MEP EP #145: The Vox in a Box Roxs

Podcast Notes

• Stephen’s Vox in a Box
  
  • Preamp Design
    • Vox AC30 preamp – 3 gain stages with a cathode follower and a phase inverter
    • Phase inverter – create differential signals for the power amp input
    • Drop signal using voltage dividers and mosfet buffers
    • 320V phase inverter to 18V buffer drive
    • Built point to point on an aluminum plate for easy modification and testing
  • Power Amp Design
    • TPA3116 Class D amplifier
    • LM311 comparator mute circuit for start up mute ~5 to 10 Seconds
    • Runs at 400KHz – Left open pads for boosting this up to 1.2MHz
      • Is there an advantage at running at higher frequencies?
  • Power Supply Design
    • TPS40210 Switching controller into a diode/cap ladder voltage multiplier
    • 7 stages of multiplication leads to ~50V per stage
    • Variable Switching from 35KHz to 1000KHz
    • PMP8621 Appnote
  • Testing the Vox in a Box
    • Been going nonstop for weeks now
    • Have taken it up to nearly 60 Watts without problems
  • Layout of the PCB
    • Multiple ground system with master start
    • Each sub circuit has its own ground return
    • 8 Master star ground returns
    • 4 layers
  • Issues with the current design
    • High Voltage SMPS does not like high capacitive load
    • Ground for signal input is very important – Nearly all noise is dominated by this single point
      • Alligator clips for signal patching are not low noise…
    • Cold Heater impedance
  • Current Schematic for the Vox in a Box
  • Fusion 360
    • New amp design
      • Learning by building a virtual amp
    • Vox in a box
      • Going to model the whole design to ensure proper fit
• R.F.O.
  • Improve your circuit toolbox – Simple designs that will save your next product
    • Chris Gammel gave a talk at the recent SuperCon about circuits you should add to your mental filing cabinet
    • Speaking of the Super Con all the videos for the talks are live on Youtube so go give those a listen to!

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