Ldmos amp kits, any good?

there are a hand full of these solid state kit amps on ebay which use high end ldmos elements at their core. mostly in push pull configurations. some of them claim to provide up to 1.5kw! but for sota this kit seems very interesting, and the seller claims its designed to work on 12-13v.

i have no idea what amp design works well for ssb versus cw (ie class a vs, push pull etc) but i want to know : has anyone been able to make a useable amp out of something like this kit?

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Here is another similar board:

Somewhat cheaper (supply your own heat sink) - it’s an SMD board however, hence the reason at the moment it’s still sat on my workbench awaiting construction. I also bought a pre-assembled switchable LPF board. It lloks like the Israel supplied board would need some LPF filters on it’s output as well.


To be legal in the US, you would need to add output filtering for each band.

Look at the Hardrock 50 kit to see a good design that is legal.


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Hi Wunder,
The board I pointed to is switched LPF filters not just one.

Hardrock 50 is a good kit as are a few others out there but that filtering is so, so importnat (regulation or not, you don’t want to be radiating where you shouldn’t be - for one thing, it’s lost power!).


As others have pointed out, this is an assembled driver amplifier module and NOT an amplifier kit. This is only one piece of what is needed to assemble a complete amplifier.

73, Barry N1EU

That isn’t a LDMOS based amplifier. Typically these devices will operate on a 48V+ supply.

12-13.8V is normally too low for MOS based devices

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For the sake of accuracy, LDMOS (Laterally Diffused Metal-Oxide Semiconductor) is a type of high-power FET (Field Effect Transistor); examples include the BLF188XRS (1400W output). There are also VDMOS FET devices (Vertically Diffused) like the VRF2933 (600W output per pair). These are generally optimized for high-power and high linearity (broadcast ?) hence they need high supply voltage, not very suitable for portable equipment. They are generally +50V devices, even if +28V parts are not uncommon. Haven’t seen an LDMOS/VDMOS designed for +13.8V operation yet (even if I’m sure they exist).

What we need for portable operations are devices optimized for linearity at 12 … 13.8V so we keep optimal efficiency. The circuit in @KN4BEV 's link uses a pair of 30W 12.5V bipolar transistors, so the max you could expect is about 50W output. It is not an amplifier but just a amplifying circut, that will still need filters and control/protection just to be usable. The builder (4Z4RB) is quite known and experienced, even if the assembly quality looks poor (mostly because of the home-made PCBs).

The link provided by @DD5LP shows an amplifier that uses FET transistors not designed for RF but for switching, which means they are cheap but there are downsides: quickly degrading gain over ~15MHz, easily damaged, etc. You also just get the amplifying stage of an amplifier and will need filters, control, protection etc.

Something practical for SOTA would be this, which is a complete solution (amp, control, filter) that puts out 40+W at 12V and 100+W at 24V:


Or if you are looking just for the PA stage:


You can find them even cheaper on Aliexpress.

These use an undervolted +28V 120W part (MRF186). The case is lightweight, it’s very compact and because it the power RF part is designed for much higher power you can expect good ruggedness/reliability.

Razvan (M0HZH).


What a wealth of knowledge in the ham community. I never stop learning new and unexpected things.

thanks to @YO9IRF and @DD5LP for such detailed responses!

@dd5lp Have you had a chance to work on these items yet Ed? The reason I ask is I was looking for some kind of HF PA and am playing with the options.

Whilst it’s nice to build stuff yourself (even if it’s a kit) I’m very time poor. So it’s a toss up between an MXP50W amp ready built and ready to go or one of the 100W MOS kits Razvan mentions and the filter board you mention plus some time boxing it etc.

Hi Andy,
I never got around to starting to build the kit, so it was one of the prizes I supplied for the SOTA Dinner at Freidrichshafen earlier this year - I don’t know who got what as all were wrapped prizes. So I don’t know how good or otherwise it is.
If the attendee from the dinner who got the kit is reading this - have you built it yet and how does it do?

Now I have the X108G with 20 watts out, the urgency to replace the modified ramsey amplifier that I used with my FT817 has gone away. The next amplifier to make any real difference from the 20W out would be my spare (yet to be repaired) RMItaly HLA300 amplifier and the problem there is not just it’s weight but also the extra battery capacity needed to run it. So for now I’ll probably stick with the 20 watts out.

Good luck with the decision Andy - as you see, I was also time limited and simply never got around to building the 70W amplifier kit. I think you can buy it already built and add your own heatsink, LPF board and case by the way - that might be a quicker option?


My friend Karl (OE6LKG) has an MXP50W. It seems to work ok. We’re taking it out again on Saturday with a SOTABEAMS 4 band dipole, so no tuner required.

Hi Matthew,

FYI John VK6NU will be on as VI6PAX from VK6/SW-039 from 0500 to 0900 UTC - if you are out at that time please take a listen for him on 20 & 40m - I am also hoping to get out. Who knows maybe we’ll get a spurt of better conditions before the snows come? John will be mostly on CW with a little SSB.

73 Ed.

I have a MiniPA50 bought from AliExpress and it works amazingly well and is a fantastic compromise between weight and power at about 40w output on HF bands. It cost me a little over £100. Used with an FT-817 and MCHF, it only needs around 2.5w to drive fully.

Regards, Mark. M0NOM

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Unfortunately, I have just come down with laryngitis, so I have to stay home where it’s warm this weekend. I will try to get up early tomorrow and chase from home, which also has a reasonable antenna.

I have a minipa50 and agree that in some conditions (most at present) it adds enough db to make a difference and contacts are much easier especially on ssb.

The amp used in conjunction with a compressor makes the signal as good as a 100w rig. However, the increased RF around the setup can cause RF feedback problems in the compressor, which is somewhat counterproductive. I have not yet debugged this problem, I suspect the preamp in the compressor has enough gain up to 1 ghz that suppressing RF on its input is essential. The compressor chip itself may well be where the oscillations occur. One solution suggested by Gerald G4OIG is to swap the cables over, use the short one between the microphone and the compressor. I have bought a short shielded CAT6 cable to attempt some RF protection as an alternative to opening up the compressor and adding bits to it, but have not tested that yet.

Part of my problem may be my antenna system, I think the coax fed antenna does not cause this problem. But the multiband antenna is so convenient…

Andrew VK1DA/VK2UH

Anyone try these with a radio with QSK fast break CW?

Kent K9EZ

I haven’t tried that but i doubt if the timing requirements would be met for long relay life. The T/R switching is driven by the radio’s PTT line so you might get away with it at low speeds, depends on whether the radio has differential keying of the PTT vs key down.

The leading edge of the transmitted signal is the problem. It probably means the amplifier relay is hot-switching. Making it do that on every element rather than every word is going to be demanding for that input relay.
Andrew VK1DA/VK2UH

I have not looked inside. So I gather mechanical TR relays.

I wonder if the amps would pull off 160 Meters just using the 80 meter low pass.

I ordered an amp from him back to June, as of now (September 22), the amp has NOT YET BEEN SHIPPED! I paid almost $1900 and all I got is all kinds of EXCUSES to postpone.
He also refused to refund, I have filed a compliant to Paypal.
I know he is a famous builder, but to me he is a con artist.

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