Building an Audio Filter Part One

Fun With a Filter Project

Over the last few weekends I've been putting together an Elecraft AF1 Audio Filter. Ordinarily, I would do the sensible thing --which I suggest you do-- and just order the kit from Elecraft.  They only want $60 for it at the time of this writing, which is truly a bargain. I scratch built it, which took ten times longer to complete and it is not nearly as pretty.

But, I have more parts than sense and I had everything I needed in the junk box. So, with some judicious substitution, I spent several blissful weekends puttering around the shop. Here are the results so far.

Starting Out

Here are my tools of the trade that I used to put the circuit together:

You'll see on the right is my solder station with the ubiquitous Radio Shack Tip Cleaner/Tinner. There's also a big blue glob of FunTak on top of the iron stand that I use to hold tricky components in place while I solder them.

Next comes the usual stuff, i.e nippers, needle nosed pliers, a hemostat, fine tip permanent marker, super glue, screwdriver, a big stash of circuit board pieces and pair of really strong reading glasses.  The glasses are a must for eyes that are 40+ years old.  There are also lots and lots of strong lights around which helps.

Colored Pencils Aren't Just for Kids

One of the things missing from the picture is my green colored pencil.  Every time I place a part on the board, I mark it on the schematic with a green colored pencil. It saves tons of time time later looking for missing wires and unconnected pins.

Marking my way to success!

Ugly Manhattan-ish Construction

I use a combination building method that is part Manhattan-style and part ugly for a result that is all ugly.  By the time the case is finished, no-one will be able to see the depths of ugly that is my completed circuit board. BUT, it worked the first time I turned it on.

You'll probably notice that I've made all sorts of marks on the board itself, notating nice things like tip and ring for stereo jacks and where pads connect to important things like Vcc and off board components. By the way, those jacks are epoxied to the board.  I have used that technique many times before and it's a great way to use through hole parts with ugly construction. My construction style also turns dead bugs upside down.  In  other words, before gluing IC's down to the circuit board, I bend the legs UP so that the part number can be seen and the pin numbering makes better sense to me.  It's a small point, but I think it makes things go faster. It certainly helps with troubleshooting as well.

The Semi-Result

Here is the front view above and the rear view on the right.  I have a very nice pine case to hold the filter. I'm experimenting with printing out a front panel legend that will be protected by plexiglass.  I figure one more weekend and this will be a lot prettier and I won't mind when it gets moved to my operating position.

How does it work?

Like a charm.  The design is simple and it works remarkably well.  In fact, I much prefer the Elecraft design to my MFJ-752C. The MFJ has many more sections and much more complexity, but it doesn't have the presence that the AF-1 does.  The Low Pass Section of the AF-1 doesn't have nearly as much cut as the MFJ and it isn't really useful with my Kenwood TS-430S, but it helps out a lot with a homebrew DC receiver that I like to use. YMMV.

I'll post the completed filter when I get it boxed up this weekend.

Enjoy and see you on the bands!

73

Kenn

KA5KXW