Hi

Over the years, I've built many AM rigs - from the early valved jobs with drifty VFOs, through crystal-controlled valve jobs, to crystal-controlled transistor efforts, then synthesised (using standard crystals) semiconductor rigs using big power FETs. I've tried all sorts of modulation methods - Heising (choke) modulation, the traditional mod transformer method, screen-grid modulation (yuk!), Doherty modulation, Ampliphase and finally, PDM Outphasing.

Practically, the old fashioned Heising Modulation was the easiest to get going. It's (relatively) inefficient, and makes plenty of heat, but the quality is hard to beat, and it's easy to align. My earliest semiconductor modulators were transistor amplifier circuits, adapted for modulation purposes. A little later, I used an op-amp driving a power transistor, which was convenient (and easy to introduce negative feedback), but this still used quite a lot of parts...

In the early 80s, Philips (and other manufacturers) introduced audio power amplifier ICs. These were (effectively) power op-amps, and many of them could be pressed into service to give a DC supply voltage that was "waggled" by the applied audio, and could be used to power the PA stage of an AM rig.

Zozo's recent little synthesised AM board uses this approach, and he chose the TDA2030 for his modulator. I've used these ICs myself, and they can sound pretty good in this context.

However, I think that there's a (slightly) better option that's often cheaper - the LM1875. This device - originally a National Semi product - has (slightly) lower distortion, will output over 4 Amps (with plenty of heatsinking) and can be run at up to 60V supply (absolute maximum). I've used them at ~40V supply to drive a rig to over 110 Watts peak, and they can be parallelled for even more "grunt".

Our old pal "Jack Donio" at Radio Morningstar in the Netherlands came up with a simple addition to the basic IC modulator, providing compression and indication of modulation levels (on a couple of LEDs). I took his idea and added it to my LM1875 modulator (see below). It's a very cost-effective way of getting control of the modulation levels, and there are no exotic components involved:

Very nice share Albert, I can confirm I do indeed use Heising Modulation on my current Valve TX and the audio really does sound superb!
I have already made a start with laying out your design here onto PCB, I think I will make the Modulator as a stand-alone board allowing the use of different RF sections of ones own personal choice.

Also I have decided to edge place the LM1875 on the PCB, this should make it easier to just bolt it onto the side of a rig chassis or another type of heat sinking.

Be interesting to see whether a Class-H amplifier could be pushed into service, something like the TDA1562 (though as this is a BTL amplifier, you could only use one half which would limit its usefulness).

The idea is that you charge a capacitor using the output of the audio amplifier and use this to boost the supply voltage when needed, so in effect you can get a nearly 24V supply from an initial 12V supply. The problem is that if the waveform is >50% modulated for significant periods, there is little opportunity for the capacitor to charge.

It might be just as easy to use a 12V -> 24V boost converter to increase the power output... in which case you could then use a Class-G modulator to reduce dissipation.

Experience shows that most of the voltage multiplier devices don't really have enough current output to drive the modulator enough. I found that the supply needs to be able to handle big pulses of current without dipping - which is why lead-acid batteries were really good for powering semiconductor-based AM rigs.

One of my most successful (and longest-lasting) AM installs used a "24V" truck battery for the supply. In reality, the terminal voltage when fully charged was 27.6V, and the battery had so much capacity that - even after several hours of broadcasting - the supply only fell by about 0.4V. The rig was running around 20 Watts carrier / 80 Watts peak, and the power droop by the end of a day's broadcasting was less than a Watt.

When I got the link receiver and remote switching working reliably, we added a charging system that could operate entirely unattended. Despite the availability of mains power at the output site, I stuck with the battery because (at the time) I couldn't entirely eliminate the hum from the carrier if it was mains powered! The mains was switched by the remote access system, so that the battery was recharged when the station was off the air. We didn't really go much into the hours of darkness, because the other, much more powerful, distant stations would drown our little signal out!

The early link was on 202.7 MHz (using a 48MHz crystal, multiplied by 4 and then adding 10.7 MHz), and the link transmitter developed about ½W, allowing remote location of the playout reel-to-reel tape player (an old Philips one from the 60s). With quadruple-play (insanely thin) tape, running at 3¾" per second, we could get about 8½ hours of programmes at a load! We recorded our programmes during the week, and a Saturday and a Sunday tape was broadcast.

We ran our little AM station - without raids - for almost three years. After the second year, I managed to get a reliable FM rig working with sufficient power to be useful, and we ran on both MW and Band II for almost a year. We carried out a listener poll to see which they were listening to, and most were listening to the FM (mono) signal, so we discontinued the AM output, which I still regret! The AM rig was given away to a friend's station in a neighbouring country, and it was used for another couple of years until it was finally "lost" in a raid.

In subsequent years I returned to AM many times, and still have a certain affection for the medium despite its obvious flaws. These days, AM presents much more of a challenge than FM does. It seems that the authorities largely leave AM stations alone - unless they cause gross interference - and a move back to AM might be a good option for some broadcasters!

On one platform, couple guys are testing this ham desing with SI5351 and modulating it with cheap chinese TPA3255 amplifier.
They used T240 ferrite instead of Wurth premade coil, it started to warm up quite fast.


https://notebook.rfcorner.in/baby-qro-r ... ifier.html

I heard them on 11Mhz and 9Mhz, and audio was fine.

Around 30€ for TX, 10€ for one band LPF filter.

That's brilliant! I haven't ever tried SIC FETs - this looks like a good idea. I found a little "PWM" amplifier that the Chinese manufacturer claims can produce 200 Watts of audio. With little modification, I found that it would fully modulate a FET amplifier at 30 Watts carrier. I used a pair of 2SK413 FETs (because I had them) and a pair loaf along at that power level. I drove them with my little CMOS synthesiser, and the most expensive parts in the whole thing were the ferrite cores for the output filter! We're looking at ~100 Watt (peak) for about £25 including the modulator. It looks like some very cheap and simple AM is on the way!

Morning, guys did the math, they call this Indian Ebay Special.

https://www.ebay.com/itm/186395160709
Ready made lowpass bank 30€

S5351, Arduino nano and other VFO parts 10€

TPA3255
300/600 Jiggawatt PMPO amplifier board 17€
https://www.ebay.com/itm/356671780886

Ampboard, 20-30€, what pcb house you use and where you get parts.

Old Ericcson 48V telcom powersupply from dump, free.

Case, what you got.

Use rotary switch to control LPF bank, or get someone do add an simple LPF switching to the Arduino Nano code.

From around 1.6Mhz to 30Mhz, even to 50Mhz AM rig, I think it is cheap at that point?

With nice heatsink it runs happily 50W carrier and peaks around 175W.

Thats what I copied from diffrent group, translated and pasted here.

This amp board works also.

€20.04
https://a.aliexpress.com/_EuN14QG

I use the TPA3116D2 class D amps as a direct modulator for a 15W carrier Tx. However for real power HALF BRIDGE PWM is fine. PCB about an inch sq, pair of SiC FETs will mod a 200W carrier with 95% efficiency, fairly linear. Fairly hefty toroids needed to remove the 150K clock. Simple as..

Str.

Here's one lashed up. 18-70V in, Dc-Dc supplies 12V for this and the drivers in the PA, will mod 10A 200V with ease. Yes tried series mod which is class A and whilst linear is so inefficient its a bit silly as you need a heatsink the size if a house! Lots of cleverer ways too but 2 chips and 2 FETs is all you need!

https://www.tinytronics.nl/en/audio/amp ... -amplifier

Link to the 24V modulator.

Has level pot, i/p connector and DC and speaker connectors.

Can run in BTL .
Just connect one side to RF stage that needs modulating. Floats at 1/2 Vcc.

Theres loads of these designs using the same chip but this one is best as it has all the extra bits and a decent heatsink.

Str