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 –