Vacuum Tube amplifier - UCF Department of EECS

Vacuum Tube amplifier - UCF Department of EECS

Group IV Stephen Nichols Jason Lambert Rafael Enriquez VACUUM TUBE AMPLIFIER Stephen DESCRIPTION A Vacuum tube audio amplifier for which the entire signal path is analog but the audio parameters are digitally controlled via a touch screen graphical user interface which also displays visualizations of the amplitude, frequency and phase characteristics of the audio signals. Stephen MOTIVATION JE-Audio, model VM60 This unit is about 5 x 13 x 16 weighs about 45 pounds and costs about $6300 per pair. Image reprinted with permission from John Lam of JEAudio Our project will be embody the analog aspects of modern commercial VTAs such as this one. The unique feature of our project is digital controlled source switching, volume, and graphic equalizer with a touch screen, LCD to display music visualizations. As far as we can determine, no other modern VTA has this feature. Jason

GOALS CONTROL PANEL POSSIBLE MUSIC VISUALIZATIONS Stephen HARDWARE REQUIREMENTS Requirement Value Condition Number of audio channels 2 (stereo) Output power rating 10 Watts Root Mean Square (RMS) Per channel at 1000 Hz without clipping Input impedance TBD Per channel Output impedance 8 ohms Per channel Bandwidth 20 Hz to 20 KHz flat 3dB

Total Harmonic Distortion, low signal level 0.5% Total Harmonic Distortion, high signal level 2.5% As measured at a moderate output level relative to the input signal level When measured at a number of frequencies 100 Hz to 5 KHz 12dB below maximum output When measured at the onset of clipping at a selection of audio midrange frequencies Stephen INTERNAL DETAIL (TOP VIEW) Input and Output Jacks AC Power Terminal Speaker Strip Relay High Voltage Power Supply Fan (if needed) Low Voltage Transform er Microcontroller, Low Voltage Power

Supply, Optocouplers LCD / Touchscreen Right Audio Processor CCA Left Audio Processor SOURCES PHONO INPUT SOURCE SELECT RIAA EQU INPUTS TAPE TUNER AUX GAIN GRAPHI C EQUALIZER PREEQU GAIN ADJ BUFF B BUFF LOCAL VOLT

REG SPEAKER TO ALL AUDIO STAGES TUBE PREAMP AND PHASE SPLITTER VOL ADJ TUBE PUSHPULL AMP ZMATCH XFMR SEL IN VR1 VR2 VR3 VR4 VR5 VR6 VR7 VR8 BUFFER A HIGH VOLT SUPPLY DIGITAL POTENTIOMETERS

A 110 VOLTS AC B OPTO COUPLERS LOW VOLT SUPPLY MICROCONTROLLER CLOCK OSC EXTER NAL USB PROGRAM INTERFACE 16 MILLION COLOR 800 x 480 LIQUID CRYSTAL DISPLAY TOUCHSCREEN Stephen AUDIO INPUT BLOCK DIAGRAM SOURCES PHONO INPUT SOURCE SELECT RIAA EQU

INPUTS TAPE TUNER AUX GAIN GRAPHI C EQUALIZER PREEQU GAIN ADJ BUFF B BUFF LOCAL VOLT REG SPEAKER TO ALL AUDIO STAGES TUBE PREAMP AND PHASE SPLITTER VOL ADJ TUBE PUSHPULL AMP

ZMATCH XFMR SEL IN VR1 VR2 VR3 VR4 VR5 VR6 VR7 VR8 BUFFER A HIGH VOLT SUPPLY DIGITAL POTENTIOMETERS A 110 VOLTS AC B OPTO COUPLERS LOW VOLT SUPPLY MICROCONTROLLER CLOCK OSC EXTER

NAL USB PROGRAM INTERFACE 16 MILLION COLOR 800 x 480 LIQUID CRYSTAL DISPLAY TOUCHSCREEN AUDIO PATH FOR ONE CHANNEL SHOWN ONLY THE OTHER IS SIMILAR Stephen From low-voltage power supply To graphic equalizer AUDIO INPUT SCHEMATIC From back panel input jacks (not shown) From MCU Stephen AUDIO INPUT PROCESSING The analog multiplexer, driven by two GPIOs from the MCU, selects one of four input sources. The output of the multiplexer is buffered by a unity-gain stage to provide a constant-impedance drive for the equalizer stage. One VR channel is used to equalize the levels of the various signals (see chart) and is set to a pre-determined value by the MCU as the sources are selected.

Source Name Processing Input Signal required to get 1 volt peak at mux output at 1KHz Phono Two-pole low-pass filter to compensate for the Recording Industry Association of America (RIAA) specification equalization applied to vinyl records when they are made 17 mV Tape None 1 Volt Tuner None 1 Volt Aux Constant voltage gain of 10 100 mV Stephen AUDIO INPUT DESIGN DECISIONS

Op-amps: The important parameters are: Noise voltage, THD, Price, availability in a DIP, and model support in NI Multisim. The Texas Instruments LM4562 was an obvious choice for all analog processing up to the vacuum tube stages. Analog Multiplexer: Four AC input sources to be selectable with the highest isolation between channels available in DIP. The physical implementation is two identical CCAs so a dual-channel switch was not considered. Four candidate parts were considered (see table); the final choice was the ADG408. Texas Analog Analog Paramete Instrument Maxim Devices Devices r s DG508 AD8184ANZ ADG408BN SN74LV4051 Supply 4.5V to 0.5 V to 7 V 4V to 6V 5V to 15V range 18V Crosstalk -45dB -98dB -85dB -68dB Noise Not rated 4.5 nV / Hz Not rated Not rated Voltage THD Not rated -74dBc Not rated

Not rated Cost, each $0.17 $5.75 $6.15 $6.31 The ADG408BN was chosen due to excellent crosstalk, compatibility with the power supply voltages and performance during simulation Jason GRAPHIC EQUALIZER SOURCES PHONO INPUT SOURCE SELECT RIAA EQU INPUTS TAPE TUNER AUX GAIN GRAPHI C EQUALIZER PREEQU

GAIN ADJ BUFF B BUFF LOCAL VOLT REG SPEAKER TO ALL AUDIO STAGES TUBE PREAMP AND PHASE SPLITTER VOL ADJ TUBE PUSHPULL AMP ZMATCH XFMR SEL IN VR1 VR2 VR3 VR4 VR5 VR6 VR7 VR8

BUFFER A HIGH VOLT SUPPLY DIGITAL POTENTIOMETERS A 110 VOLTS AC B OPTO COUPLERS LOW VOLT SUPPLY MICROCONTROLLER CLOCK OSC EXTER NAL USB PROGRAM INTERFACE 16 MILLION COLOR 800 x 480 LIQUID CRYSTAL DISPLAY TOUCHSCREEN AUDIO PATH FOR ONE CHANNEL SHOWN ONLY THE OTHER IS SIMILAR

Jason GRAPHIC EQUALIZER DESIGN DECISIONS Equalizer topology Sallenkey Gyrator Order 2nd 2nd Constant Q Yes No Adjustable center frequency No No Number of op-amps per band 2 1 Complexity 7

4 8 3 10 being high Familiarity 10 being high Jason GRAPHIC EQUALIZER Jason GRAPHIC EQUALIZER Jason DIGITAL POTENTIOMETERS DESIGN DECISIONS key parameters AD8403 Number of channels 4 per chip Number of positions 256 Serial interface 3 pin min Availability in a DIP Yes

Min cross talk 65 dB VA = VDD, VB = 0 V Min THD 0.003 VA = 1 V rms + 2 V dc, VB = 2 V dc, f = 1 kHz Daisy chainable Yes % Jason DIGITAL POTENTIOMETERS DIFFICULTIES A loading effect occurred on the data lines which caused the serial data input to the digital potentiometers to possibility change during the data hold time. Stephen VTA BLOCK DIAGRAM SOURCES PHONO INPUT SOURCE SELECT RIAA EQU

INPUTS TAPE TUNER AUX GAIN GRAPHI C EQUALIZER PREEQU GAIN ADJ BUFF B BUFF LOCAL VOLT REG SPEAKER TO ALL AUDIO STAGES TUBE PREAMP AND PHASE SPLITTER VOL ADJ TUBE PUSHPULL AMP

ZMATCH XFMR SEL IN VR1 VR2 VR3 VR4 VR5 VR6 VR7 VR8 BUFFER A HIGH VOLT SUPPLY DIGITAL POTENTIOMETERS A 110 VOLTS AC B OPTO COUPLERS LOW VOLT SUPPLY MICROCONTROLLER CLOCK OSC EXTER

NAL USB PROGRAM INTERFACE 16 MILLION COLOR 800 x 480 LIQUID CRYSTAL DISPLAY TOUCHSCREEN AUDIO PATH FOR ONE CHANNEL SHOWN ONLY THE OTHER IS SIMILAR Stephen VTA SCHEMATIC From graphic equalizer To speaker, via relay (not shown) 5 Volts AC for tube heaters From High Voltage Power Supply Stephen VTA ARCHITECTURE SELECTION

Four candidate architectures were investigated early in the project to select the design approach of the power amplifier: Singleended and push-pull configurations and with or without global feedback, see table. Design 1 was chosen as offering the best frequency response and highest power at the lowest distortion. Design 1 2 3 Topology Max Max THD THD THD THD THD Vout Power at at at at at (Vpk) (Wrms) 1KHz -6dB -12dB -18dB -24dB Phase splitter, no feedback 18 Phase splitter, with feedback 15.3 Single end, no feedback Single end, with 9.2

20.24 2.95 1.3 0.61 0.293 14.63 3.8 0.96 0.42 0.27 5.29 3.7 1.98 0.4 0.17 Freq Response 18.3 dB, 20Hz-100KHz down <1dB at 0.165 ends 18.3dB 500Hz-100KHz 0.16 down 3dB @ 92Hz -1.2dB, 20Hz-100KHz down <1dB at

0.09 ends -0.5dB, 90Hz-100KHz Stephen VTA DISTORTION VTAs are favored by many musicians and high-end audio enthusiasts for their mellower sound and low-distortion characteristics. This effect, known as tube sound, is believed to come from the soft clipping characteristics of vacuum tube amplifiers which emphasize even-order harmonics, as opposed to solid-state designs that tend to produce odd-order harmonics when they sharply clip during musical peaks. During the architecture selection, the distortion characteristics of the various configurations were analyzed with NI Multisim. In general, the even-order harmonics tended to be of higher amplitude than the next odd-order harmonic (see Table 1). Note that even-order harmonics are simply the same musical note at a higher octave (see Table 2) Table 1 Frequency Harmonic Number Musical Note 440 Hz 1 (fundamental) A in 4th octave 880 Hz 2nd

A in 5th octave 1320 Hz 3rd Approx. E in 6th octave 1760 Hz 4th A in 6th octave 2200 Hz 5th Approx. C in 7th octave Table 2 (Music notes are per the Equal Tempered Chromatic Scale) Stephen VTA DESIGN DECISIONS V3 is a dual-triode tube configured as a phase splitter. Various reference designs used type 12xx7 tubes so several were analyzed in NI Multisim. The type 12BH7A was chosen due to slightly lower THD characteristics V2 and V1 are beam power pentode tubes configured as a push-pull amplifier with a center-tapped transformer as their plate load. Type 6L6 tubes were chosen due to almost universal use in reference designs.

Impedance Transformer: The model 125E was chosen due to being specifically designed for this application, flexible impedance ratio and availability. It provides six taps on the secondary ranging from 3K to 22.5K. A value of 5.6K ohms was chosen because it provided the best combination of maximum output power and THD. Stephen AUDIO PROCESSOR DIFFICULTIES The optimal configuration of the push-pull amplifier was challenging due to several conflicting factors: Maximum output power occurs with the highest plate voltage. High plate voltages unfortunately run the risk of exceeding the 6L6 maximum plate voltage rating of 500 volts. Lowest distortion was achieved with lower values of cathode resistor, however this resulted in higher plate voltages. Stephen AUDIO PROCESSOR SUCCESSES The audio input circuits, analog multiplexer and graphic equalizer were prototyped and worked. A few minor schematic issues were discovered during this process, which have been resolved.

Stephen HV POWER SUPPLY BLOCK DIAGRAM SOURCES PHONO INPUT SOURCE SELECT RIAA EQU INPUTS TAPE TUNER AUX GAIN GRAPHI C EQUALIZER PREEQU GAIN ADJ BUFF B BUFF LOCAL VOLT REG SPEAKER TO ALL AUDIO STAGES

TUBE PREAMP AND PHASE SPLITTER VOL ADJ TUBE PUSHPULL AMP ZMATCH XFMR SEL IN VR1 VR2 VR3 VR4 VR5 VR6 VR7 VR8 BUFFER A HIGH VOLT SUPPLY DIGITAL POTENTIOMETERS A 110 VOLTS AC

B OPTO COUPLERS LOW VOLT SUPPLY MICROCONTROLLER CLOCK OSC EXTER NAL USB PROGRAM INTERFACE 16 MILLION COLOR 800 x 480 LIQUID CRYSTAL DISPLAY TOUCHSCREEN AUDIO PATH FOR ONE CHANNEL SHOWN ONLY THE OTHER IS SIMILAR Stephen HIGH VOLTAGE POWER SUPPLY 450 Volts DC to the VTA 360 Volts AC from a transformer Room provided on the circuit board, if required

Rafa LOW VOLTAGE POWER SUPPLY DIFFICULTIES Item Symbo l Volt (V) Power DVDD AVDD VGH VGL +3.3 +10.4 +16.0 -7.0 Input Signal VCOM +3.6 Input Logic High VIH - Input V

Logic Low IL - Amplifier + Vah +12 Amplifier - Val -12 Rafa LOW VOLTAGE POWER SUPPLY BEFORE RECEIVING DISPLAY PANEL Rafa LOW VOLTAGE POWER SUPPLY DIFFICULTIES Item Symbo l Volt (V) DVDD +3.3 AVDD +10.4

VGH +16.0 VGL -7.0 Input Signal VCOM +3.6 Input Logic High VIH - Input Logic Low VIL - Amplifier + Vah +12 Amplifier - Val -12 MCU 5

Vdd +5.0 MCU 3 Vdda +3.3 Power Item Symbo l Volt (V) Power DVDD VGH +3.3 +5 Amplifier + Vah +12 Amplifier - Val -12

MCU 5 Vdd +3.0 MCU 3 Vdda +2.2 Resistor values had to be changed to match standard values. Rafa LOW VOLTAGE POWER SUPPLY SUCCESS Jason MICROCONTROLLER TOPOLOGY DESIGN DECISIONS For simplicity we decided to go with a monolithic microcontroller design instead of multiply low performance controllers. Eliminates the need for inter micro controller communication bus Simpler hardware footprint Easier to synchronize multiple interrupts Jason

MICROCONTROLLER SELECTION DESIGN DECISIONS Microcontrol ler Stellaris STM32F3 GPIO 100 100 60MHz 72MHz ADC 12 12 FPU Yes Yes Well supported Yes Yes Dev board available Yes

Yes Peripheral library Yes Yes Availability No Yes Min 63 Clock frequency Min 50 MHz Min 12 bit res Rafa DISPLAY REQUIREMENTS Item Requirement Screen Dimension 6 to 7 diagonal Refresh rate at least 50ms Cost $100 max. Interface

digital Documentation sufficient documentation must be available Availability Ability to receive product within 30 days of purchase Rafa DISPLAY OPTIONS Item Option 1 Option 2 Cost $57 $86 Screen dimension Panel dimension (width x height x depth) Resolution 7 diagonal 6.2 diagonal 6.4 x 3.8 x 0.7 6.1 x 3.5 x 0.2 800 x 480 800 x 480

Colors 16 million 16 million LCD Controller LCD Controller documentation Touch Screen controller Touch Screen controller documentation Availability SSD1963 (integrated) Not documented available Not documented XPT2046 (integrated) Not documented available Not documented within 20 days Within 20 days Rafa DISPLAY DECISION Item Cost Screen dimension

Panel dimension (width x height x depth) Resolution Colors LCD Controller LCD Controller documentation Touch Screen controller Touch Screen controller documentation Availability Option 1 $57 7 diagonal 6.4 x 3.8 x 0.7 800 x 480 16 million SSD1963 (integrated) available XPT2046 (integrated) available within 20 days Rafa DISPLAY DIFFICULTIES Difficulty Pin out was contradicting Power up sequence Power down sequence Overcome Once display panel arrived, pin out table was generated based on hardware present. Discovered that it is not needed.

Rafa DISPLAY SUCCESSES Success LCD communication LCD control Description Communication between MCU and LCD screen. MCU is capable of turning ON/OFF and sending data to LCD screen. Rafa SOFTWARE DIAGRAM Digital Equalizer Visualization State Detector Sound Analyzer EQ Values Updater Set mode Graphics Generator I Graphics Generator II Graphics Update

Display Rafa IMMEDIATE PLAN FOR COMPLETION Item Date Finish building and test Low voltage power supply June 7th Display panel connector June 14th Program background. Using existing libraries as necessary. June 14th GUI images (buttons, bars, etc). Using existing June 21st libraries as necessary. Stephen AUDIO PROCESSOR PLAN FOR COMPLETION Event Expected date Finish adding VR chips to the prototype, and write a test program. June 8 Prototype and test Vacuum Tube Amplifier June 10

Layout of PC board June 10-12 Respond to any manufacturability issues noted by vendor (Advanced Circuits) June 12-14 Finalize PC board order June 14 PC assembly June 27-28 Begin PC unit testing Week of July 1 Begin integration with MCU PC assembly Week of July 8 Begin assembly of completed project demonstration Week of July 15 Completion of project build and test July 20 to 22 Presentation July 26 Rafa CURRENT PROGRESS GUI MCU-board Visualizations

Equalizer (Hardware) Power-Amp Not Started In-Progress Completed Pre-Amp Low Volt. Power High Volt. Power Research Overall 0% 20% 40% 60% 80% 100% Stephen OVERALL BUDGET & FINANCING DETAILS This project is self-funded by the group, with Stephen providing 90% of the funds The original budget of this project was $500 As of now, approximately $500 has been spent Item Cost Comments Audio processor PC $66 2 double-sided boards at $33

The following costs remain: board each MCU PC board $66 1 multi-layer board Miscellaneous parts $30 Estimated QUESTIONS

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