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Re: PWM Controller question

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, June 23, 2004 8:22 PM
Subject: Re: PWM Controller question

 > Original poster: Sue Gaeta <sgsparky-at-prodigy-dot-net>
 > ...and that was with a lousy modulation transformer!
 > I showed my project to a friend of mine who is a broadcast engineer, and a
 > high fidelity freak. He is also an amateur radio operator with a home
 > brew  high fi AM transmitter. Just for sh**s and grins, he connected his
 > modulator and power supply to my VTTC. The quality of the audio was
 > unbelievable!
 > The mod transformer that I have in there now is out of an old military
 > transmitter which is known for poor quality audio (an ART 13). If I wanted
 > good fidelity I would have to use a transformer that was 4 times as heavy,
 > and would not fit in my modulator deck! Good high side modulation makes
 > a very bulky, and heavy rig, but the results are indeed cool!
 > I have this crazy idea that bigger sparks will give better bass response,
 > so I would love to see Steve Ward give his interrupter a rest, turn down
 > the rail voltage half way, and high side modulate that sucker! People say
 > it can't be done, but it is done with 50, 60, 100, and 120 Hz all the
 > Finding a suitable mod transformer that can handle that much current, and
 > has the correct impedance match would be challanging, and maybe even
 > impossible though. Someone once suggested substituting the mod transformer
 > with an SCR or something. If someone could come up with a way of high side
 > modulation with active power devices in place of a modulation transformer
 > it would be great! Even if he settled with driver side modulation, I would
 > love to see what kind of audio comes out of a 10 foot arc! The nice thing
 > about high side, high fidelity modulation is that throbbing base will
 > produce long streamers if you are fully modulating (ie: the rail voltage
 > will peak at double the! supply voltage with no modulation, and then dip
 > 0 volts during an audio cycle).

Rather than using transformer plate modulation, why not use a DC supply that
is modulated.  There are several schemes for doing this, all of which would
be familiar to AM transmitter designers.  After all, what you are basically
building is a AM transmitter, no?

There are some nice IGBT based PWM DC supplies that have 20 kHz modulation
bandwidths around.  Of course, you quickly find that the design challenges
for the supply are more than for the RF PA.  I ran into this when designing
a SSB PA using Class E PA stages. The PA was easy, the power supply was not
(because essentially, you're trying to make a 3kW hifi audio amplifier)  In
any case, going to PWM means that the modulation transformer (that which
boosts the voltage up to match the high load impedance of your plate
modulated amp) gets much smaller.

What about something along the lines of the Harris digital broadcast amps.
Essentially a giant power DAC.

 > Sue
 > Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: "Eastern Voltage Research Corporation"
 >  > > I think Dan's PlasmaSonic design is good, but I could see some
 >  > > improvements in using (possibly) rail modulation. Real high power
 > plasma
 >  > > audio modulation will be seriously cool! I have seen it while arcing
 >  > > something "semi-grounded" to a AM radio station tower many years past
 >  > > (don't ask!). My grid modulated VTTC has been the closest I have seen
 >  > > yet, but quality was unreliable.
 > Absolutely. This *is* the way to do it, although high-side modulation is
 > more involved and requires a lot more
 > equipment.
 > Sue Gaeta's high-side audio modulated VTTC was stunning and definitely out
 > performed the PlasmaSonic as far as audio quality goes.
 > Dan