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Re: SSTC Break-Through - 5MHz operation for Ion Tweeter system



Original poster: "sundog by way of Terry Fritz <teslalist-at-qwest-dot-net>" <sundog-at-timeship-dot-net>

Hi Dan,

   I'm am *very!* Impressed!!!    I agree that it's much harder to get 
solid state to run at that speed due to switching losses.  Now it may 
become clear why so many broadcasting stations still rely on large vacuum 
tubes to drive their signals.  Yes, some stations have switched to solid 
state, but usually they have banks of lower powered SS devices running in 
parallel.   Large power triodes (slowest of the Vacuum Tubes) still operate 
perfectly fine up to around 30mhz or so at full power, and usually you can 
operate them at even higher frequencies at reduced power levels. (IE, I 
have a 4kv, 500mA tube rated to 120mhz at full power.  Above 120mhz the 
plate voltage should be reduced to 2kV to reduce heating and stress on the 
plate and grid.)  Tetrodes and Pentodes effectively reduce the C between 
parts of the tube, allowing them to run at higher frequencies at full power 
than triodes can.

   I'm curious about how much drive power the SS requires. I know that with 
the higher frequencies you're still pushing the gate C.  What about 
parasitic oscillations?  Tubes tend to get freaked out when pushed above 
their frequency range, because of the internal capacitance of the various 
assemblies.  Those oscillations are practically impossible to snub.  Does 
SS suffer the same problems?  How bad is the Ozone production?

   How are you going to audio modulate the setup?  I'm not really sure that 
you'll hear a whole lot of difference between the solid state and vacuum 
tube setup.  So long as the plasma generator is run at a few MHZ (usually 
around 3-5mhz), and has ample drive power, the sound is mostly determined 
by the preamp, and amp stages.  Most plasma tweeters use tubes for the 
preamp because it's much easier to interface tubes to tubes (vs SS to 
tubes), and tubes produce those even order harmonics and much softer 
clipping than SS preamps.

   Make sure you have plenty of spare drive power for the plasma generator, 
and keep the audio signal well filtered and most importantly, well below 
the clipping level of any of the SS preamp/amp and it should sound good.  A 
well made Plasma Tweeter has startling clarity and quality, and a medium 
powered unit can produce ear-splitting volumes as well.  In conjunction to 
a well designed and balanced stereo, they can sound so good it brings tears 
to your eyes.


Shad


At 12:22 AM 8/6/2003, you wrote:
>Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><dhmccauley-at-spacecatlighting-dot-com>
>
>Tonight I made a big step towards a Hi-Fi solid state ion tweeter (aka audio
>modulated SSTC).  The key to creating a Hi-Fi ion tweeter is to run at a
>frequency of at least
>3MHz.   The corona formation of anything less than 3MHz can be typically
>considered "noisy" or "hissy", but once the 3MHz+ limit has been hit, corona
>discharge is practically silent.
>Typically tubes are used to easily get switching frequencies well over 12MHz
>for ion tweeters, but solid state has always been limited to low frequencies
>(<300kHz typically) due to the
>inherent complexities of switching a high voltage at relatively high current
>at high frequencies.  To my knowledge, the highest frequency obtained yet
>for a solid state ion tweeter
>has been by a group of doctorate students at Villanova University,
>Pennsylvania where their SSTCs were operating at 3MHz.
>
>However, my initial tests this evening have proven reliable operation at
>5MHz with some very quiet, continous corona.  I haven't audio modulated yet,
>but this is the first step.  My goal,
>however challenging, is to get operation up to about 10MHz switching 170VDC.
>At these frequencies, I think i can create a ion tweeter approaching Hi-Fi
>characteristics although probably
>never achieve the quality (at least in an audiophiles opinion) that tubes
>can produce.
>
>This is exciting news and I will be keeping the group posted.
>
>The Captain
>

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Shad (Sundog)
G-5 #1373
The Geek Group www.thegeekgroup-dot-org
"Ever stop to think, and forget to start again?"
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