A Do It Yourself Plasma Tweeter
Because the plasma tweeter community seems to be quite small I write this page in English. The rest of my website is mainly in Dutch.
Contents:
First Experiment begin may 2005
This chapter is about the first steps I I'm setting on the road to a DIY plasma tweeter. Started may 2005.
In this stage I'm playing around with a function model, the first experiments to gain some experience with the technology.
Of course I still have much questions. I summarized them in a separate chapter
I would be very pleased to come into contact with other enthousiatics working or playing in this field. Please send email to hifi @ breem.nl (remove the spaces)
In this startup phase
I contacted a former colleague who has a long time
experience with amateur radio transmitters. His first reaction was: In building
transmitters we always put a lot of efford in preventing corona's. And he
is making music with it......
Top
Plasmatweeter.de Ulrich Hauman (german / partly english)
HardcoreAudio.de (german)
Roger Russel's page (english)
Electric Fluid Mechanics (english)
BaroneDiego (english)
Quick and dirty (english)
When you search the internet on terms like "Ionophone", "Plasma Tweeters" or "Plasma Loudspeakers" you will find electronic circuits like below, of course with some variations.
Very often the power
tube is an EL519 or one of its family members, a popular line-stage driver in
the latest (now late) tube-based generations of color TV's.
I did'n have one in stock so my first experiment used an 807, a well known
transmitter and audio tube.
Fig 1. The first circuit I've built and that worked.
L1 is a single layer coil of around 200 turns 0.3 mm lackered wire on a piece of 5/8" electricity tubing. According to the air coil calculator it is around 140 uH.
L2, often named the "Tesla Coill" has 20 turns of 1 mm daim. lackered wire on a thin plastic body of 45 mm diam. The length of the coil is around 20 mm The inductance should be some 20 uH
L3 also is in a single layer, has around 90 turns on a ceramic body of 9 mm diameter. According to the calculator 20 uH.
I used the chassis and power supply of an old mono tube amplifier I've built over 12 years ago, just for fun. It never played a role in my audio system, but now it came in handy.
The first working
circuit was in fact slightly different; I omitted R1 and C3, because I
thought they had no function. Also C2 was not present.
When I added a C2 of several nF the oscillation was killed. Probably L3 is to
small.
Later on a radio (transmitter) amateur told me that C3 may block DC from the ionized area. But I used a piece of wire with high voltage insulation at quite a distance from the flame, so that problem seems to be unlikely.
I do not yet have
reliable measurements of tube current and so, because my multimeter got wild in
the strong RF field of this unshielded setup. The multimeter even beeped
sometimes when it was switched off
Some pictures
The very improvised setup. You see L1 dangling from the anode connection. L2, sits on top, having the blue plastic can from some coffee sweetener.
The red wire ending in the white loop is the feedback electrode.
Here you see the tesla
coil more in close-up. Unfortunately the flame is somewhat hiding in the
background.
On the oscilloscope (yeh, some of these oldies still go strong) you see the
modulated RF signal picked up by the probe just left of the flame.
Here the corona discharge is very well visible. Unbelievable that this produces a substantial amount of a very convincing sound.
At first I used an ordiray sewing needle, but -as te be expected- the tip burned away very fast. A few days later I could lay hands on some pieces of 0.4 mm diam. tungsten wire which appeared to last much longer.
Here the flame is seen on the tip of the tungsten wire. The flame is about 5 mm high. Notice the small piece of the wire glowing red.
I measured near 70 dBa sound pressure level at 1 meter distance. This is nice, but to low for a practical sound system. Further work has to be done.
In the mean time I've obtained an EL519. I changed the circuit such that an EL84 is supplying the screen current for the final tube. An ECC83 provides the signal amplification in two stages. (schematics will follow) The choice of the tubes is just becaus I had them at hand.
The results: Wow!. The
flame now starts by itself and can be adjusted much higher. I measured 85 dBa
sound level on 1 meter distance for a continuous tone of some 6 kHz.
The tungsten wire does not appear to be the perfect solution. It burns away
quite rapidly when the flame gets white. A thick copper wire performed better. Explanation
is in the Q&A
It is also clear to me that I have to build the RF section into a very well EMI-tight case. The fieldstrength produced by this open setup is such that most of my measuring equipment gets wild, so I can't do any reliable measurements.
Yet another thing became clear to me: Intermodulation distorsion is a very serious issue. I played around with some dual tones of frequencies between 4 an 48 kHz and in most cases difference tones were clearly heard. The oscilloscope also clearly showed assymetric sinewave modulation. If it is so visible we're talking about over 10% IM.
Here I list questions which came up to me. I'll put answers along when I found them.
About the basic process of converting electrical (RF) energy into sound pressure
Q: Is this process linear? That is: if we have a perfect AM modulated signal without any intermodulation distorsion, will the produced sound also be without intermodulation distorsion?
Q: Does this conversion process have an upper (audio) frequency limit? Why?
Q: Will the maximum undistorted Sound Pressure Level increase proportional to the average RF power? Or will there be nasty things when the power is increased (besides Ozone)
Q: I've seen designs working with rather different RF frequencies. Do the physics of the conversion process indicate a preference ?
Q: What is the best material to use. (for this one I have an answer, but contradict me when I'm wrong)
A1: I'd guess Tungsten,
official name Wolfram, chemical symbol W, because it is the metal with the
highest known melting temperature of 3410 ºC. It is widely used for filaments
in incandescent lamps, including halogen lamps and automotive car lamps. You can
easily harvest some usefull pieces of tungsten wire from such lamps, even if
they are burnt out.
A2: After experimenting with the higher power provided by the EL519 it
turned out that the 0.4 mm diam tungsten wire burns away quite rapidly, while a
1.8 mm diam copper wire (2.5 mm2 installation wire) performed well
over a substantial time, produced a less luminous flame of larger size, and even
produced a higher sound level.
The explanation may be that copper has a thermal resistance which is 2.3 times
lower than tungsten. The heat entering the tip is much faster sinked through the
material keeping the very tip at a lower temperature. It also helps that the
copper wire I used was much thicker than the tungsten wire.
The ideal electrode is perhaps a copper body of several mm diameter,
ending in a shap tip of about 45 degrees top angle and maybe supported on a
heatsink. The tip might be covered by a thin layer of tungsten, but I don't know
how to do that. The heatsink may be quite large, but it should not impede the
Quality factor of the Tesla coil.
Q: What are other people's experiences with the life time of the electrodes?
Q: How sharp should the tip be? Will it be sharp enough when the wire is cut under an angle with an electrical wire cutter? Does the sharpness have an effect on the automatic starting of the corona?
About the Tesla coil
Q: What material to use for the body? Glass and ceramics (any ceramic?) will do probably the best. What about the many sorts of plastic?
A: A tip from a radio amateur: Put it in the microwave oven for some time. If gets hot it is wrong. (But do not use stainless steel, although that also does not get hot, hi hi)
Q: Where to buy suitable coilformers?
Q: Does it matter if the turns are adjacent, or is a spacing preferable? If so, does the spacing have to be very regular?
Q: What is the influence of the wire diameter. Does it make sense to use Litze ?
Q: The number
of turns largely determines the RF frequency. What other properties of the
system will change when you use different numbers?
A: Of course I can and will do some experiments, but input from experienced
people is always welcome.
Q: In a more final design I 'd like to see a separation capacitor at the foot of the coil. Touching the tip will never become comfortable I'd guess, but at least the dangerous DC voltage is blocked. Has anyone an opinion on this?
About the modulation technique
Q: In most tube based circuits screen (G2) modulation was used. Is this the best way? Probably it uses the fewest components, but I noticed quite some none-linearity at higher modulation depths.
More questions will
come.......
(.........when I find time to continue this project........)