Cheap and easy PWM modulator for small AM transmitters

[redhat]

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

[Stretchyman]

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.

[redhat]

The aforementioned link would eliminate the need for a mod transformer, 24V boost onboard.

+-RH

[Stretchyman]

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.

[redhat]

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

[Stretchyman]

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.

[JimIO]

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).

[Stretchyman]

er... No thanks, it won't fit in my case!

[W9RAN]

Brilliant suggestion, Redhat - I've been thinking about evaluating these amps as well.

Has anyone found a source for the one linked on Amazon which is no longer available?    I've looked at a bazillion of 'em on eBay but haven't found one yet that has the built-in boost converter, which would be a real plus.

73, Bob W9RAN

[aurora]

The DROK site (Chinese) states they are still in stock:

https://www.droking.com/audio-amplifier/Audio-Amplifier-DC-12V-24V-Subwoofer-Amplifier-Board-TPA3116D2-150W-High-Power-Pure-Bass-Car-Amplifier-Module?limit=75

[W9RAN]

Duh, go to the mfgr's website, what a radical idea ;-)   Thanks!

Bob

[KaySeeks]

Question for anyone who has used the Droking amplifer modules:

They are meant to drive subwoofers, which implies an LPF is involved, perhaps on the front end. From the photo on the website and the reviews, it seems like it could be used as a "regular" audio amplifier (for frequencies greater than 50 Hz, for example). Is that what the tritone switch setting allows? I'm seeking regular low frequency (arbitrary low cutoff) to >5 KHz audio response.

Thanks in advance.

[redhat]

That's my understanding.  The pot adjusts the cutoff frequency in LPF mode.  Tritone should really be called fullrange (20-20,000).

+-RH

[KaySeeks]

Thanks RH.

Also, answering my own question: if you look under the red rectangle label item #4, you can see "LOW" and "FULL" positions in the PCB silkscreen.