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Easy Class D PWM exciter




 
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Author Topic: Easy Class D PWM exciter  (Read 931 times)
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km6sn
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« on: July 31, 2020, 01:02:39 PM »

Hi,
Please take a look at my build of a class D PWM exciter to
drive my 3-500 one-holer.

See attached files.

Rod

* build.pdf (969.57 KB - downloaded 188 times.)
* support-docs.pdf (141 KB - downloaded 112 times.)
* tech-note.pdf (329.83 KB - downloaded 104 times.)
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Steve - K4HX
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« Reply #1 on: July 31, 2020, 01:35:09 PM »

Very nice and well documented. Thanks!
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K5IIA
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« Reply #2 on: August 13, 2020, 02:32:53 AM »

Was it easy to obtain 100 percent modulation or a lil more with the carrier levels listed.   I noticed the 88 percent mod figure and just wanted to make sure that modulation levels at and above 100 percent were easy to obtain.  Did you happen to notice the max modulation headroom, or is it simply limited by how much voltage you feed it and have available?  Thanks and sorry for newbie question but i
Am 100 percent a tube guy and even though I am learning some here and there  my ss knowledge level is about that of a 6 year old during recess with a ton of sugar running threw him. 

73 and have a good day
K5iia
Brandon
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Be kind, for everyone you meet is fighting a hard battle.

73, Brandon K5iia
km6sn
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« Reply #3 on: August 14, 2020, 06:10:55 AM »

I have not acheived 100% modulation. That is offset by the ease of implementation.
I am getting reports of broadcast-quality audio and the 3-500 loafs along at 140W carrier.
Rod
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km6sn
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« Reply #4 on: August 14, 2020, 06:14:15 AM »

Brandon, the modulation % is determined by the characteristics of the stereo class D amplifier.
I run it at 24v, and it supplies  plenty of power to drive the 3-500 one-holer.

Rod
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WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


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« Reply #5 on: August 14, 2020, 09:46:57 PM »

Rod,

Did I not see it?
Seem to have not found the full schematic or a block diagram of the design?
Found the mic preamp schematic.

Btw, Parts Express sells a number of inexpensive higher and lower powered
stereo and mono Class D amps. For anyone reading this.

                 _-_-bear
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_-_- bear WB2GCR                   http://www.bearlabs.com
km6sn
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« Reply #6 on: August 14, 2020, 10:21:15 PM »

Hi Bear,
in the original post there are 3  attached docs, one of which shows the mic preamp and the RF deck.
I did not post a full schematic: however, if you look at the three docs you can see how it
is done.

Thanks for the tip on Parts Express: I will sure take a look.

Regards,

Rod
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km6sn
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« Reply #7 on: August 14, 2020, 10:30:11 PM »

Hi Bear,

The issue with selecting class D audio amps is finding one that will supply some undistorted power at a 12.5 ohm impedance level. They are all spec'd at 4 ohms, but that does not tell the tale at 12.5 ohms, a level I selected to
1. minimize losses, and
2. simplify construction.

Regards again,
Rod
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steve_qix
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Bap!


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« Reply #8 on: August 15, 2020, 02:22:51 PM »

Hmmmmmm.. I wonder why you aren't using an analog series modulator rather than class D ?  I say this, because, using this to drive a linear, the total efficiency is already low.  Dissipating a few more watts in the driver will not amount to much, and the audio will be perfect.

Anyway, just my 2 cents  Cheesy

Regards,  Steve
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High Power, Broadcast Audio and Low Cost?  Check out the class E web site at: http://www.classeradio.org
km6sn
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« Reply #9 on: August 15, 2020, 04:53:57 PM »

Steve,

good idea. I may switch. Analog modulation may
simplify things at this power level.

The project was originally driven by the desire to play
with class D modulation.

Rod
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