amplifier at 8-10W as a maximum recommended “playing volume”
• This rating is somewhat flexible due to tone desirability from overdriving pentode
• This power output is expected as it is biased near 20W with an expected 50%
efficiency in a single ended setup
PLAYING DEMONSTRATION
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Note lack of hum:
• Quality DC voltages, requires high-fidelity supply design
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Note frequency response:
• Special consideration given to sizing biasing and coupling capacitor
• Special consideration given to audio transformer saturation current
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Note the gain/distortion:
• Smooth gain, subtle yet full. Additional overdrive will square out signal more
dramatically
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Lacking Tone Design:
• Equalizer will improve audio quality. Capacitors may need to be decreased to reduce
signal drift and popping
PRELIMINARY PARTS LIST
Part
Cost
12AX7 Ruby
$12.57
6L6GC
$17.50
5 x 100uF 330V caps
$6.00
3x4.7K, 2x200K, 2x51K, 680, 910, 200, 510 Resistors
$10.00
500K & 1M Logarithmic Potentiometer
$3.00
Power Transformer 269AX
$45.00
Output Transformer 125ESE
$70.00
Wood & Screws
$50.00
400W Peavey Scheffield
$30.00
Android Droid
$50.00
20 x I2C Potentiometers
$25.00
I2C Switches
$10.00
Effects Components (R,L,C, Op-amps)
$150.00
Amplifier PCB
$33.00
Effects PCB
$33.00
MSP420 Dev. Kit
$3.50
Approximate Total:
$548.57
UPDATED SCHEDULE - HARDWARE
UPDATED SCHEDULE - SOFTWARE
DIVISION OF LABOR
Task
Derek Glass
Cody Dinges
Greg Entzel
Build and test filters
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Build power supply
X
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Build amplifier
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Android initialization
Design User Interface
X
MSP430 data
Interfacing
X
Foot Pedal
Interfacing
X
X
Construction of Amp
X
Altium Design
X
Miles Blair
X
X
X
X
X
CURRENT HIGH RISK FACTORS
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Hardware:
• Power Supply Stability
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Software:
• I2C Address Space
• UART Capability of MSP430 vs. ARM M0
• Flash Memory Capacity
QUESTIONS?