Some More Sproutie MK II Videos

Since finishing The Sproutie MK II and publishing the blog-post on it a few weeks ago, I have been listening to it, winding an extra coil or two, and also attempting to tweak the active audio filters. Coverage of The Sproutie is now up to 18.3MHz, and while I know from previous experience that it will cover up to 30MHz, I am going to leave the upper limit where it is for the time being. Any new coils will most likely be wound for more limited coverage on specific bands under 18MHz. PS – I just spent part of the morning testing out the upper limit of the newest coil by listening to SSB on 17M, and it’s working great – quite stable too.

I had wanted to give the CW and SSB active audio filters more gain, to compensate for the fact that in those modes, the RF gain needs to be wound down to prevent oscillator pulling. Because the narrower filters, even if they have the same gain as the wider filters, give the perception of lower volume, I wanted to design them with higher gain to compensate. Currently, the narrowest filter, a 700Hz low-pass, has a gain of 20dB. I tried building 700Hz low-pass filters with gains of 46 and 34dB, but they both oscillated, putting out a square wave with 10V amplitude at a frequency of somewhere in the region of 100-150Hz. I made sure to keep the Q below 3 – in fact the highest Q stage in the 34dB filter was just a little over 2, but this didn’t help any. For the time being at least, I have decided to keep the current filters as they are. If you view the videos, you’ll see that The Sproutie does indeed work on SSB and CW. If receiving a weaker station for which the set could use a little more gain, plugging headphones in helps and at this point, it’s a compromise I’m willing to make. Trying to build the perfect regen is a rabbit hole from which it sometimes feels as if there’s no escape, so I decided to draw a line in the sand and leave things as they are.

Once again, I feel as if I should apologize for the quality and resolution of these videos. I just entered the 21st century a few months ago with the acquisition of my first smartphone, a first generation Moto G. It’s a budget model, so doesn’t have the best video. It is an improvement on the videos I used to post from my decidedly old Canon Powershot A80 though. The one thing the videos do achieve, I think , is to give you some kind of feel for what the receiver is like to operate. For detailed views, the still photos are the way to go.

This one shows how a regen, if you nudge it into gentle oscillation, can provide some carrier injection for reception of weak AM stations –

Here’s the 25M SW broadcast band –

And another video on the 40M amateur band on CW and SSB, with a special brief guest appearance by Jingles the blind kitty –


12 thoughts on “Some More Sproutie MK II Videos

  1. Wow ! What an impressive regen. I’m just getting started building my 1st regen. Using a circuit from EMRFD. I have a couple of National venier dials to use.

    Gene, W5DOR

    1. Thanks Gene. I saw those National dials on your site. Good luck with the regen build! By the way, someone posted a link to your site on the G-QRP mailing list the other day, citing it as an excellent source of links and info. I couldn’t agree more.

      73 for now,


    1. Thank you Roger. I stubbed my little toe quite badly about 10 days earlier and was making sure there was nothing broken or infected. Nothing serious – thanks for noticing,


  2. Dave, your work is exquisite as always! It appears that because of the superior reduction mechanism of the HRO dial, you really didn’t need the fine tuning. Have you found this to be generally true?

    1. Yes Cliff – in fact there are two controls that I could have left out, and wouldn’t have missed much. The fine tuning is one, and the “bass” switch the other. I do use the fine tuning on AM stations, to verify that I have indeed tuned the station on the nose. Every time, I gently rock it back and forth, only to find that I tuned it accurately using the main dial and drive, and then return it to the center position. On SSB and CW, it is completely unnecessary, as the regeneration pot acts as a fine tuning control. For background, I tend to display a fair amount of repetitive behavior, doing things like checking every room 3 times before leaving my apartment to ensure that everything is as I want it (it nearly always is, but I go ahead and check again anyway). I was pretty sure, before beginning construction, that I wouldn’t need these 2 controls, but I put them in anyway, “just in case”. This is in itself, a form of redundant behavior!

      The gentle effect of the bass switch is only really noticed when using headphones or earbuds, especially ones that are designed to accentuate the bass. Every time I have used it, I flip it to the “extra bass” position, think something like, “Oooh, that’s nice”, and flip it straight back. I have never left it on.

      Hope that helps,


  3. Dave, your work is inspiring! Thank you very much for sharing all this great projects with us. I have two questions… 1. Do you think the Sproutie and a tuned magnetic loop are a good combination and 2. do you think the substitution of the varco with varcaps (like in your wbr receiver) leads to worse performance? Thanks in advance and best wishes from germany.


    1. Mike – thank you for the compliment – it’s much appreciated. Regarding your questions, the answer to both of them is – I don’t know. Regarding the use of a tuned magnetic loop, even though the antenna is isolated from the detector by the RF preamp, I’d still want to do whatever I can to preserve that isolation. Therefore, with an indoor antenna, it might be a good idea to build your regen in a fully-shielded enclosure. Actually, that is never a bad idea.

      As for using a varicap, one of the problems I have seen reported is RF from the oscillator getting into the diode and modulating the frequency. The back to back diodes in the WBR get over this, as the effect is cancelled out. You might want to try looking at the “Solid State Radios” forum over at The Radio Board, and the Solid State Regenerative Receivers Group on Google+, as I’ve seen conversations on this topic in both groups. If my memory serves me correctly, QRP-Gaijin, who is in both groups, has experience of this, and so does Vasily Ivanenko in the G+ group, of which he is the founder.

      The use of a varicap may well reduce the Q of the tank circuit but with an FET detector, that many not be such a bad thing. The impedance of the FET, and the resultant Q of the tuned circuit, is so high that the nose selectivity can cause the audio to sound a bit restricted. A varicap may broaden out the nose of the selectvity curve a bit and improve the audio.

      73 for now,


  4. Thank you for sharing your work. That is a beautiful hand built machine. I’m currently trying to learn what I need to know to build like this. It is always inspiring to see.

    1. Thank you Josiah. I just got a camera with slightly better video capability and would like to make a better quality video at some point. Whether I will ever get around to it though, is a different matter! Hopefully, these videos at least give you a rough idea what this receiver is like.

      73 for now,


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