On a sunny day (22 May 2006 10:29:39 -0700) it happened "Telstar Electronics"
wrote in
.com:

Your statement about having the switching freq much greater then the
output freq I agree with. But if you wanted to make an amp for 27MHz...
what switching freq would you have? 2X, 5X, 10X ?\

Look at that example link, it uses 100kHZ (KILO Hertz) in the PWM AM modulator,
leaved the RF stage as is, but the PWM modulates it supply voltage.
that way the output.

In case of direct pwm to the RF, yes, I mentioned 1nS switching time,
could be done.
I like the digital idea better.....


Also, with a high
switching rate... with square-waves... you have tremendous harmonic
content. You also would require much higher frequency amplifers...
along with the quantum leap in complexity of combining all those
amplifiers,
no no, MOSFETS can switch that fast....

A/D conversion,

Any PC gives 16 bit audio, just some shift registers or some i2c io expanders,
no need for an AD converter, we are going to drive digital now :-)!!!

etc... I think your idea would work... it's
just not practical in my mind.

Remember the modulation frequency is in the audio range, at that speed you
have to change the PWM.
You need a pi filter lowpass in ANY case to get good harmonic surpression.

In the digital case (swithing 8 amplifiers), there are some more advantages.

Problem is adjusting the exact power level (so a ramp (sawtooth) waveform has
equal steps), adding (summing) the output takes inductors couplers...
impedance matching.
Hey, I have done that DA thing for up to 10 MHz or so....
the big boys can do it, why not we?

Probably will be worth a fortune on ebay 50 years from now.

"Lancer" wrote in message
m...
Telstar Electronics wrote:

JSF... you're right about those "big amps". They are nothing but
hay-wired prototypes, mostly designed and constructed by persons having
no electronic background. I have looked at many of them over the years
and have found that they have major flaws such as inadequate cooling,
improper tuning, and instabilities/oscillations caused by improper
layout and long wires running around all over inside. Because of these
issues, their owners must keep a steady stream of expensive output
transistors flowing through them... if you know what I mean... LOL

www.telstar-electronics.com

Kind of like building an amp and running it with class C bias?

You know what that means?

I think some of those Amps are getting close to class D now, they drive 250
Watt amps with 5 watts in to a 50 watt driver 10- 13 DB gain then a pair of
finals with 10-13 DB of gain, no bias on the base , I can just see the crap
the poor transistors go through. Then they mod their radios to give SWING
which does nothing but make the wattmeter swing like SSB but their no
louder, but I will say it helps keep the poor finals cooler.

In the new Commerical Broadcast AM transmitters their Audio is Class D audio
amps driving the finals saving allot of AC power.

"Jan Panteltje" wrote in message
...
On a sunny day (22 May 2006 07:47:53 -0700) it happened "Telstar
Electronics"
wrote in
.com:

Jan,

Sure you can run class D at 27MHz... the problem is distortion. Class D
is very efficient but is really only meant for digital signals.

www.telstar-electronics.com

Hi, you know class D audio amps exist I suppose?
There are even class D power amp chips for audio.
These use PWM (pulse width modulation).
You can make any output waveform with PWM (as long as the switching
frequency
is much higher then the output frequency).

This links shows some basics for a modulator (driving class D for
example):
http://www.netway.com/~stevec/ham/pd...lid_state.html

I can also think of a 'digital' solution, say you have 8 RF amps,
1W, 2W, 4W, 8W, 16W, 32W, 64W, and 128W,
now simply switching these on with an 8 bit digital audio signal (one on
each
bit) and summing the RF output together, will give you a 255W AM modulated
transmitter with less then 1/255 (0.4 percent) distortion (have not
tried)?
Since these RF amps can be class C (no need for A, AB, B, any linear!!)
you
should get good efficiency.
The stages are only 'on' or 'off'.

So, maybe someone could combine some :-)
Nice for experiments....
hehe



Class D audio amps are only designed for low frequency as they would sound
like crap for the higher freqs.

On a sunny day (Mon, 22 May 2006 19:02:24 -0500) it happened "Dr.Death"
wrote in :


Class D audio amps are only designed for low frequency as they would sound
like crap for the higher freqs.

Can we have some math please?
Or did you design the one for the full audio range?

On a sunny day (Mon, 22 May 2006 17:44:40 -0500) it happened "JSF"
wrote in :


In the new Commerical Broadcast AM transmitters their Audio is Class D audio
amps driving the finals saving allot of AC power.

Exactly, and no excessive distortion in the audio either.
I see no reason why it cannot be done for CB AM (at high power too).

Jan, please contact us when you have a working prototype.

www.telstar-electronics.com


Jan Panteltje wrote:
On a sunny day (Mon, 22 May 2006 17:44:40 -0500) it happened "JSF"
wrote in :


In the new Commerical Broadcast AM transmitters their Audio is Class D audio
amps driving the finals saving allot of AC power.

Exactly, and no excessive distortion in the audio either.
I see no reason why it cannot be done for CB AM (at high power too).

On a sunny day (23 May 2006 04:34:52 -0700) it happened "Telstar Electronics"
wrote in
.com:

Jan, please contact us when you have a working prototype.

Why do you not use the one from
http://www.netway.com/~stevec/ham/pd...lid_state.html

and pay the guy something for his design?
You can then use a FET (at the higher voltage) for the RF power stage.

I, myself have a Ranger RCI-2970, so am in no hurry to build something
atm.
But I would help you with the design for fun.
Dunno everything, but I did my thing to 500W in the past.

The reason of all this, is, that when I look at you power amp,
(and the Ranger also has this rather large 'cooling block' at the bottom),
those huge fins say: Waisting Power.
Any watt dissipated is a watt not transmitted.

I think I can do this thingy for AM, CW or course, and likely DSB..
For SSB things would be more complex, you need 2 DSB versions, and the
audio / RF 90 degrees phase shifts.

But it would be SSB class D + C :-)

Jan, I read the entire article by WA1Q1X. It's very interesting but
doesn't change my mind. One of the major problems that I see... and why
it could never replace external amplifier blocks such as our SkyWave
amplifier is that this digital system is high level modulated. This
does not lend itself to being added into an existing tranciever
system... like CB or Ham. This design is something that would have to
be incorporated inside the transmitter where a seperate audio signal is
available.

www.telstar-electronics.com

On a sunny day (23 May 2006 05:42:22 -0700) it happened "Telstar Electronics"
wrote in
.com:

Jan, I read the entire article by WA1Q1X. It's very interesting but
doesn't change my mind. One of the major problems that I see... and why
it could never replace external amplifier blocks such as our SkyWave
amplifier is that this digital system is high level modulated. This
does not lend itself to being added into an existing tranciever
system... like CB or Ham. This design is something that would have to
be incorporated inside the transmitter where a seperate audio signal is
available.

Agreed.
Your amp sure has a lot of merit on its own.

My remark was actually triggered somehow by yours about 'flying wires',
This picture of the RF stage: http://www.classeradio.com/2module.htm
and somebody elses suggestion to have more tech.... postings.

Regards
Jan

By all means keep tech posting. It's much better than the usual
bickering/name-calling that accounts for 95% of the posts here.

www.telstar-electronics.com