The AM Forum

I have been using a ricebox and a 3-500 one-holer at 150W carrier, and have been quite pleased with the results. Only needs about 6W carrier drive.

To get the fidelity of PWM, I have been wanting to build Class D PWM for some time. Joe, W6ISJ, gave me a brilliant suggestion: use a Class D stereo amplifier as the modulator and power supply for the RF deck! (DUH, palm slaps forehead, why didn't I think of that?!!)

I had been playing with the idea of making a 50W carrier Class D rig, to run either barefoot or for driving the one-holer. After Joe's suggestion, I have decided to move ahead with it. I ordered
https://www.parts-express.com/tpa3116d2-2x50w-class-d-stereo-amplifier-board-with-volume-control--320-699

For ten bucks you get a complete PWM generator and modulator already built!! it uses a well documented TPA3116D2 IC. Has lots of built in fault protection and shutdown. Furthermore, it appears to be DC coupled throughout, with the attendant advantages.

Joe cautions run coax from the stereo amp to the RF deck, and filter the PWM products at the RF deck.

I will run the RF deck at 12.5 ohms and use a 30 v (Meanwell 28v switching power supply, cranked up a bit) to
run the rig. I may have to drop the RF deck impedance to 5.5 ohms (50/9) to get enough power out of
the stereo amp. Who knows at this point. I will keep you informed as things progress.

73, Rod

The TPA3116D2 is spec'd for 24V and will work at 28V but not 30V.
The heatsink on the model you show is way too small and even with the ones that run the full width will still need cooling if run flat out.

Also if you intend to run thus as a direct pwm modulator (you'll need a mono/pbtl version) you'll only have half the supply on the o/p so 12/14V so you'll be limited to a 10-15W carrier.

You can of course use the amp to drive a modulation transformer but that kinda defeats the purpose but does work well.

Been doing it for years as have members of the 'other' community.

Regards.

JB.

I've modulated a class E transmitter using a solid state audio driver, similar to the arrangement that you're suggesting. I used a 'modulation' transformer in between the RF deck and the audio driver. This transmitter was 50 watts carrier.

I also drove another class E transmitter that was 400 watts carrier using a higher powered solid state audio amp. I didn't use a 'modulation' transformer on this one. Instead I directly drove the RF deck via a modified Heising circuit.

Both designs sounded awesome! There are minor technical quirks to both, but overall the whole setup is very forgiving.

The topics are posted ad nauseam on this site.

Jon

** FWIW, I've settled my AM station design around a homebrew 15 watt tube transmitter with a one-hole 3-500 linear for about 125 watts carrier. I'm still building the amp. Feng-shui reigns.

Hi Jon...

I noticed that ......
I'm looking forward to your testing of that TX .... particularly the squarewave one.....  ;D


Wayne

Hey Wayne,

Seriously. 100 watts can be done for $5 and two FETs, while asleep at the soldering iron.

This amp is going to have a new 1957 Chevy appeal. Chrome shine, dim internal lighting, silent mega-fan, meters, and a remote LED head for measuring Ep, Ip, & Power.

When someone asks me what is radio, I want them to immediately fall to their knees as they step into my shack from the presence of Marconi angels singing hosanna. GE Compactrons be praised!

Now, I return you to your originally scheduled programming.

Jon