Re: Mini Audio modulated self resonant SSTC

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

Original poster: Finn Hammer <f-h-at-c.dk> 

Jim,

This is not going to work.

Using the antenna/comparator to time the oscillation, you only need one 
gate driver chip, TC4428 works well, to drive tha beefy gate driver from 
Aron/Justins site. This method (from antenna -> streamer) implys that your 
gate signal goes directly from positive to negative.

The audio modulation of the TL494 works this way: The gate signal goes 
positive for less than 180degrees, then to zero til 180 degrees. Then 
negative for less than 180 degrees and back on zero untill 360 electrical 
degrees at Fres. has passed. The output of the coil is determined by pulse 
width modulation, and the ratio btwn. pos./neg. (called on time) and zero 
(called dead time) is varied, so that when the dead time is large, the 
output is low.
To make this work, you need 2 driver chips, and the TL494 set for 
differential output.

Furthermore, with the TL494/antenna-comparator circuit, you have the option 
of using either of the 2. when you swich to antenna self tuning, the TL494 
fixed frequency circuit (with all it`s sofistication) is bypassed.

Modulate the rail! Particularly since you are shooting for low power.

But size the series element right:
It must be able to pass the full current, and block the full voltage. It is 
no problem getting a semiconductor that can do that, the IGBT`s of the 
bridge do that all the time. However it must also be able to dissipate the 
power that is set up in it, when it is biased at it`s operating point. 
That`s where it is "idling along" without any audio in, and should be able 
to both swing positive and negative by equal amounts. So half of the supply 
voltage is across the device.

At that time, it will have to dissipate 50% of the full power: full 
volts*full current.

I have forgotten how to justify this statement, but I am sure others will 
help me out.

So what you have to look for is an IGBT with the voltage rating of the 
supply, and a current rating much higher, by looking at the dissipation 
ability of the device.

We are not going to get both loud and fidel audio at the same time right 
yet, but imagine the jaws of the (hopefully) buying audience, if you start 
the presentation by playing the start of "An der schonen blauen Donau" 
That`s the starting melody of "Year 2001, A space odyssey" And it does like 
this:

da da da da daaa  bam bam   bam bam

And the "da`s" are audio modulated CW audio, whereas the "BAM`s" are short 
streamer producing bursts, of course delivered with the appropriate breakrate.

Boy do I love this hobby.

Cheers, Finn Hammer


Tesla list wrote:
>Original poster: "Jim Mitchell" <electrontube-at-sbcglobal-dot-net>
>Hello Group, and fellow SSTC'ers:
>Last night I built the miniature IGBT H-bridge and wound the gate drive 
>transformer for my MAMSRSSTC (Mini Audio Modulated Self Resonant SSTC 
>;-)))  Anyway, I am not doing true self resonant audio modulation, there 
>is going to be a TL494 on the input to the comparator via a switch, you 
>flick the switch on, and it overrides the feedback from the antenna, 
>allowing the TL494 to control the coil.  I will have the TL494 set up to 
>take in audio and have a variable duty cycle/frequency.  My question is, 
>how do I set the TL494 up for audio modulation?  It shows no schematics at 
>Richie's website...
>A crappy picture,
> 
><http://www.hot-streamer-dot-com/electrontube/DSC01221.jpg>www.hot-streamer-dot-com/electrontube/DSC01221.jpg 
>
>
>Regards - Jim Mitchell
>