Re: audio modulated SSTC

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

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

Maybe even a small pentode?  You could feed audio to grid two, and the
signal from the comparator to G1, that would probably work but you might
need a booster to amplify the voltage so it will be enough to turn the grids
on and off.

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
 >
 >