Monday, 1 April 2019



The Elecraft KX3 is a beautiful radio. Fabulous ears!! But as a QRP Rig for outdoor use, it can't really be expected to be much use at home as a base station. Not unless you can add a power amplifier. The trouble is, there's not many options out there for the KX3.

The only real solution is to buy the 100W Elecraft KXPA100 which integrates seamlessly with the KX3, but it costs a staggering amount of money!  And that's for the basic amp. If you want to add the internal ATU (which you obviously would) then you're looking at almost £1700 with carriage October 2021. That's a whopping £17 per Watt.

😮 Staggering Cost New 😮

The obvious answer is to wait for some poor schmuck to buy new and later sell it used. That way you can get one for almost half that price. Still incredibly expensive I say, but thanks to a certain someone, I now have one sat in my shack (ready for when I upgrade my licence).

The ATU built into the KX3 is disabled when connected to the KXPA100. The amplifier's built-in ATU is almost as good as the KX3's but you gotta bear in mind that the KXPA100 ATU is dealing with around 10X the RF.

The amp is pretty compact in size and thankfully doesn't have a fan. Instead it relies on a huge heatsink atop the case. There are two Antenna Outputs (if you have the ATU installed) and these are switchable from the radio. The amp can also be tuned from the radio. In fact, you can pretty much hide the amp away if you want to (but why would you hide something that cost you so much? I feel more inclined to give it pride of place in the lounge!) 😂

As with all radio equipment, there's a tangled mass of cables go to and from the amp. Whoever comes up with some radio kit which only has one control lead will win a Nobel Prize!

With a Non-Elecraft transceiver, the integration disappears apart from the Yaesu FT-817 which has a dedicated interface cable available for it (for an eye-watering £56). All other radios will have to rely on RF-Sensing unless you make your own interface lead.


With my current QRP radio, ICOM IC-705, it's quite easy to make up a lead by building an RCA-to-TRS (Phono to 3.5mm stereo jack). I'll detail that later.

Like I said previously, the amp is phenomenally simple to use if connected to an Elecraft radio - the level of integration is just fabulous (apart from the cables) and with a third-party radio it's still very simple once you've set it up.

There's amazing levels of self-protection built into this amp too and I'm guessing you'd have to be a bit of a moron to do it any harm. That's so comforting for newcomers to the hobby who aren't experienced in these things. I'm guessing it's comforting to a few old hands too 😊

I contacted Harry Bloomberg (W3YJ) in Pennsylvania about using the KXPA100 with an IC-705 because I'd spotted on a forum that he was using this combination and I wondered if he'd interfaced the two units with a home-made lead. Harry very kindly wrote back and told me that he used the IC-705/KXPA100 without any interconnecting lead and simply relied on the amps RF sensing, saying that there was no need for a lead. Harry said that the KXPA100 was a very intelligent design and as long as you controlled antenna changes and tuning from the amp instead of the radio, all was fine!

Now that I'd got some assurance on the safety of using the IC-705 "bareback", I will give it a go this weekend. I'll insert an SWR/PWR meter and try working without an interface lead.

But I also want to try making a lead up. I'm thinking that it's better to let the amp know what's about to happen by making it aware that I've pressed the PTT rather than surprising it with 5W of RF and letting it react and adjust.

Is there an advantage?? I don't know but I feel that there is. Maybe a few milliseconds of signalling in advance from the IC-705 to the KXPA100 makes things more efficient or operationally safer 😐

From what I understand, a lead can be constructed quite simply by using an RCA (phono) at the amp end and a 3.5mm stereo plug at the other end. I just need to find out if I use the Tip and Sleeve or the Tip and Ring with the 705.

I'll report back.


Well it turns out that without any interface lead, the KXPA100 does NOTHING! Using the Elecraft Utility software, I could see that it was receiving RF from the IC-705 but was only outputting a couple of Watts.

It also wouldn't tune-up when I tapped the TUNE button on the amp and transmitted on the 705 in RTTY mode, 5W. This contradicted what Harry had told me. I started to wonder if there was something wrong with my KXPA or IC-705. I checked the KXPA100 firmware to make sure it was the latest version - it was.

I decided to power everything down and make an interface lead. I started off by just using the tip of a 3.5mm stereo jack plug. 

Using good quality connectors and shielded two-core cable, I made the lead with RCA (phono) at one end for the amp and a stereo TRS plug at the other for the radio. 

With this simple lead connecting the two units together (ALC-SEND to PA-KEY) everything worked a treat!! Straight away the radio tuned and using a dummy load, I was able to push out 100W using 5W from the radio. HAPPY DAYS!

Using the IC-705's internal battery and therefore showing a maximum output power of 50% (5W), the Amp will deliver over 100W. Here's what it outputs if you turn down the power on the IC-705...

A good setting for an Intermediate Licensee

NEVER feed the amp with more than 10W

These power figures were tested on 20M and 40M. Results may vary on other bands. While I'm very pleased with how this turned out, there's still the issue of having to manually switch to a constant carrier and back in order to tune, but it's a small price to pay until someone comes up with a proper interface.

It amazes me how all the big retailers who sell these amps, can't be bothered to come up with interfacing solutions - even if they're just simple diagrams on how to do it yourself. I'm equally amazed at how many ADVANCED operators can't figure this stuff out too. 
Elecraft KXPA100-AT & Icom IC-705

I had the same problem a year ago when I was trying to find out how to interface a Yaesu FT-817 with an ELAD DUO ART amp. The advertising blurb said it had an 817 interface built-in but there was no interface lead available and no information on how to make one! I spent an age trying to figure it out and eventually managed to make one
 (and I'm as thick as two short planks)!

UPDATE 2 : Harry has emailed me to say he had forgotten it needed a signal from the 705's SEND socket. He'd not used his amp for a while. 😊 Well at least he'd responded to my initial query and made the effort to try and help out a newbie. Thanks Harry.

73, Tom, M7MCQ.

Connecting Elecraft KXPA100 to ICOM IC-705
Can you use the KXPA100 with the IC-705
Interface lead for KXPA100 and IC-705

Sorry that you struggled with this. There are a number of explanation of this that have been posted in the forum going back to just after the 705 was released. Many of us just used 1/2 of an old stereo 3.5mm to dual RCA adapter.

Please edit your blog to indicate that you are using the SEND (tip) not the ALC (ring) connection. ALC has no function in this set up. In Elecraft lingo, the RCA connection is PA KEY.

If you still have the the KX3 to KXPA100 adapter cable, keep it! It can act as a key line "relay" to other equipment (SDR protector switch or a high power amp). I have successfully  daisy-chained my 705 to the KXPA100, then to a KPA500/KAT500 combo. I use that setup occasionally to step power from QRP to QRO and when I need extra power while on HF D-Star. This flexibility has made the 705 a very versatile back-up radio.

73, Jack - KD4IZ


Good work Tom.  Looking at your numbers I see that amp is going into compression above 70 W.  Doubling the input only gives 25% increase in output.  It's usually acceptable to have 1 dB compression so 80 W is as hard as I would push it on SSB.  For carrier modes like CW that are not audio derived 100 W could be run but the difference compared to 80 W is small and pushing the amp that hard is not imo a good idea.


Hi Tom,


Great on the mods.  But I have a Sepulvedaism “ which is “Everything is simple once you know the answer”.



Bill, K5LN



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