Ultimate3 LPF build


I decided to start with the low pass filters (LPFs) as they are relatively simple builds. Until I tipped the components out the packet I had no idea how minuscule these things actually were. The ferrite cores for the torroids are slightly bigger than the miniature Polo mints they did a few years ago. Having heard first hand how tricky winding transformers and the like could be this was rapidly becoming a task that wouldn’t feature highly on my bucket list.

I didn’t realise I’d bought 4 filter kits (10, 20, 30 & 40m flavours) by mistake but that was a bonus as my logic suggested if I made my first attempt on one for a wavelength I was unlikely to use, if I loused it up I could get a replacement at a later date, but would be on the money when I built the ones I needed. 

To effectively wind torroids you need hands the size of a seven year old, the finger dexterity of a concert pianist and the eyesight of a neurosurgeon. Having fingers that the Quality Controller at the Wall’s Sausage Factory would reject on the grounds of being “too porky” and eyes in need of a trip to SpecSavers meant this wasn’t going to be a cake walk. 

After much F’ing and Jeffing and wishing I’d got a jewellers tool kit with micro fine tweezers and ultra needle nosed pliers I admitted defeat, put what I had in my hands down, reached one foot to the left beneath the work bench and fished out my jewellers tool kit. The first torroid was completed in 5 minutes.

I was really happy with the finished product, beautifully spaced and so forth, until I tried to mount them on the PCB, when I discovered I’d wound them the wrong way round. Oh how I laughed as I unwound them (yes all three) and started again.

Above, How not to do it!

And the right way!

Learning points – 

1/ Start winding the torroids from the five o’clock position on the circumference, passing the wire into the core and work around in an anticlockwise direction. 

2/ Label them (the torroid cores before you start winding)! There’s only one turn of wire difference between them in places and it will make your life easier. 

3/ Hans’s construction notes give a good tip on boiling off the wire enamel with a blob of solder. It works a treat. Having tried scraping enamel off of wire with a pen knife and wet and dry paper as a kid when I made a crystal radio, the solder boiling technique wins hands down! To clean things up, removing the contaminated solder with a desoldering pump then re soldering the joint gave good peace of mind.

4/ The completed torroid isn’t that robust and soldering it to the PCB was tricky. It moved around without realising and I found that the windings had overlapped and were loose which isn’t what you want. I revisited it several times to ensure they were anchored as securely as possible while retaining the windings at the correct spacing.

5/ Label the completed filters. They’re like a little batch of clones and it’s easy to pick the wrong up in a hurry

Et voila! One 10m LPF


Having started work on the 20m filter today, things are much easier and you quickly get into a routine. To be honest, the more windings on the torroid, the easier as they are more substantial and don’t wiggle around as much.

One and a half down, two and a half to go!