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Re: PWM Controller question

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

Oddly, we were discussing just this at lunch today, in connection with 3885
You could build one of the Class E amps a'la Rutledge, then audio modulate
the DC power supply.
We discussed various techniques for doing plate modulation w/o transformers
(or with physically small transformers), particularly for doing SSB.
PDM rigs or PWM rigs?  Pulse Duration Modulation, I suspect.
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, June 24, 2004 5:52 PM
Subject: Re: PWM Controller question

 > Original poster: Sue Gaeta <sgsparky-at-prodigy-dot-net>
 > Jim,
 > Several years ago, a new breed of AMers started to show up on the 75 meter
 > ham band with "PDM rigs" that use Mosfets in their designs. They really
 > sound great, and I know they aren't modulating them with mod.
 > but to be totally honest here, I haven't done enough research to find out
 > exactly what they are doing. If they sound that good, using transformers
 > modulate is not only bulky, and inconvinient, but absurd! The only good
 > thing about them is that you get complete isolation, and the circuitry is
 > less complex. However, I don't think complexity is an issue for those guys
 > that are pioneering this whole SSTC (and it's varients) movement.
 > I am a follower rather than a leader. Show me a schematic and I will build
 > it! Although my friend has built many transmitters with vacuum tubes, and
 > is perfectly capable of installing and operating the most modern
 > transmitter equipment all the way up to 100KW, he has never delved into
 > inner workings of those solid state transmitters, so I can only get vacuum
 > tube help from him!
 > Sue
 > Tesla list <tesla-at-pupman-dot-com> wrote:
 > Original poster: "Jim Lux"
 > ----- Original Message -----
 > From: "Tesla list"
 > To:
 > Sent: Wednesday, June 23, 2004 8:22 PM
 > Subject: Re: PWM Controller question
 >  > Original poster: Sue Gaeta
 >  >
 >  > ...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
 > &g!
 > t; good fidelity I would have to use a transformer that was 4 times as
 >  > and would not fit in my modulator deck! Good high side modulation makes
 > for
 >  > a very bulky, and heavy rig, but the results are indeed cool!
 >  >
 >  > I have this crazy idea that bigger sparks will give better bass
 >  > so I would love to see Steve Ward give his interrupter a rest, turn
 >  > the rail voltage half way, and high side modulate that sucker! People
 >  > it can't be done, but it is done with 50, 60, 100, and 120 Hz all the
 > time.
 >  > Finding a suitable mod transformer that can handle that much current,
 >  > has the correct impedance match would be challanging, and maybe even
 >  > impossible though. Someone once suggested substituting the mod
 >  > with an SCR or something. If someone could come up with a way of high
 >  > modulation with active power devices in place of a modulation
 >  > it wou! ld be great! Even if he settled with driver side modulation, I
 >  > love to see what kind of audio comes out of a 10 foot arc! The nice
 >  > about high side, high fidelity modulation is that throbbing base will
 >  > produce long streamers if you are fully modulating (ie: the rail
 >  > will peak at double the! supply voltage with no modulation, and then
 > to
 >  > 0 volts during an audio cycle).
 > Rather than using transformer plate modulation, why not use a DC supply
 > 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
 > (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 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
 >  > > > something "semi-grounded" to a AM radio station tower many years
 >  > > > (don't ask!). My grid modulated VTTC has been the clo! sest I have
 >  > > > yet, but quality was unreliable.
 >  >
 >  > Absolutely. This *is* the way to do it, although high-side modulation
 >  > more involved and requires a lot more
 >  > equipment.
 >  >
 >  > Sue Gaeta's high-side audio modulated VTTC was stunning and definitely
 >  > performed the PlasmaSonic as far as audio quality goes.
 >  >
 >  > Dan
 >  >
 >  >
 >  >
 >  >