Crawls Backward (When Alarmed)

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Full-Range Bass Reflex Speakers - Proof of Concept Build

Now that I've built a low-power single ended tube amp, I need some efficient speakers to use with it.

I've wanted to embark on this project for many years and have read a lot on the Internet and done a fair bit of research.  What really drove my desire to build some fullrange speakers was the fact that I wanted to build a single-ended triode amp powered by 2A3s or 300Bs.

Fullrange speakers, as the name implies, use just one driver for the entire frequency spectrum, instead of using a crossover(s) to split up the range into bass, midrange and treble.  Why do this?  There are several reasons: including the fact that it eliminates the crossover electronics, imaging is improved since the sound is coming from one point.  There are a lot of resources about fullrange speakers on the net, including the link above.

I have some 2A3s, but decided as a dry run/proof of concept, I'd build the '42 amp first.  Now that it went well, I'm moving on to a set of speakers to use with it.

What I really want to do is build some fullrange back-loaded horns, but since that's a bit more complex in terms of construction, I decided to just build a simple (for some...maybe not me...) bass-reflex design, but use some efficient speakers in it.

The speakers that are most popular, it seems, are Fostex.  I see a lot of folks on the internet using Fostex speakers, which see to offer a good value for the money.

There are a lot of DIY horn plans around, but not a lot of bass reflex designs.  Fostex themselves have "recommended" designs on their site, so I decided to go with that.

I chose the FE-126En speaker.  This is a 4 inch fullrange driver and is 93db efficient.  Considering your average home speaker is probably in the range of 85db, this is a big jump.  I would think (hope) that they'll be pretty loud driven by my little amp.  It also won't break the bank - mine cost $43 each from Madisound.

The plan from Fostex is a box that's just 295 x 295 x 180 millimeters.  Just a bit less than 12 inches square and about 7.5 inches wide.  A good manageable size, I think.  The design is tuned down to about 61 Hz, so it won't be tons of bass but should be adequate.  Again, this is a proof of concept, so we'll see.

So armed with my new jigs, I procured a piece of 4 x 8 x 3/4 birch plywood from the Despot.  I had them hack it crosswise into thirds so I could fit it in the car.

Now the fun starts.

Here's a hunk of plywood on the ol' Workmate about to be sliced up into pieces.  One adjustment I had to make was for the thickness of the birch.  The Fostex plan calls for a 15mm thick board.  Here in Merica, we have 1/2 inch (about 12mm), or 3/4 inch (about 18mm).  So I went with the thicker 3/4 inch.  I just had to compensate some of my dimensions to ensure the internal volume would be the same.

I won't go into the details, but I really just had to add 3mm here and there to outside dimensions.

I had a heck of a time using my rip jig.  So frustrating, in fact, that I took no pictures.  Basically, the idea of running the saw in a longitudinal channel was problematic - the height of the channel slanted the saw and my edges were not square.  After a couple cuts, I figured that out, and just went with the straight side of the jig.

At least one clever sage took a look at the jig and emailed to point out the error of my ways.  Hats off to him. (If you look at his site you'll see he knows which end of a hammer to pound on).

The crosscut jig, on the other hand, worked a charm.  Nice square cuts.  The screws on the channel on one side tended to hang up the saw, but I put smaller screws in and the saw floated right through.

At least I got something right.  Proof of concept, you know.

Here are the final boards stacked up on the workbench.  Now I need to pair them up and make sure they're all square.

After that, I'll cut the speaker, port and terminal holes and stick 'em all together.

I should add that I've never heard these speakers, so this is truly a forage into the unknown.

 
 
 
 

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