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The Blue Box - CDR

The Blue Box - CDR. Miles Blair Cody Dinges Greg Entzel Derek Glass. Project Update. Class A Amplifier – Analog Board Update: Thorough understanding of tube theory and single-ended design 30W design not economically feasible with single ended design

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The Blue Box - CDR

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  1. The Blue Box - CDR Miles Blair Cody Dinges Greg Entzel Derek Glass

  2. Project Update • Class A Amplifier – Analog Board Update: • Thorough understanding of tube theory and single-ended design • 30W design not economically feasible with single ended design • Currently driving 10W using 250VDC • Plan to drive closer to 15W signal power. • Foot Pedal & Effects Circuitry – Digital & Analog Board I/O • Larger SPI potentiometers set with good accuracy at 8-bit precision • Plan to implement effects/preset command signals using digital logic • MSP430 – Digital Board Update: • SPI drivers set potentiometers with ease • Plan to transition to I2C with surface mount potentiometers • Plan to focus on interfaces, buffers, and database before UI

  3. FUNCTIONAL DECOMPOSITION – LEVEL 0

  4. Functional decomposition – level 1

  5. Foot pedal – level 2

  6. Smartphone – level 2

  7. Digital board – level 2

  8. MSP430g2553 Microcontroller • Inexpensive • UART connection to Bluetooth modem • I2C drivers for digital potentiometers and effect bypass switches • 20 pin designs offer enough general purpose pins for the foot-pedal buttons • 16 kB non-volatile flash memory to store presets and performance sets

  9. 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

  10. Digital board system context diagram

  11. Analog board – level 2

  12. Analog board – level 2

  13. Single-ended amplifier schematic Pre-Amplifier (Twin Triode 12AX7): 45 X 45 = 2025 Gain Power Amplifier (Power Beam Pentode 6L6GC): 10W Signal Output

  14. Pre-amp Design • Triode (12AX7): • Anode/Plate: • Determines operating point of tube. • Delivers output signal of gain stage • Cathode: • Determines sensitivity to input. • Grid: • Input signal. • Heater: • Improve cathode conductivity. • Minimize effects of any gas.

  15. Pre-amp data sheet consideration

  16. VSUPPLY=250V • VPLATE=175V • VIN,MAX=100mV • VCATHODE=1V • ITRIODE=1.65mA • VR_P=75V • RP=VR_P / ITRIODE • 46 Kohm • Plate Resistor • RK=VCAT. / ITRIODE • 610 Ohm • Cathode Resistor

  17. Mutual Conductance: gm = 1.950 mA/V Plate Resistance: rp= 53 KOhm

  18. Preamp equivalent circuit • rP = 53 Kohm • RP= 51 Kohm • RLOAD = 200 Kohm • RK = 680 Ohm • RTOT =rp|| RP || RLOAD = 23 KOhm • gm = 1.950 mA / V • VIN,MAX = 100 mV • ∆iP = gm * VIN,MAX = .195 mA • VOUT = RTOT * ∆iP • AV = VOUT / VIN,MAX = 45

  19. PRE-AMP SIMULATION RESULTS • VPP = 7.6V • VP = 3.8V • AV = 38

  20. pre-amp test results • VPP = 8.6V • VP = 4.3V • AV = 43

  21. pre-amp gain / DISTORTION results • First gain stage outputs 4.5V peak. • 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

  22. Power output tube • 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.

  23. 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

  24. Critical output design

  25. Power output • 388 VP! • A lot of energy stored in the output transformer • Transformer Input Waveform • Speaker Waveform • 17.5VPP indicates near 5W output • VRMS = 6.189 • P = VRMS2 / R • P = 4.8W

  26. NOTE ON POWER OUTPUT • There is no standard for determining ratings for amplifiers • 5W was only obtained at 100Hz and 100mVp • Used a guitar with humbucker pickups (generate a 200mVP signal) • Drove the speaker to VPP = 23 V (a 10W output) without noticeable distortion • Drove the speaker to VPP,MAX = 35 V (near 20W output) with noticeable distortion • 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

  27. Playing demonstration • Note lack of hum: • Quality DC voltages, requires high-fidelity supply design • Note frequency response: • Special consideration given to sizing biasing and coupling capacitor • Special consideration given to audio transformer saturation current • Note the gain/distortion: • Smooth gain, subtle yet full. Additional overdrive will square out signal more dramatically • Lacking Tone Design: • Equalizer will improve audio quality. Capacitors may need to be decreased to reduce signal drift and popping

  28. Preliminary parts list

  29. Updated schedule - hardware

  30. Updated schedule - software

  31. Division of labor

  32. Current high risk factors • Hardware: • Power Supply Stability • Software: • I2C Address Space • UART Capability of MSP430 vs. ARM M0 • Flash Memory Capacity

  33. questions?

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