RE: audio modulated SSTC

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Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com> 


Herwig,

1.  Push-pull operation (as defined from the TL494) is NOT required for
audio modulation.  In the TL494 controller, push-pull mode has 50%
deadtime and maximum duty cycle of your output pulse is 25%.  The only
reason push-pull mode was
selected in my designs was to reduce total power by at least 50%
dissipated in the FETs.  In single-ended mode in the
TL494, you can run a maximum of approx. 50% duty cycle.  This will work
fine for audio modulation too, except heat dissipation will be twice
that of push-pull mode of the TL494.  (assuming 25% vs. 50% duty cycle)
The terms "push-pull" and "single-ended" are not general terms.  They
are terms which refer to their respective operations
of the TL494 controller itself.

2.  CW operation means that your RF envelope is basically CW at the
resonant frequency.  Basically, the input to your full-bridge or other
switching circuit is DC.  You are then chopping this DC at some chopping
frequency.  In non-CW SSTCs, you typically have a half-rectified AC
input which is chopped up.  The sharper rise times associated with this
type of AC RF envelope result in longer arcs, but the low frequency
(60Hz for half-rectified) hum in the arcs is too loud for audio
modulation.

Again, please don't confuse push-pull and single-ended mode to some sort
of general operational characteristics.  They are terms which are
defined by the TL494 only where Push-Pull results in 25% maximum duty
while Single-Ended results up to 50% maximum duty cycle.

3.  Yes, you may have problems with a self-resonant system and audio
modulation using existing schemes.  However, in my experience, a
self-resonant system is more a luxury than a requirement and may
actually hurt you in audio modulation.
I find that I need to tune my SSTC a little off maximum output to get
good audio modulation.

4.  Plasma Tweeters typically are tube-based and operate at frequencies
above 3MHz.  It has been proven that output
arcs above 3MHz exhibit almost zero "hissing" noise from the arc.  3MHz
is easy to obtain using a tube-based switching
circuit, however, in solid state, its much more difficult.  I have been
experimenting (although not lately) on a variety
of high frequency drive circuits at 3MHz+, but still have a ways to go
to get something working well enough to say its
a success.  Most of this work is actually going for high frequency DC-DC
converter design, but an offshoot is to use it
to develope a Hi-Fi type solid state audio modulation system switching
at least 5MHz.

Dan



List,

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?

2. " Self-resonant systems adjust the driver frequency using the
detected maximum output power of the resonator. 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."

Plasma tweeters use capacitive feedback mostly and  produce a very good
audio quality. However the operating frequency is much higher than in
our TC systems. And the output power is relatively small. Is the a.m.
statement wrong in principle?

Any answers and comments are very welcome.

Cheers,

Herwig