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Author Topic: Cheap and easy PWM modulator for small AM transmitters  (Read 944 times)

Offline redhat

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Cheap and easy PWM modulator for small AM transmitters
« on: February 26, 2018, 0434 UTC »
I've suspected this was possible before, but tried it today and verified my suspicions.  Buy a TPA3116 amp board off ebay or amazon (about $12).  Feed 24-28VDC into the amp board, apply audio, and take the L+ terminal to your RF amplifier.

Why it works;
These chips are PWM audio amplifiers.  In order to produce symmetrical output, at rest the output of the amplifier is one half the supply voltage, minus a diode drop or two.  Thus, with 25V of input, the output pins rest at around 12V.  This is perfect for a small grenade/lulu/etc type AM transmitter.  All the benefits of PWM with none of the hassle.

As a side benefit, the chip has built in thermal and over current protection.

Better yet, get this guy, as it has built in 12 to 24V boost  so no need for 24V power source.
https://www.amazon.com/dp/B0767JLR84/ref=twister_B079QY7999?_encoding=UTF8&psc=1

+-RH
« Last Edit: February 26, 2018, 0440 UTC by redhat »
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Offline Stretchyman

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Re: Cheap and easy PWM modulator for small AM transmitters
« Reply #1 on: February 26, 2018, 0708 UTC »
Yep I've been using these amps for a couple of years albeit with a mod tranny. They're a great design, cheap, efficient and compact, what more do you need!

There are many versions, even one with a balanced input and hard wired to PBTL (mono mode).

I use one that has a volume knob and 3.5mm audio connector on the PCB as it makes construction somewhat simpler.

Str.
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Offline redhat

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Re: Cheap and easy PWM modulator for small AM transmitters
« Reply #2 on: February 26, 2018, 0812 UTC »
The aforementioned link would eliminate the need for a mod transformer, 24V boost onboard.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Stretchyman

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Re: Cheap and easy PWM modulator for small AM transmitters
« Reply #3 on: February 26, 2018, 1128 UTC »
Mod tranny still needed to boost voltage to 48V! (from 24V i/p)

I can get 50W from the same design I used to get 10W from.

I designed a MiNi PWM for my Tx's ages ago but the design complexity and extra cost negated it entirely.

However For 10W carrier crowd this amp is ideal and may use it as such if anyone still wants ONLY 10W!

I harped on about PWM at this power level for ages and how many takers did I get?

ZERO!

I'm even getting a UK company to build a mod tranny just for the 40W design as it is the simplest way to get to a higher voltage (in my case 48V)

Sure I'm using PWM for a higher power design as a mod tranny would be silly, VERY silly indeed!

Good stuff RH, well spotted BTW.

Str.
'It's better to give than receive' so why RX when you can TX!

                            Buy one from me, NOW!

Great discounts on ALL my transmitters if purchased via HFUnderground


                                              ;)

Offline redhat

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Re: Cheap and easy PWM modulator for small AM transmitters
« Reply #4 on: February 26, 2018, 1324 UTC »
Very well, I was only considering the 20W and under crowd, hence small AM rigs.  Anything above that best done with purpose built stuff.  I suppose your could do the class d/e thing with a 4:1 output transformer and push pull, that would get you more power from low supply voltages.

Thank You!  :)

+-RH
« Last Edit: February 26, 2018, 1326 UTC by redhat »
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Offline Stretchyman

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Re: Cheap and easy PWM modulator for small AM transmitters
« Reply #5 on: March 10, 2018, 1747 UTC »
Must say I had to try it and of course it works well.

The quality of the A/F amp is a bit crap with heatsinks falling off and the somewhat useless front end (LPF for sub woofers).

I may trace out the circuit and mod it for 7KHz. Otherwise nice n simple!

Str.
'It's better to give than receive' so why RX when you can TX!

                            Buy one from me, NOW!

Great discounts on ALL my transmitters if purchased via HFUnderground


                                              ;)

Offline JimIO

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Re: Cheap and easy PWM modulator for small AM transmitters
« Reply #6 on: March 11, 2018, 2041 UTC »
Try one of these: 120-240 VAC or 12-24 VDC power, Bluetooth, FM radio, USB of Micro SD, RCA input. 50 Watt mono output. Volume, Bass and Treble. Remote Control.
For about $12 off ebay.

Note 1: Probably not 50 Watts RMS.

Note 2: The mains side of the SMPS is exposed through holes in the enclosure (ask me how I know).

Offline Stretchyman

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Re: Cheap and easy PWM modulator for small AM transmitters
« Reply #7 on: March 11, 2018, 2159 UTC »
er... No thanks, it won't fit in my case!
'It's better to give than receive' so why RX when you can TX!

                            Buy one from me, NOW!

Great discounts on ALL my transmitters if purchased via HFUnderground


                                              ;)