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Old 12th July 2004, 01:34 AM   #11
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here you go

http://gilmore.chem.northwestern.edu/ionovac1.gif
http://gilmore.chem.northwestern.edu/ionovac2.gif

I have a couple of pairs of these plus some spare parts.
Without the horn the efficiency is just unusable.
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Old 12th July 2004, 05:54 AM   #12
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Thanks, Kevin. These look similar to most of the DIY designs I've found, and it appears many commercial ones too.

The Hill speakers are unique in that they do not use a corona discharge, but a lower voltage helium plasma between electrodes. I came across this page:
http://members.cox.net/jayphil/plasma.html
The curves shown here look flatter and show lower frequency response than what I've seen for corona discharge tweeters. What I find most interesting is his claim that they do not use RF to pump the plasma, so that's why I was trying to find info on the driving circuit. I tried contacting the webmaster of that site, but he said he was busy with other stuff and didn't provide any info.

I wonder if this approach would work with air plasma, as messing with helium tanks doesn't look fun or cheap.
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Old 15th July 2004, 08:13 AM   #13
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Well, at least I found Hill's patent (thanks to someone at audioasylum). It's 4,219,705. Fascinating. PDF version can be retrieved using http://free.patentfetcher.com/
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Old 15th July 2004, 12:29 PM   #14
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I am curious what the DC bias voltage and the amplitude of the AC signal is in Hill's speakers. Obviously the AC low peak cannot be nearly as much as the DC bias, since the total voltage would drop too low on the low peaks and extinguish the plasma.

I tried an experiment: amplifier output through a 12V step-down transformer BACKWARDS to get a couple of hundred V, then biased the transformer output with rectified mains (340V). I hooked up a lit propane torch to +, and held the - electrode at the flame end. The low voltage means small gap, and with unsteady hands I was mostly getting noisy arcs. However, on occasion I was able to get sound for at most a second at a time. I was about to build a voltage multiplier ladder to try higher voltage, but the arcs are almost shorts and overheated the small transformer -- POP! and that was the end of my experiments. Also, my eyes are messed up from the UV emitted by the arcs.

It's probably hard to get a stable air (or in my case, mostly CO2) glow discharge at atmospheric pressure even with sufficient voltage and shaping of the gas flow, but my search on Google revealed that researchers have used microhollow cathode discharge (MHCD) to get stable plasma at these conditions, so that may be one way to improve performance instead of getting mostly arcs.

Edit: I forgot to high-pass the signal, which must have made thing less stable. Also, if anyone else should try this, the torch should be on low output as fast flow disturbs the discharge.
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Old 15th July 2004, 03:43 PM   #15
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I have come across these two papers on stable air plasmas using a microhollow cathode discharge as the cathode (i.e. the cathode is a discharge itself): "Direct Current Glow Discharges in Atmospheric Air" (IEEE Transactions on Plasma Science, Vol. 30, No. 1, February 2002) and "Formation of Large-volume, High-pressure Plasmas in Microhollow
Cathode Discharges" (Applied Physics Letters Vol. 82, No. 19, 12 May 2003).

I'm going to try to build the system described in the latter article, though drilling clean submillimeter holes in the copper/mica sandwitch doesn't seem easy. With this type of desing, it is possible to have large glow discharges by increasing voltage without transitioning to arcs.

The problems with using air instead of helium as in Hill's design are that it is probably more difficult to get a good response curve and linearity (I'm not sure if different gasses in air have differing thermal relaxation behavior), and the generation of O3 and especially NO2 (too bad it's NO2 instead of N2O, the latter being laughing gas ). The first problem can be dealt with circuit and mechanical desing. The thermal relaxation is affected by the cooling plates (see Hill's patent), for example. The second problem can be either dealt with a catalyst coated cage, or by using a natural gas flame (much cheaper than helium), or by reversing the fan in Hill's patent diagram to suck out the gases through a duct.

Any design, including Hill's (and even the corona discharge type tweeters), however, suffers from a serious problem that I don't see any way to avoid: UV emission. There's no UV shield I can think of that won't affect the sound. Even when horn loaded this is a problem, since you have to stand in front of the horn.
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Old 18th July 2004, 05:42 PM   #16
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I bought a 2.1kV 0.7A MOT from eBay; when I'll get it I'll post more results.
BTW there's a photo of the PlasmaSonic ion headphones:
http://www.audioasylum.com/cgi/m.pl?...t=&r=&session=
These are also corona discharge, but it would suggest that full range glow discharge drivers are workable for headphones, and could probably put the Stax OmegaII to shame
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Old 28th July 2004, 04:10 AM   #17
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I got a 1.5 kW MOT; one end of the HV winding is connected to the transformer case. Would there be any problems with disconnecting that end so I can use a bridge rectifier?
Also, where do I buy 3 kV rectifiers and capacitors? I know I can put regular ones in series but that's not as good due to uneven distribution of the voltage, and using resistor dividers as some people do to fix that results in a lot of components and a mess.
The heater winding puts out about 3 V RMS. I'm wondering if I use a full wave doubler to get 6 V and rectify that, if it can work as heater supply for tubes. I don't know how to figure out how much current it can supply (the winding only has about three turns).
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Old 28th July 2004, 04:43 AM   #18
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Prunce, interesting journey. Thanks.

Is the use of helium rather than air possibly also to prevent electrode burn out? Could something like argon be profitably used? Since it's heavy, one could essentially curtain the electrodes from above. I think the tradeoff might be the energy necessary to make the gas jiggle because of its higher density.
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Old 28th July 2004, 09:35 AM   #19
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Well, all the corona-type ionic tweeters have an electrode in air at similar temperatures. Electrodes shold be tungsten or something similar. However, one of my thoughts is to use C02, that is, burning natural gas (butane or propane). Fully burned natural gas shouldn't contain but trace amounts of any oxidizers. That has another use, too, since the injected gas needs to be heated (see Hill's patent). The amount of natural gas used would still make this much cheaper than helium, especially if plumbed in (I'd have to run a line from the kitchen though... )
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Old 6th September 2004, 08:33 AM   #20
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Stable DC glow discharge, 3 kV at 100 mA:
http://diyaudio.com/forums/showthrea...140#post469140
I'll try full power (2 x 200 mA) when I get the rest of the filter capacitors and chokes. MHCD not shown in the pictures, just plain electrodes, but I did try it with good results.
Now I need to figure out what sort of amplifier will drive this...
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