The Ultimate Tone Vol. 4

Table of Contents and List of Figures


Chapter 1: SONIC BOOM
THE TREE IN A FOREST
SOUND
      Frequency
      Amplitude
      Phase
      Symmetry
      Duration
WAVE SHAPE ENVELOPE
      Attack
      Sustain
      Release
TIMBRE
      “Timber! I’ve lost my timbre”
      Room Reflections
      Audience Participation
HUMAN PERCEPTION OF SOUND
      Dynamic Range & Resolution
      Apparent Loudness
      Aural Compression & Illusionary Sound
UNSOUND REASONING
      “That’s my Sound”
LIVE SOUND PARADOX
      Sound By Bliss
      Use it or Lose it Philosophy
      Human Overload
      Lower Than Low
BETTER LIVE SOUND IN OUR FUTURE
      The Culture of Short-term Goals
      Too Quick to the Gate
      TUT for Everyone

Chapter 2: ATTENUATORS
LOUDNESS REDUCTION
      Inefficient Yet Too Efficient
AC SIGNALS
      Frequency
      Amplitude
      Symmetry
      Duration
      Phase
      Phase Shift
E-I PHASE RELATIONSHIP
      Resistive E-I Phase
      Inductive E-I Phase
      Capacitive E-I Phase
      L-C-R Circuit E-I Phase
MORE SPEAKER DETAILS
      Impedance Curve
      Frequency Response
      Adding it Up
AMP/SPEAKER INTERFACE
      Driver Damping
      Amplifier Damping
      Modelling the Output Stage
      Modelling the Speaker
SPEAKER COMPRESSION
      Magnet Influence
SONIC PERCEPTION
ATTENUATOR EVOLUTION
SAMPLING ERRORS
ATTENUATION TO MONITOR SPEAKER
      Series Attenuation...
          ...by Power Ratio
          ...by Resistance Ratio
          ...by SPL Reduction
          ...by Transformer Step-down
AMP LOADING
      To Load or Not to Load
      Constant-Load Resistive Attenuation
      Reactive Loading
      The Road Block?
      Mr. Zobel to the Rescue
      Other Load Forms:
          A fan of your own
          Half a speaker; half the sound?
ATTENUATOR ALTERNATIVES
      Master Volumes
      Multiple Amplifiers
      Common Amp Voicing
      Closed System
      Active Attenuators
      Modified Line-out From Attenuator
      Variable-Boost Super Scaler
      Power Scaling
OTHER PARTS OF THE ATTENUATOR

Chapter 3: POWER SCALING
SOUND BASIS
      Power Scaling Definition
ATTEMPTS TO CONTROL LOUDNESS
      ...with Master Volumes
      ...Using Low-wattage Amps
      ...Using Active Attenuators
      ... by Integrated Power Control
HOW LOUD IS “QUIET”?
      LOUDspeakers are Called That for a Reason
      “Quiet” with Small Tubes
      “Quiet” with Large Tubes
      Curved Sound
POWER CONTROL
      Indirect Audio Paths
      Basic Principle of Power Scaling
SUPPLY IMPEDANCE FALLACY
      Stock Supply Impedance
      Adding the Ideal Power Scale Regulator
      “Ideal” Oversight
      Capacitance Issues
SONIC IMPACT OF SUPPLY IMPEDANCE
REGULATOR TYPES
      AC Regulators
      DC Regulators in General
      DC Shunt Regulator
      DC Series-pass Regulator
      Linear Regulators
      Switching Regulators
      Conduction-Controlled Regulators
OTHER REGULATOR CONSIDERATIONS
      Heat and Other Noise
      PCBs & Hand-wiring
      Alternatives to Regulators
POWER SCALE CIRCUIT FORMS
      Single Transformer with Variac
      Separate Transformers
      Clean Power
      Variable Output Power Conditioner
      Adding Linear Regulators
DIRECT CONTROL
      Dual-ganged Potentiometer Control
      Tracking DC Regulators
      Tracking Switching Regulators
COINCIDENCE POINT
      Refined Coincidence
      Power Range
      Mosfet Protection
      Selecting a Mosfet
      Proper Grounding
      Raw Bias Ideal
DISPARATE RAILS
      Intrinsic Triodes
      Power Extreme
      Direct Control
      Tracking Plate Regulator
TONE CONSTANCY
      Limit Control
      Automatic Limiting
      Alternative Auto-limits
POWER SCALE ON-OFF
      Instant Headroom
      Fast-transition Circuit
      Power Scale Boost
MULTIPLE POWER SCALE CONTROLS
      Power Scale ×2
      Interfacing Power Scale Control Selection to Jfet Audio Switching
      Power Scale ×3 and Up
      Limit Compensation...
          ...For Schmitt Splitters
          ...For Concertina Splitters
SWITCHING MULTIPLE LIMIT CONTROLS
      Jfets
      LDRs
      Relays
      PVAs
      Physical Switch Location
CURRENT-MODE REGULATORS
      Current Mirror Reflections
      Turning the Current Mirror into a Current-mode Regulator
      Current mirror Output Interfacing
      DAC to CMPS
      Potentiometer Replaces DAC
      Inverting the Control Function
“AMPLIFIED DC”
      Multiple Power Scale Controls with the Linear Amplifier Regulator
PURE DC CONTRO
      Amplified Follower
MASTER POWER SCALE CONTROL
      MPS Added to Proportional Regulators
      Adding MPS to Current-mode Regulators
      MPS for the Amplified Follower
      MPS and the Linear Amplifier Regulator
UNIVERSAL APPLICATION
WHAT TO POWER SCALE?
      Output Stage
      Output Stage, Splitter & More
THERMAL LAG
CLASS FALLACY
PLUG & PLAY POSSIBILITY?
      Interrupting Power Tube Plug-in...
          ...Adding Cathode Resistance
          ...Adding Plate Resistance
          ...Adding Variable Voltages
Plug-and-Play that works
BIAS-MODULATED TREMOLO
      Bias-Modulated Tremolo in Fixed-Bias Amps
      Bias Modulation in Cathode-Biased Amps
PENULTIMATE WORD
      Triode-Pentode Switch
      Tube Selector Switch
      Low-Power Switches
      Power Dampening
      Carvin Power Selector
      Wattage Control
FINAL WORD
      “What about Bob?”
      “What about blocking distortion?”
      “What about speaker break-up?”
      “What about transformer saturation?”
      “Where is Bob, then?”

Chapter 4: SAG
SOUNDS UNIMPRESSIVE
      Internal Resistance
      Inherent Regulation
      Signal Envelope
SAG TIME CONSTANTS
ALTERING REGULATION
ACTIVE LINEAR REGULATORS
       Linear Mosfet, BJT or Tube Regulators
SWITCHING REGULATORS
      Relay-based “Vibrators”
      Low-frequency Inverters
      Medium- to High-frequency Regulators
      Thyristors
      IGBTs
      “To be or not to be”
PASSIVE SAG CONTROL
      Output Stage Sag Only
      Complete Amp Sag
COUNTERINTUITIVE SAG CONTROL
      Historic Precedents...
          ...Sound Craftsman
          ...HH Scott
COMPRESSED ALTERNATIVES
      Compressor Feedback Loop
      Push-pull Amp as a Compressor
      Splitter Modulation
      Screen Modulation
      SE Sag Effects
CURRENT-CONTROLLED SAG
      Active Dynamic-current Response...
          ...at the Supply Rail
          ...at the Ground-end of the Output Stage
          ...at Ground but “New & Improved”

Chapter 5: MASTERFUL CONTROL
MASTER OF YOUR DOMAIN
      Tone Generator
      Master Volume
      Transparent Amplifier
      Transparent Speaker
      System Hurdles
TRADITIONAL MASTER VOLUMES
      Conventional MV
      Improved Conventional MV
      Bootstrapped MV
      Improved Bootstrapped MV
      Bootstrapped MV for Preamp Channel
POST-PI-MASTER-VOLUMES
      Simple Post-PI-MV
Post-PI-MV-DON’T!
      Isolated Dual-Section Post-PI-MV
      Isolated Single-Section Post-PI-MV
      Concertina Uniqueness
      See-Saw Possibilities
MODERN MASTER VOLUMES
      Impedance Modulation
      Current-limited Splitter
TRANSPARENT POWER TRANSFER
      Power Amp Requirements
      Speaker Limits
      Using Conventional Guitar Equipment
      Direct Sound

Chapter 6: AMP VOODOO
“Stupid Amplier tricks”
MESA BOOGIE
      Series Gain Stages for Two Sounds
      Simulclass
      Triode-Pentode Switching
      How is this Patentable?
      Output Tube Selector
      Rectifier Selection
      Mains-tap Power Reduction
VIRTUAL POWER REDUCTION
      Post-PI Attenuator
      Damping Differences
BACK TO THE FUTURE
      Output Transformers for Solid-state Power Amps
“HIGH-GAIN” OUTPUT TRANSFORMERS
      Power Stage 101
      OT Manipulation Results
AUTOMATIC BIAS
      Self Bias
      Active Bias Control Circuits
      Bias Balance
      Active “Cathode” Bias
      Current-source Bias
      Active Control over True Self Bias
SWEEPABLE FIXED-TO-CATHODE BIAS
       ...via Rheostat
       ...via Variable RK
AUTOMATIC NOISE REDUCTION & SPEAKER PROTECTION
      Lamp Without a Genie
      Lamp as Distortion Reducer
      Lamp as Compressor
      Lamp as Speaker Noise Quietener
      Lamp as Speaker Protector
SPLITTER REUNION
      Stacked-Schmitt
      Concertina
      See-Saw
NOTHING FOR SOMETHING
      Value-Added & Upgrades
SUPPLY AGING
EFFECTS LOOPS
      Effects Loop Control Options
      Multiple Effects Loop Controls
      Multi-Voice Preamp with Individual Loops
      Stereo FX Returns
MULTIPLE POST-PI-MVs
YELLOW JACKETS
CONJUNCTIVE FILTERS
      “Current Tracking Technology”
TRANSTUBE AMPS
SRV-TONE “NON-MOD”

Chapter 7: SUPER SCALERS
LIVING LARGE
AMPLIFICATION
      Unity Gain
      Gain Beyond Unity
      Gain Below Unity
      Negative Gain
DOMAIN MATRIX
      Voltage Domain
      Current Amplifiers
      Power Amplifiers
SIMPLE TUBE SUPER SCALER BASICS
      Getting Power Output from a Tube
      Grid Drive
      Cathode Drive
      Screen Drive
      Cascaded Super Scalers
      Power-Grid Super Scaler
      The SV572 Alternative
DESIGNING SIMPLE TUBE SUPER SCALERS
      Transformer Availability
      Screen Drive Input Transformer Selection
      Designing the Cathode Input Transformer
      Designing the Power-Grid Input Transformer
VARIABLE BOOST RATIOS IN SIMPLE TUBE SUPER SCALERS
      ...Using Output Transformer Taps
      ...Using Input Transformer Taps
      ...For Screen Drive
      ...For Cathode Drive
BJT SUPER SCALERS
      BJT Characteristics
      Common-Emitter Connection
      Emitter-Follower Possibility
      Common-Base Option
      BJT Issues
      BJT-Tube Cascode
GOOD CIRCUITS BUT NOT SUPER SCALERS
      Open-Loop UL Stage
      Transformer-Driven Emitter Follower
      Mosfet Follower with Transformer Input

Chapter 8: G-M-X
ACTUALLY GM X
      “The ‘G’ stands for Conductance”
“TRANS” FORM
      Tube gm
      BJT gm
      Mosfet gm
Gm X CORE
      GmX Hybrid
      Practical GmX Hybrid
BENEFITS OF THE GmX APPROACH
      ”Let’s get Large...”
          ...Using the 1650T
          ...More Power From the 1650T
          ...Using the 1650W
CUSTOM GMX OUTPUT TRANSFORMER
      Traditional OT
      Modern OT Approach
      Multi-tier Efficiency
DEALING WITH OTHER OUTPUT STAGE ATTRIBUTES
      Cathode Bias
      Triode and UL Wired Output Tubes
      Single-ended
ALL-TUBE GmX
GmX or NOT?

Chapter 9: Z-M-X
TRANSIMPEDANCE AMPLIFIERS
      Passive Potentiometers
      Direct Volume and Tone Controls
      Direct Panning Pots
      Direct Balance Control
      Direct Effects & Reverb Controls
INDIRECT CONTROL
      Trans-gm Tradition
      Simulated Resistor
      Device Limitations
      Level Control Applications
      Compressor Applications
      Politics of Compression
ALTERNATIVE ZMX CIRCUITS
      DC-only ZmX Circuit
      Controlling R in the RmX Circuit
      High-voltage Signals
NON-INTUITIVE CONNECTIONS
      Bilateral Switch in Linear Mode
      Parallel Extreme
SWEPT BIAS USING ZMX

Chapter 10: POWER MANAGEMENT
POWER PLAY
POWER SOURCE
      Mains Noise Filtering
      Spike Suppression
      Broadband Noise Suppression
      AC Current Limiting
      Active AC Current Limiting
RECTIFICATION
      Long Live Valves
      Solid-state Contradictions
      Mixing Vacuums with Sand
TRANSFORMER TAPS
      AC Taps Used as AC
      DC Conundrum
      But We See This Done Elsewhere!
VOLTAGE RESTORATION
VOLTAGE CLAMPING
      DC Noise Filtering
      DC Current Limiting
HEATER SUPPLIES
      DC Heaters
ACTIVE HEATER REGULATION
      Heater Voltage Regulators
      Heater Current Regulators
      Active Current Balancing
POWER AMPLIFIERS
      SE
      PP
      Bias Conditions...
          ...Class-A
          ...Class-B
          ...Class-AB
      Bias Methods...
          ...Cathode Bias
          ...“Fixed” Bias
      Alternative Bias Circuits
      PSRR

Chapter 11: DESIGN PHILOSOPHY
“I plink, therefore I amp”
Fear and Loathing in the Music Shop
      Music-heads
      Gear-heads
      Decision perspective
      Context
      “Things appear smaller than they actually are”
DSP
The Point
HIERARCHICAL DESIGN PROCESS
      Decisions / Organization / Baby-steps
      Over-all Format Decision
      Active Element Choice
      Power Amp Topology Choice
      Resistor Choice
      Capacitor Choice
      Capacitor Contentions
      Wiring Choice
      Chassis Choice
OVER-BUILT
      Who is qualified?
      Adequate vs. Ample: the Chassis Example
      Electronic Component Safety Margins
DESIGN COMPROMISES
      1645 + 273CX
      1650K + 272HX
      1650K + 272JX
      1639 + 275X
      1650G + 273BX
      50W Comparisons
300W DESIGN OPTIONS
      6× 6550
      4× 6550
      2× 811A
      Comparison of 300W Options

Chapter 12: PLUG-N-PLAY
“We already have one”
      Passive Attenuation
      Active Attenuation
      Active Loads
      Benign Loads
      Compression
      Loudness Compensation
CHAIN
NOT SO “ULTIMATE”
      Flaws One and Two
      Power Issues
NULL METHOD
      Thermal Management
      Optimizing the Null Amp
      Tone Amp Loading
SUPER SCALERS
      Back in The Day...
      Dohs into Dos
ACTIVE LOADS
      BJT Load
      Mosfet Load
      Turning the Active Loads into Active Attenuators
FLOATING ACTIVE ATTENUATOR
      Synchronous Rectifier Note
LIVE POST-PROCESSING
      From Studio to Stage
      Attenuator Revolution
      Fender’s Transimpedance Attenuator


List of Figures

Fig. 1-1: Sound wave characteristics
Fig. 1-2: Relative phase of sound
Fig. 1-3: Sound envelope characteristics
Fig. 1-4: Timbre and harmonics
Fig. 1-5: Room reflections
Fig. 1-6: Audience effect on room sound
Fig. 1-7: Fletcher-Munson curves
Fig. 1-8: Basic compression and spectral compression effects
Fig. 2-1: Speaker efficiency measurement
Fig. 2-2: Perceived loudness related to absolute loudness
Fig. 2-3: Electrical signal characteristics
Fig. 2-4: Other wave shapes and symmetry
Fig. 2-5: Duration of sounds
Fig. 2-6: Signal phase as absolute and relative terms
Fig. 2-7: Phase shift
Fig. 2-8: Vector diagram for a pure resistance
Fig. 2-9: Voltage stimulus and current consequence for an inductor
Fig. 2-10: Vector diagram of inductive current-voltage relationship
Fig. 2-11: Current stimulus and voltage consequence for a capacitance
Fig. 2-12: Vector diagram of capacitive current-voltage relationship
Fig. 2-13: Composite phase relationship for LCR circuit
Fig. 2-14: Mechanical parameters and influences over speaker performance
Fig. 2-15: Typical electrical impedance curve for a raw driver
Fig. 2-16: Frequency response of raw driver
Fig. 2-17: Electrical damping of the speaker by the amplifier
Fig. 2-18: Damping factor for different tube power amplifier configurations
Fig. 2-19: Model of a tube amp output stage
Fig. 2-20: Output transformer power bandwidth
Fig. 2-21: Electrical model of a dynamic loudspeaker
Fig. 2-22: Loudspeaker as both motor and generator
Fig. 2-23: Driver compression
Fig. 2-24: Loudspeaker magnetic assemblies
Fig. 2-25: Real-world verification of “illusionary” sound
Fig. 2-26: Speaker attenuator evolution
Fig. 2-27: Resistive series attenuator
Fig. 2-28: Series-resistance attenuator using power ratios to determine circuit values
Fig. 2-29: 100W amp 8O series-resistance attenuator example
Fig. 2-30: Loudness for each of the steps in the 100W 8O series-resistance attenuator
Fig. 2-31: Designing the series-resistance attenuator using multiple resistance values of the load
Fig. 2-33: Transformer attenuation
Fig. 2-34: Constant load series-parallel attenuator
Fig. 2-35: Output power compared to voltage and current phase relationship
Fig. 2-36: Zobel network gets rid of “fizz”
Fig. 2-37: Electric fan as reactive load with resistive attenuation to speaker
Fig. 2-38: Crippled speaker as load
Fig. 2-39: Master-volume amp with no electrical loudness or quiet restrictions
Fig. 2-40: Different sized amps for different loudness situations
Fig. 2-41: Closed-system options
Fig. 2-42: Line output options
Fig. 3-1: Loudness perception
Fig. 3-2: Transfer curves and operating points
Fig. 3-3: Power supply interaction with signal
Fig. 3-4: Power Scale essence
Fig. 3-5: Simplification of essential control
Fig. 3-6: Power supply impedance fallacy with respect to varied voltage
Fig. 3-7: Power supply noise rejection by amplifier
Fig. 3-8: Capacitive reactance vs. frequency
Fig. 3-9: Inherent power supply regulation
Fig. 3-10: Current modulations and noise
Fig. 3-11: Thyristor characteristics
Fig. 3-12: Thyristor regulator circuits
Fig. 3-13: SCR-based AC regulator
Fig. 3-14: Mosfet AC regulator
Fig. 3-15: BJT AC regulator
Fig. 3-16: Tube AC regulator
Fig. 3-17: IGBT AC regulator
Fig. 3-18: DC shunt regulators
Fig. 3-19: DC series-pass regulators
Fig. 3-20: Integrated circuit regulators
Fig. 3-21: Switching regulators
Fig. 3-22: Size difference between linear power supply and switching power supply
Fig. 3-23: DC switching regulator
Fig. 3-24: Conduction control principle
Fig. 3-25: Tapped transformer secondaries for two power levels
Fig. 3-26: Multi-tapped secondaries for various power settings
Fig. 3-27: Multi-tapped primaries for various power settings
Fig. 3-29: Multiple transformers with AC regulator
Fig. 3-30: Bilateral mosfet AC regulator
Fig. 3-31: DC regulators used to control AC
Fig. 3-32: Proper way to use a variac for variable power
Fig. 3-33: Power conditioner circuits
Fig. 3-34: Linear power condition for dedicated amplifier
Fig. 3-35: Basic DC regulator Power Scale circuit
Fig. 3-36: Intuitive dual-pot Power Scale circuit
Fig. 3-37: Choosing the correct pots
Fig. 3-39: Tracking regulators with direct control over bias supply
Fig. 3-40: Tube bias tracking error with direct-controlled bias regulator
Fig. 3-41: Direct-controlled plate/screen voltage and tracking bias regulator
Fig. 3-42: Switching regulator for high voltage with linear tracking bias supply
Fig. 3-43: Coincident point of Power Scale regulator options
Fig. 3-44: Simplest Power Scale regulator
Fig. 3-45: Simplest Power Scale regulator with practical refinements
Fig. 3-46: Circuit operation and waveforms
Fig. 3-47: Setting the range of power control
Fig. 3-48: Active current limiting
Fig. 3-49: Soft current limiting
Fig. 3-50: Capacitor charge currents and turn-on surge
Fig. 3-51: Load current vs. Power Scale regulator current
Fig. 3-52: Grounding the Power Scale regulators
Fig. 3-53: Ideal raw bias supply and alternatives to achieving the ideal
Fig. 3-54: Bias supply loading
Fig. 3-55: Protecting the bias supply
Fig. 3-56: Adding Power Scaling to amps with widely different screen and plate voltages
Fig. 3-57: Intrinsic triode within tetrodes and pentodes
Fig. 3-58: Plate current dependance on screen voltage
Fig. 3-59: London Power’s 700W output stage
Fig. 3-60: Waste power in a conventional output stage and effect of changing the screen voltage
Fig. 3-61: Mosfet power sharing issues
Fig. 3-62: Tracking plate supply regulator for disparate rail amplifier
Fig. 3-63: Tracking plate supply operation
Fig. 3-64: Signal levels at a high-power level
Fig. 3-65: Using preamp volume control to attempt drive compensation
Fig. 3-66: Proper drive compensation
Fig. 3-67: Overdrive possibilities using Power Scale, Limit and Volume controls
Fig. 3-68: Automatic limiting method with Schmitt splitter
Fig. 3-69: Automatic limiting with concertina splitter
Fig. 3-70: Tracking switch for manual or automatic distortion limiting
Fig. 3-71: Power contained in sine and square waves
Fig. 3-72: On-off for the Power Scale regulator
Fig. 3-73: Fast-transition circuit
Fig. 3-74: Integrated instant headroom switch
Fig. 3-75: Power Scale boost control
Fig. 3-76: Control interaction dependence on settings and pot values
Fig. 3-77: Boost and Power Scale controls that are 10× different in value
Fig. 3-78: Multiple Power Scale control concept
Fig. 3-79: Dual Power Scale controls selected by DPDT
Fig. 3-80: Cascading regulators for multiple Power Scale controls - basic approach
Fig. 3-81: Detailed approach for two cascaded Power Scale regulators
Fig. 3-82: Jfet-controlled fast-transition circuit for dual Power Scale controls
Fig. 3-83: Multiple Power Scale selection using relays
Fig. 3-84: Multiple Power Scale selection using shunt jfets and mosfets
Fig. 3-85: Fast-transition coupling with jfet-shunt-compatible control and three or more Power Scale controls
Fig. 3-86: Multiple limit controls in power amp with Schmitt splitter
Fig. 3-87: Multiple Limit controls in amps with Concertina splitter
Fig. 3-88: Series jfet switches and the correct gate control voltage window
Fig. 3-89: LDRs used to select Limit controls
Fig. 3-90: Relay-selected limit controls
Fig. 3-91: PVA pitfall in audio switching circuits
Fig. 3-92: Digital-to-analog converter with current output converted to produce voltage output
Fig. 3-93: Current mirror basics
Fig. 3-94: Output voltage from input current
Fig. 3-95: Current-mode Power Scale regulator
Fig. 3-96: Cascoded current-mode Power Scale regulator
Fig. 3-97: DAC connected to current-mode Power Scale regulator
Fig. 3-98: DAC resolution as number of bits versus steps of output change
Fig. 3-99: The pot and programming resistor determine input control of the current mirror
Fig. 3-100: Multiple Power Scale controls tied to the current-mode Power Scale regulator
Fig. 3-101: “Normalized” control function
Fig. 3-102: Linear amplifier as Power Scale regulator
Fig. 3-103: Multiple Power Scale controls in the linear amplifier regulator
Fig. 3-104: Pure-DC Power Scale approach
Fig. 3-105: Amplified follower variation of current-mode Power Scale regulator
Fig. 3-106: Eliminating the dead spot at the X-end of the Power Scale control
Fig. 3-107: Adding a Master Power Scale control to a relay-switched proportional regulator
Fig. 3-108: Adding a Master Power Scale control to jfet-selected multiple Power Scale regulator
Fig. 3-109: Master Power Scale added to current-mode regulator
Fig. 3-110: Adding a Master Power Scale to the inverted-function current-mode regulator
Fig. 3-111: Adding a Master Power Scale to the amplified follower with multiple Power Scale controls
Fig. 3-112: Master Power Scale applied to linear amplifier regulator
Fig. 3-113: Power Scaling just the output stage
Fig. 3-114: Power Scaling the output stage and splitter
Fig. 3-115: Power Scaling the output stage, splitter and last preamp stage in typical Marshall or Fender circuits
Fig. 3-116: Power Scaling the entire amp
Fig. 3-117: Thermal compensation options
Fig. 3-118: Plug-and-play connectivity
Fig. 3-119: Variable resistance added to cathode and plate circuits respectively
Fig. 3-120: Transconductance amplifiers used to compensate signal and bias levels
Fig. 3-121: “Practical” plug-and-play option with limited application
Fig. 3-122: Connecting bias-modulated tremolo to a Power Scaled amplifier
Fig. 3-123: Grid-modulated tremolo for cathode-biased output stage is actually modulating tube bias
Fig. 3-124: Triode-pentode switch effect on amplifier characteristics
Fig. 3-125: Tube switching for power reduction
Fig. 3-126: High-low power switch on Fender “The Twin”
Fig. 3-127: Variable cathode resistor in Schmitt to reduce signal output
Fig. 3-128: Carvin’s power reduction switch from 1980s models
Fig. 3-129: Typical coupling between splitter and power stage, and the impedance change with signal drive
Fig. 3-130: Cathode follower easily drives grid with low distortion
Fig. 3-131: Normal speaker cone break-up with frequency
Fig. 3-132: Transformer core flux density at different frequencies
Fig. 3-133: Tube characteristics and how saturation resistance limits drive to the output transformer
Fig. 4-1: Internal resistance of a power supply
Fig. 4-2: Inherent supply regulation
Fig. 4-3: Envelope of an audio signal
Fig. 4-4: Change of signal as it passes through the power amp
Fig. 4-5: Performance spread caused by imperfect regulation
Fig. 4-6: Supply voltage variations in a typical push-pull amplifier
Fig. 4-7: Typical SE and high-bias amp supply voltage variations
Fig. 4-8: Integrated circuit linear regulator
Fig. 4-9: Voltage concerns with linear regulators
Fig. 4-10: Typical tube voltage regulator
Fig. 4-11: Typical BJT voltage regulator
Fig. 4-12: Typical mosfet voltage regulator
Fig. 4-13: Vibrator-style regulator
Fig. 4-14: Low-frequency BJT transformer inverter
Fig. 4-15: SMPS inverter
Fig. 4-16: Switching regulator
Fig. 4-17: Fan reduces heat-sink size
Fig. 4-18: Tube and semiconductor thyristors
Fig. 4-19: Typical SCR application as a regulator
Fig. 4-20: IGBTs
Fig. 4-21: Passive sag added to output stage
Fig. 4-22: Passive sag added to complete amplifier
Fig. 4-23: Passive sag added to screen supply
Fig. 4-24: Active sag control approach using long-loop regulation
Fig. 4-25: Sound Craftsman varo-proportional power supply for headroom expansion
Fig. 4-26: H.H.Scott 265-A basic power amplifier with dynamic power monitoring
Fig. 4-27: Compressor basics
Fig. 4-28: Adding compression side-chain to push-pull amplifier
Fig. 4-29: “Insert” type compression circuit example from LAB amp
Fig. 4-30: Modulating the splitter output using a dual triode
Fig. 4-31: Screen modulation problems
Fig. 4-32: Mosfet and tube screen modulators
Fig. 4-33: Reducing redundancy in the screen modulator
Fig. 4-34: Series-pass screen modulator reduces losses compared to shunt approach
Fig. 4-35: Adding compression side-chain to SE amp
Fig. 4-36: Adding compression to a preamp gain stage
Fig. 4-37: London Power Sag Control Kit in B+ line
Fig. 4-38: Ground-referenced current limit for fixed-bias output stage
Fig. 4-39: Low voltage drop ground-referenced current clamp in fixed-bias output stage
Fig. 4-40: Boost amp options for ground-referenced current clamp
Fig. 5-1: Guitar sound system built upon a true master-volume approach
Fig. 5-2: Tone generators in synthesizer systems
Fig. 5-3: Master-volume locations in various guitar amp circuits
Fig. 5-4: Ideal transparent amplifier
Fig. 5-5: Ideal transparent speaker
Fig. 5-6: Alternative method using the PA and monitor system as the output-end of the MV-system
Fig. 5-7: Traditional and later metal backline stage setups
Fig. 5-8: Clean stage setup with monitor wells and flying PA cabinets
Fig. 5-9: Marshall 800 MV amps as examples of “conventional MV”
Fig. 5-10: Reducing the conventional MV’s tendency to frequency roll-off over its sweep
Fig. 5-11: Ground-referenced MV problem and how to fix it
Fig. 5-12: Bootstrapped MV
Fig. 5-13: Improved bootstrapped MV
Fig. 5-14: Bootstrap MV achieved by cathode-follower reallocation
Fig. 5-15: Cascaded bootstrap-MVs allow the implementation of a series effects loop without tone impediment
Fig. 5-16: Universal dual-pot post-PI-MV
Fig. 5-17: Post-PI-MVs that are NOT recommended
Fig. 5-18: Capacitively-coupled dual-pot post-PI-MV
Fig. 5-19: Coupling-cap value implications
Fig. 5-20: Capacitively-coupled cross-line post-PI-MV
Fig. 5-21: Concertina bootstrapped-MV
Fig. 5-22: MV for see-saw inverter
Fig. 5-23: Seymour-Duncan’s “Juice” control
Fig. 5-24: Current-limit used as MV in stacked-Schmitt splitter
Fig. 5-25: Electronically controlled splitter current controls headroom and loudness
Fig. 5-26: Voltage control of the splitter as a way to reduce output
Fig. 5-27: MV-system using full-range amps and cabinets
Fig. 5-28: MV-system using guitar amps and cabinets
Fig. 5-29: Multiple amps and cabinets
Fig. 5-30: Small OT restricts power bandwidth and applicability in a MV-system
Fig. 6-1: Early boost circuitry used by Mesa
Fig. 6-2: One form of Simulclass
Fig. 6-3: Radiotron Designer’s Handbook defines simultaneous class operation
Fig. 6-4: Simulclass output stage as originally drawn
Fig. 6-5: Corrected Simulclass drawing
Fig. 6-6: Triode-pentode switching
Fig. 6-7: Tube selection in Mesa-Boogie
Fig. 6-8: Power level switch in Trace Elliot 400W bass amp
Fig. 6-9: Power tube selection in Fender’s PS
Fig. 6-10: Rectifier selection and effect on power supply
Fig. 6-11: Voltage reduction by mains-matching tap
Fig. 6-12: Attenuated drive to output tubes
Fig. 6-13: Comparison of output impedance of small-tube amp and large-tube amp
Fig. 6-14: Tube OT with parasitic elements shown
Fig. 6-15: Vox’s mosfet output stage with output transformer
Fig. 6-16: Typical 50W output stage
Fig. 6-17: Reflected plate load impedance and its effect on grid drive and clip points
Fig. 6-18: Self bias as “automatic” bias
Fig. 6-19: Cathode-bias connection options for EL
Fig. 6-20: Basic active-bias arrangement
Fig. 6-21: Active bias-balance control from Yorkville Sound Custom 40
Fig. 6-22: Active “cathode bias” that isn’t
Fig. 6-23: Current-source tube biasing
Fig. 6-24: Active bias control over cathode-biased amplifier
Fig. 6-25: Active bias balance in cathode-biased push-pull amp
Fig. 6-26: Active balance of fixed-bias push-pull stage via tube screen-grids
Fig. 6-27: Primitive approach to sweepable fixed-to-cathode bias
Fig. 6-28: Variable RK in sweepable cathode-to-fixed-bias circuit
Fig. 6-29: Further improvement of the active swept bias circuit
Fig. 6-30: Lamp resistance characteristics
Fig. 6-31: Lamp used to control amplitude and thus distortion in an audio-frequency low-distortion oscillator
Fig. 6-32: Lamp used to control audio signal amplitude in compressor circuit
Fig. 6-33: Lamps as speaker-shunt noise reducers
Fig. 6-34: Lamp as speaker protector
Fig. 6-35: Stacked-Schmitt splitter
Fig. 6-36: Concertina splitter
Fig. 6-37: Traditional see-saw inverters
Fig. 6-38: Pre-tweed style amp with see-saw middle
Fig. 6-39: Pre-tweed revisited with modern-voiced preamp and see-saw middle
Fig. 6-40: Method for introducing the sound of aging caps or vintage performance
Fig. 6-41: Budget effects loop without send and return controls
Fig. 6-42: FX loop with preset return gain but manual send and return controls
Fig. 6-43: FX loop with active adjustable return-gain
Fig. 6-44 Dual-controls for an effects loop with intuitive switching of direct controls
Fig. 6-45: Multi-path preamp where each path has its own loop
Fig. 6-46: Multi-voice single-path preamp with individual channel loops
Fig. 6-47: Stereo FX loop
Fig. 6-48: Dual amp with shared output stage
Fig. 6-49: Dual amp selection
Fig. 6-50: THD Yellow Jacket tube adapter
Fig. 6-51: Conjunctive filter wired two different ways
Fig. 6-52: Typical solid-state power amp design where a tube-like output impedance is desired
Fig. 6-53: Phil Abbott’s “external amp mod” to achieve SRV tone
Fig. 7-1: Unity-gain amplifier is also a power amplifier by definition
Fig. 7-2: Typical single-gain-element buffers
Fig. 7-3: Conventional non-inverting amplifier using multiple gain elements within a feedback loop
Fig. 7-4: Common-grid amplifier, common-base amplifiers and common-gate amplifiers used to achieve non-inverting gain above unity
Fig. 7-5: Fractional-gain non-inverting amplifier using attenuator and conventional non-inverting gain stage
Fig. 7-6: Fractional-gain non-inverting amplifier using a common-grid stage
Fig. 7-7: Fractional-gain non-inverting amplifier using cascaded fractional-gain inverting gain stages
Fig. 7-8: Fractional-gain non-inverting amplifier comprised of a voltage follower driving an attenuator
Fig. 7-9: Inverting amplifier
Fig. 7-10: Conditions that satisfy the presence of only voltage amplification
Fig. 7-11: Voltage source characteristics
Fig. 7-12: Current amplifier characteristics
Fig. 7-13: Currents in various gain elements
Fig. 7-14: Current mirror as current amplifier
Fig. 7-15: Typical current-mirror applications
Fig. 7-16: Input impedance and signal conditions around the current mirror
Fig. 7-17: Complementary current-mirror circuit to handle bilateral currents
Fig. 7-18: Current amplifier using vacuum tubes
Fig. 7-19: AC current amplification with tubes
Fig. 7-20: AC current amplification with tubes that does not require a floating load
Fig. 7-21: Achieving current gain without power gain
Fig. 7-22: Input power “required” versus “absorbed”
Fig. 7-23: Typical power stage with voltage input and power output
Fig. 7-24: Cathode-driven Super Scaler basics
Fig. 7-25: Screen-driven Super Scaler basics
Fig. 7-26: Cascaded Super Scalers for higher boost ratios
Fig. 7-27: Power-grid tube used in Super Scaler
Fig. 7-28: 811A plate and grid curves
Fig. 7-29: 812A plate and grid curves
Fig. 7-30: 572B power-grid tube used as Super Scaler
Fig. 7-31: Methods of balancing hum in filamentary tube output stages
Fig. 7-32: SV572-160 Super Scaler
Fig. 7-33: Changing power output via output transformer tap selection
Fig. 7-34: Adding a line-matching transformer to the output to increase the number of effective taps
Fig. 7-35: Looking at the obvious and hidden voltage ratios of a typical output transformer to use as an input transformer
Fig. 7-36: “Step-able” boost ratio in cascaded Super Scaler
Fig. 7-37: BJT current relationships
Fig. 7-38: BJT common-emitter connection
Fig. 7-39: Changing the collector load changes the power gain
Fig. 7-40: Common-emitter power gain
Fig. 7-41: Adding matching transformers to the common-emitter stage to reduce power gain
Fig. 7-42: Emitter-follower impedance transformation
Fig. 7-43: Common-base power gain
Fig. 7-44: Adding an emitter resistance to control gain and raise input impedance of a common-base stage
Fig. 7-45: BJT-tube cascode used in Music Man amps
Fig. 7-46: Music Man power amp with op-amp front-end shown
Fig. 7-47: Open-loop UL with transformer splitter
Fig. 7-48: Emitter follower with 1:1 input transformer and 1:2 input transformer
Fig. 7-49: Mosfet source-follower output with transformer input
Fig. 8-1: Parallel tubes become a composite tube
Fig. 8-2: Parallel resistances across a voltage source
Fig. 8-3: Series resistances
Fig. 8-4: Tube transconductance
Fig. 8-5: Bipolar junction transistor transconductance
Fig. 8-6: Mosfet transconductance
Fig. 8-7: GmX amplifier using tubes
Fig. 8-8: GmX tube amp taken to an extreme
Fig. 8-9: GmX hybrid amp basics
Fig. 8-10: Increasing gm multiplication factor
Fig. 8-11: Practical hybrid GmX amp
Fig. 8-12: Waste heat and power sharing in the hybrid gmx output stage
Fig. 8-13: Cost benefit in other support windings and power supplies by using gmx approach
Fig. 8-15: Juggling gmx to reduce VSAT for better efficiency
Fig. 8-16: 200W from the 1650T
Fig. 8-17: Big power from the 1650W
Fig. 8-18: “Traditional” split-winding OT for high-power gmx amp
Fig. 8-19: Bridge-style gmx hybrid amp
Fig. 8-20: Multi-tier approach to the gmx output stage
Fig. 8-21: Intuitive gmx signal sample points that do not work
Fig. 8-22: Universal method for interfacing gmx circuitry to cathode-biased output tubes
Fig. 8-23: Adding gmx circuitry to individually cathode-biased output tubes
Fig. 8-24: Triode-pentode-ultralinear impact on gmx
Fig. 8-25: SE amp with gmx
Fig. 8-26: 1W-25W SE gmx amplifier
Fig. 8-27: Intuitive tube gmx
Fig. 8-29: A gmx possibility that isn’t: this is a hybrid amp chain
Fig. 9-1: Single-section pots used in ground-referenced and floating circuits
Fig. 9-2: Multi-section pot applications
Fig. 9-3: DJ source-selection pan-pot
Fig. 9-4: Left-to-right panning of a signal in a mixing board
Fig. 9-5: Passive balance control for stereo signals using single-section pot
Fig. 9-6: Reverb blend and mix controls
Fig. 9-7: Effects loop panning controls
Fig. 9-8: Selection of direct controls using relays
Fig. 9-9: Indirect signal control via remote foot-switch and jfet interface
Fig. 9-10: LM3080 transconductance op-amp
Fig. 9-11: LM13600 transconductance op-amp with linearizing diodes
Fig. 9-12: Transconductance op-amp wired as a voltage-controlled resistance to ground
Fig. 9-13: Current and device conditions in the transconductance op-amp configured as a ground-referenced resistance
Fig. 9-14: Using multiple transconductance op-amps to alter the levels within a parallel effects loop while accommodating multiple-channel panel controls
Fig. 9-15: LAB compressor circuit
Fig. 9-16: Direct-current electronically controlled resistance
Fig. 9-17: Using a jfet to vary the effective resistance
Fig. 9-18: Using a transconductance op-amp to vary the larger electronic resistance circuit
Fig. 9-19: Mounting the semiconductor case
Fig. 9-20: Discrete transconductance op-amp to handle very large signals
Fig. 9-21: Mosfet conduction characteristics
Fig. 9-22: Bilateral switch current and voltage limits
Fig. 9-23: Bilateral switch turned into a linear resistive element
Fig. 9-24: Open-collector output accommodates voltage differences between equipment
Fig. 9-25: BJT switch as audio mute, and the limitations to signal control
Fig. 9-26: Parallel complementary BJTs as possible linear resistance
Fig. 9-27: Parallel complementary BJT open-collector circuit concept incorporating synchronous rectification to take advantage of collector voltage ratings
Fig. 9-28: Example output stage in fixed bias and cathode bias
Fig. 9-29: Transconductance versus control current for the 3080
Fig. 9-30: Composite electronic-RK circuit with amended values
Fig. 9-31: Protecting the output tubes from a zero-bias condition
Fig. 9-32: Controlling the bias supply in the swept cathode-bias to fixed-bias circuit
Fig. 10-1: Sinusoidal mains voltages
Fig. 10-2: Inductive and capacitive loading effects on power factor
Fig. 10-3: Effect of abrupt removal of load from power source
Fig. 10-4: Lightning and solar flares as electromagnetic noise sources picked up by the mains distribution system virtual antenna
Fig. 10-5: Spark gap as spike suppressor
Fig. 10-6: Capacitive spike suppression
Fig. 10-7: Varistors as non-ideal spike suppressors with unpredictable life expectancy
Fig. 10-8: LC and RC filters
Fig. 10-9: Line-balancing transformer
Fig. 10-10: Steady vs. peak currents and fuse ratings
Fig. 10-11: Tube heater start-up and operating currents
Fig. 10-12: In-rush current to charge main filter capacitor
Fig. 10-13: Thermistor characteristics
Fig. 10-14: Thyristor power control approach
Fig. 10-15: Bilateral mosfet current-limiter approach
Fig. 10-16: Setting up the floating supply with automatic mains range detection
Fig. 10-17: Single mosfet in full bridge controls AC current
Fig. 10-18: Smooth conduction characteristics of tube rectifier
Fig. 10-19: Centre-tapped plate winding discontinuous conduction issue
Fig. 10-20: Full bridge with tubes
Fig. 10-21: Internal resistance of tube rectifier
Fig. 10-22: Mechanically securing rectifier tubes in position
Fig. 10-23: Semiconductor junction diode operation and conduction characteristics
Fig. 10-25: Leakage currents in solid-state diodes
Fig. 10-26: Cleaning up solid-state diode switching noise with inexpensive passive components
Fig. 10-27: Alternative rectification scheme with centre-tapped plate winding and solid-state diodes
Fig. 10-28: Cross-conduction failure mode when diodes are unloaded, and how to fix it
Fig. 10-29: Avoiding cross-conduction failure with full bridge
Fig. 10-30: Tube/solid-state switching in B-52 Stealth 100
Fig. 10-31: Three rectification modes using full-bridge supply and tube rectifier
Fig. 10-32: AC taps on primaries and secondaries
Fig. 10-33: Full-wave bridge over secondary makes taps look like centre-taps
Fig. 10-34: Conceptual explanation of tap behaviour
Fig. 10-36: Voltage rise problem with replacement of tube rectifier with solid-state diodes
Fig. 10-37: Resistive method for voltage restoration is fine with constant load currents
Fig. 10-38: Zener diode for voltage restoration
Fig. 10-39: Amplified zener and how to use it in cathode-biased amps and fixed-bias amps
Fig. 10-40: Buffered proportioning regulator used as voltage restorer
Fig. 10-41: Voltage clamping circuit
Fig. 10-42: Noise on the DC supply
Fig. 10-43: Inrush-limiting resistor does double duty as part of noise suppression filter
Fig. 10-44: Tying the chassis ground to the circuit ground
Fig. 10-45: Current limiting for high-voltage supplies as an independent circuit
Fig. 10-46: Foldback current-limiting characteristic
Fig. 10-47: BJT and mosfet safe operating area curves
Fig. 10-48: Noise coupling to heater winding
Fig. 10-49: Typical reference methods for heater supply
Fig. 10-50: Heater noise path within each tube
Fig. 10-51: DC-stand-off reference
Fig. 10-52: Simple DC heater supplies
Fig. 10-53: Simple method for viewing capacitor charge and discharge currents with oscilloscope
Fig. 10-54: RC filtering for heater supply
Fig. 10-55: Mix of AC and DC heaters in modern high-gain guitar amps
Fig. 10-56: Referencing options for DC heater supply
Fig. 10-57: Typical ways to wire 6V and 12V heaters on a DC supply, and the option of dual supplies that allows use of dissimilar power tube heater ratings
Fig. 10-58: London Power’s DC heater supply method for accommodating 6V and 12V heaters with dissimilar power tube heater ratings
Fig. 10-59: Typical voltage regulator circuits for heater supply
Fig. 10-60: Heater current regulator
Fig. 10-61: Series-string current-regulated heater supply with open-heater annunciation
Fig. 10-62: Current-balancing circuit for London Power’s 6/12V DC heater supply using current mirrors
Fig. 10-63: Alternative current monitoring approach to Fig. 10-62 using op-amps
Fig. 10-64: Power gain at every stage
Fig. 10-65: Ultra-simplified power amplifier concept
Fig. 10-66: Single-ended tube power amplifier
Fig. 10-67: Solid-state single-ended power amplifier variety
Fig. 10-68: Typical push-pull tube amp with plate-driven output transformer
Fig. 10-69: Obsolete solid-state push-pull form
Fig. 10-70: Common solid-state push-pull power amplifier forms
Fig. 10-71: Satisfying a class-A signal condition in a SE tube amplifier
Fig. 10-72: Satisfying a class-A signal condition in a push-pull tube amplifier
Fig. 10-73: Satisfying a class-A signal condition in solid-state SE amplifiers
Fig. 10-74: Satisfying a class-A signal condition in push-pull solid-state amplifiers
Fig. 10-75: Ideal class-B signal condition
Fig. 10-76: Class-AB signal conditions
Fig. 10-77: Cathode-bias methods for SE and push-pull
Fig. 10-78: Signal condition through the conduction transitions of a cathode-biased amp with shared bias resistor
Fig. 10-79: Trying to stabilize the bias point in cathode-biased amps
Fig. 10-80: Using a separate power supply between the grid and cathode to stabilize the signal bias condition
Fig. 10-81: Achieving a fixed-bias signal condition in a tube amp
Fig. 10-82: Amplified-zener as a “cathode bias” form
Fig. 10-83: Current source in place of cathode-bias resistor
Fig. 10-84: Power supply noise compared to supply noise that appears on the audio output
Fig. 10-85: Raw hum balancing of push-pull output stage and issues with UL stage
Fig. 10-86: Feedback loop in SE amp helps increase amp PSRR
Fig. 10-87: Push-pull power supply noise injection and balance points
Fig. 10-88: Improving performance of splitters with further decoupling and larger RC values
Fig. 11-1 Over-all format decision tree
Fig. 11-2: Active element decision tree
Fig. 11-3: Power amplifier topology decision tree
Fig. 11-4: Resistor selection decision tree
Fig. 11-5: Capacitor selection decision tree
Fig. 11-6: Wiring method decision tree
Fig. 11-7: Chassis decision-tree
Fig. 11-8: Adequate vs. Ample chassis thickness
Fig. 11-9: 1645 + 273CX amp
Fig. 11-10: 1650K + 272HX amp
Fig. 11-11: 1650K + 272JX amp
Fig. 11-12: 1639 + 275X amp
Fig. 11-13: 1650G + 273BX amp
Fig. 11-14: Filter cap differences for the 50W amplifier examples
Fig. 11-15: Six 6550 amp
Fig. 11-16: Four 6550 amp
Fig. 11-17: 811A amp
Fig. 12-1: Loudness compensation
Fig. 12-2: Typical amplifier chain
Fig. 12-3: Follower-type attenuator with added current limiting and DC protection for the speaker
Fig. 12-4: Nulling the speaker signal and thus reducing sound output to zero using two amplifiers
Fig. 12-5: Waste heat in the null-amp with zero null signal and thus zero attenuation
Fig. 12-6: Sine wave power versus square-wave power
Fig. 12-7: Power distribution and waste heat during various attenuations
Fig. 12-8: Carver commutating output stage
Fig. 12-9: Yorkville Sound commutating output stage
Fig. 12-10: London Power’s self-cascoding multi-tier output stage
Fig. 12-11: Null-amp output stage with optimized supply rails
Fig. 12-12: Tube amp load during attenuation
Fig. 12-13: Nominal speaker impedance vs. actual impedance curve vs. tube amp loading
Fig. 12-14: Adding EQing to provide tonal compensation to attenuated tone
Fig. 12-15: Original Super Scaler concept can go louder or quieter
Fig. 12-16: Non-master-volume amp driven clean then clipped
Fig. 12-17: MV amp driven to point of output clipping
Fig. 12-18: Complementary parallel BJT rmx circuit adapted to be high-voltage active load
Fig. 12-19: Mosfet bilateral switch modified to be an electronic-controlled resistance
Fig. 12-20: Mosfet control voltage reference shift at opposite cycle signal peaks
Fig. 12-21: Turning the active load circuit into an active attenuator
Fig. 12-22: Floating active attenuator
Fig. 12-23: Post-production technique used in recording studios
Fig. 12-24: “Embedded” effects sound using front-loaded effects, and the “split” amp with FX-loop
Fig. 12-25: Dry-wet multipath stage setup
Fig. 12-26: Live post-processing using zmx circuit or null-method
Fig. 12-27: So-called “transimpedance” attenuator

List of Tables

Table 3-1: Variable VS
Table 3-2: Variable VS & VA