Re: Successful SSTC Audio Modulation - SWEET!

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

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>

Basically your parts aren't heating up because I think you are running with
half-rectified input.  Average power is relatively low.
For the audio modulation, you'll need to have a DC rail on the full bridge
circuit.  This is where things get real interesting.  Basically, you'll need
to
get a nice big smoothing capacitor (i'm currently using 3500uF, 450VDC,
although smaller should work), and a bridge rectifier rated for the voltage
and the peak currents expected.  Running at full 50% duty cycle with no
deadtime even heats up my huge heatsinks with the IXFN24N100 FETs in a
matter
of seconds.  Running with the 25% deadtime between pulses really cuts down
on the average power and also allows for better audio modulation as well.

However, you can try for now just using a high impedance source such as a CD
player and connecting this to the deadtime comparator pin through a 1-10uF
capacitor (block DC).
Also, you can also put a 10k across the CD output just to provide some sort
of load for it.

Putting actual speaker output levels to the pin is a bit risky.  You need to
keep the levels down below a few volts, and also need to convert to a high
impedance otherwise the
amplifier will load the TL494 down.  10k resistors in series works in this
case.

Another interesting way, is to feed your audio signal directly into the
frequency determining pin of the TL494.  Again put a 10k resistor for a load
on the output, then series through a 1-10uF capacitor for DC blocking and
also through a 22k resistor to keep FM compression down.  This way works
well, and provides FM modulation.

Good luck!

Dan




 > Hey Dan,
 >
 > Question, Do you need the goofy deadtime settings????  Ive got lots of 50A
 > parts that dont heat up too easily, and im also building my next SSTC
right
 > now (its going to use a 494).  I would love to be able to crank some audio
 > into it if i only have to add a cap and lift a pin from the board!  That
 > would be really awesome!  Is that all that is necessary for audio modding?
 > I really want to do this.
 >
 > Thanks
 >
 > Steve Ward
 >
 >
 > >From: "Tesla list" <tesla-at-pupman-dot-com>
 > >To: tesla-at-pupman-dot-com
 > >Subject: Successful SSTC Audio Modulation - SWEET!
 > >Date: Thu, 29 May 2003 07:46:39 -0600
 > >
 > >Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>"
 > ><dhmccauley-at-spacecatlighting-dot-com>
 > >
 > >After a day of extreme frustration (yesterday gate drive problems) and a
 > >simple re-design of my gate drive topology, i finally got my SSTC to run
 > >from the push-pull outputs from the
 > >TL494.  So now, I have 25% on-side1, 25% deadtime, 25% on-side2, 25%
 > >deadtime.  This is required as operating in CW mode consumes mucho power,
so
 > >by running the coil
 > >this way, you can reduce total output power and improve reliability of
the
 > >h-bridge switch.
 > >
 > >Audio was provided by a CD player (high impedance output) and fed into
the
 > >deadtime comparator pin of the TL494 through an 0.1uF capacitor.
 > >
 > >Audio output was very clear and almost zero noise otherwise from other
 > >harmonics in the arcs.  Very impressive, similar to Sue Gaeta's high side
 > >modulation audio quality on her 811A
 > >VTTC.  Also, with fan cooling, the heatsinks and FETs never even really
get
 > >warm.
 > >
 > >One note to add, is that to improve audio quality, it is necessary to
 > >draw-out a continuous white flaming arc from the toroid.  This can be
done
 > >by any metallic object and permanently set at about
 > >a 2-3 inch gap depending on output power levels.  The result is stunning
 > >audio.  (at least stunning when considering its coming from a hot flaming
 > >multi-kilovolt arc ! ! !)
 > >
 > >Also, volume control can be provided by adjusting this discharge gap
 > >distance.  To get really loud audio, just try to stretch the arc as long
as
 > >possible.  I guess this provides much more heating of the air and
therefore
 > >more volume.
 > >
 > >Awesome!
 > >
 > >The Captain
 > >
 >
 >
 >
 >
 >