Re: audio modulated SSTC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Mitchell" <electrontube-at-sbcglobal-dot-net> 

Well I for one would say audio modulating an SSTC isn't worth it since you
only get mediocre sound quality.  Tube driven tweeters run in the 10s of
megahertz range.  Having built a tube driven plasma tweeter, I can tell you
that the coil that actually steps the voltage up is only 15 turns of 16awg
solid wire, so it's resonant frequency will be very high.  Yes they do use
grid modulation, but the tube is already running at around 30mhz to generate
the plasma flame, then you just modulate the signal to the coil.  I wonder
if there is such a thing that you could use to build a self resonant audio
modulated sstc by feeding the output of the comparator to another chip which
would also take the audio input.  That way, it could amplitute modulate the
input signal from the comparator, which I think would allow audio
modulation,  is there such a thing?  I think he was getting his info from
dan's site, www.spacecatlighting-dot-com...

Regards - Jim Mitchell
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, February 03, 2004 3:07 PM
Subject: Re: audio modulated SSTC


 > Original poster: Jan Wagner <jwagner-at-cc.hut.fi>
 >
 > Hi,
 >
 > On Tue, 3 Feb 2004, Tesla list wrote:
 >  > Original poster: herwig.roscher-at-gmx.de
 >  > Trying to improve quality and output power of my audio modulated
 >  > SSTC, I need to understand the theory of operation better. There are
 >  > two statements, I do not fully comprehend:
 >  >
 >  > 1. "For audio modulation, you must use push-pull mode *since audio
 >  > modulation requires CW operation*."
 >  >
 >  > Does "CW operation" mean, that both half waves need to have an
 >  > *equal* length? And, that pulse width modulation of a single ended
 >  > load will not lead to good-quality-audio-modulation?
 >
 > I don't know where you found these statements, but they appear
 > to be quite wrong. Firstly, you don't need push-pull at all - a simple
 > half- or full-bridge is sufficient. Second, whether the load is
 > single end tied or not, simply does not matter, for example compare to
 > power amps driving speakers: single ended or cross-over.
 >
 > Third, CW usually means "continuous wave" (no carrier
 > deviation/moedulation) e.g. radio amateur morse broadcasts, but with a
 > SSTC it is taken to mean "continuously powered", that is, you don't pulse
 > the 300VDC/whatever supply voltage (or blank out the SSTC driver) as for
 > example in 10ms ON then 10ms OFF, or similar (aka staccato, interrupter,
 > interruptor).
 >
 >  > 2. " Self-resonant systems adjust the driver frequency using the
 >  > detected maximum output power of the resonator."
 >
 > Actually they just keep the phase difference between drive signal
 > voltage and TC current draw at zero degrees. As a consequence, the
 > TC runs efficiently and at maximum power, though the control system
 > itself doesn't "detect" maximum power.
 >
 >  > Since audio modulated
 >  > circuits are being power modulated, the output power of the resonator
 >  > and the control signal to the driver will vary according to the audio
 >  > being fed into the TC system. Therefore audio modulation will not
 >  > work with self-resonant systems."
 >
 > Not really correct, either - up to date, the SSTC audio mod circuits up to
 > date use pulse width modulation and a fixed drive frequency, because this
 > is so trivial to build. The simplest self-resonant system doesn't use
 > PWM, but this does definitely NOT mean that audio modulation would not
 > work with self-resonant systems, too!
 >
 > Vacuum tube driven tweeters are (second?)grid modulated (AFAIK), similar
 > to if you modulated the supply rails of a self-resonant SSTC. So, there
 > you already have one way to do the audio modulation: modulate the supply
 > rails. You could add a class A/B prestage, class D prestage (buck
 > step-down or sepic), etc.
 >
 > Or, leave the rails like they are, and add a voltage dependend phase shift
 > (delay) to the feedback signal. Phase modulation accomplishes the same as
 > PWM modulation. Although with the TC load the result isn't linear enough
 > and will lead to audio compression, so you'll need to pre-compensate for
 > that or add an outer feedback loop (microphone? base current averaging?
 > etc)
 >
 > Phase modulation is bound to be much better than PWM modulation,
 > efficiency and EMI/EMC wise. I've some ideas on this but haven't yet found
 > the time to do real experiments, got too many other projects right now,
 > but hopefully sooner than later...
 >
 > cheers,
 >
 >   - Jan
 >
 > --
 > ****************************************************
 >   Helsinki University of Technology
 >   Dept. of Electrical and Communications Engineering
 >   http://www.hut.fi/~jwagner/ - jwagner-at-cc.hut.fi
 >
 >