Class E Plasma Tweeter / Projects / Forums

Forums

4hv.org :: Forums :: Projects

« Previous topic | Next topic »

Class E Plasma Tweeter

Move Thread

LAN_403

Weston

Sat Mar 14 2015, 10:22PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

I have been working on a class E plasma tweeter system for the past year so I finally decided to share it. The system is designed to operate in the 6.78MHz ISM band to reduce interference.

The current iteration of the system is based around the STM32F332 microcontroller, which is designed for SMPS. It has high resolution PWM and high speed ADC. The STM32F334 generates the drive signals for a differential class E amplifier that drives the resonator and a class D stage that modulates the class E amplifier. Everything is controlled directly by the STM32F334.

Here is a photo of the completed system in operation:

The resonator is ~120 turns of 19AWG magnet wire on a 2" diameter teflon coil form. The primary winding is three turns of 14AWG wire. A screw for a breakout point is a must, otherwise the magnet wire will melt very quickly.

Currently the system is stable but needs force air cooling. It draws ~150W and audio quality is decent. I expect audio quality to improve as I update the modulator .

Here is a close up of the PCB. Both the Class E stage and the Class D modulator are on this PCB. The PCB is 4 layers. I also have a header for a small OLED display on the board. I hope to use it to eventually display real time operating parameters and power draw.

1426371509 1316 FT0 Img 20150311 015948 Lowrez

The class E stage is a differential configuration. This cancels some of the harmonics, reducing EMI, and also allows me to use smaller power transistors. This is mainly important in regards to gate capacitance as gate drive chips get really sad at the 6.78MHz operating frequency.

I am using ixdd609 gate drive ICs and they dissipate close to 1.5W each. Additionally, their internal delays are almost equal to 180 degrees at 6.78MHz. Here is a scope capture of the gate drive IC input (yellow) and output (blue).

The Class D modulator operates at 400KHz and is controlled by the STM32F334. It samples the audio input with its ADC and generates PWM drive signals proportionally to the audio input. I have the connections required for a feedback loop as well as overcurrent protection and am working on a controller for that. However, its a bit difficult as I only have around 100 clock cycles between ADC readings/PWM updates.

To Do's for the project are:
RF Cage for the Resonator
Laser cut fan mount
PID Modulator Firmware
Mount project on base

If you want to look at the code/PCB files the project is on github:

Dr. H.

Wed Mar 18 2015, 06:19AM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Hi guys,

I don't know why no one is commenting. One great project and beautiful design. A video would be nice

.

Keep up the good work.

Cheers

E.TexasTesla

Thu Mar 19 2015, 02:16AM

Registered Member #4362 Joined: Sat Jan 21 2012, 03:44AM
Location: Texas
Posts: 98

Looks very nice. I like the board and layout.
Could you explain why your running it at such a high frequency?
Is interference at a more sane speed (~1 mHz) a problem ?

Weston

Thu Mar 19 2015, 05:10AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

Thanks for the feedback. This is rev 4 of my solid state plasma tweeters after a while of playing with vacuum tube based ones.

The 6.78MHz operating frequency is two fold. A higher frequency means less secondary turns and a smaller coil, which is nice because I am lazy and live in a dorm.

More importantly is the fact that 6.78MHz is an ISM band. The coil can be a powerful RF emitter and having it in the ISM band allows it to interfere with less. For some reason a lot of class E tesla coils operate at 4MHz where they have the chance to interfere with a lot more.

To give an idea on how much RF the system can emit, I have a RF suppressor core between the plasma tweeter and my laptop as an audio source which gets hot to the touch.

I am taking a class on feedback systems right now that has a midterm project that starts after spring break. I am going to try to use my plasma tweeter for the project, so hopefully I will have a well compensated modulator with a flat frequency response sometime soon!

I will try to take a video sometime this weekend.

Mads Barnkob

Thu Mar 19 2015, 07:08PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

That turned out to be a neat little coil. Even though you mention the high frequency is because you want a smaller coil, it could yet be larger and still sit on top of your heat sink assembly.

One of the problems I have earlier encountered when doing audio modulation directly from a analogue output is that the device by time takes some damage.

I have killed a toy keyboard and a ipod shuffle from being connected to a audio modulated flyback. Do you have any additional protection besides the ferrite bead? That it gets warm sounds worrying to say the least :)

Weston

Mon Mar 30 2015, 06:22AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

And now the system is stereo!

1427696266 1316 FT169764 Img 20150330 004822

I mounted the two plasma tweeters on the acrylic plates a water jet for a previous version.

In stereo the audio quality is pretty good. However, I can hear a beat frequency between the two coils, and if I bring them close enough I can see plasma flickering to the beat frequency too. I need to synchronize the clocks between the two tweeters or enclose them individually in a shielded cage.

I am going to make a video of the system in the stereo configuration as soon as I figure out what songs will not get taken down by youtube. Can anyone recommend a test song?

Teodor Zafiroski

Mon Mar 30 2015, 01:45PM

Registered Member #48007 Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33

The Super Mario and Zelda themes are a popular choice for small coils, The Imperial March is also a popular choice, and it sounds fascinating played by a DRSSTC... Don't worry, songs played by you or your contraptions won't make you any problems...

My video of "Nothing Else Matters" played by my plasma speaker is sitting there for a year now...

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob