• It takes form...

    Jon Dansie • 12/31/2014 at 03:49 • 0 comments

    So I know I said I was going to focus on the electronics. And I've had a play with that - I'll write about it later. I've also got the body of the radio up and going, so that's my focus here. The body started as a big chunk of wood pulled from the scrap bin at a local timber recycler. It was in said bin because it had a bunch of nails in it; not many, but enough to make it uneconomical, so they would've cut it off the end of a longer, more salable beam. At a guess, it's possibly a wharf post or something? Hard as the nails embedded in it....

    I cut a thick slice off the beam end (with a band saw. Neither easy nor precise...) Managed to avoid nails in the cut path, but there's one still buried in the final piece. I then routed out a spot for the speaker to go.

    Again, I can't claim this was precisely done... There's a few over-routed bits, plenty of extra chiseling required and not a pretty finish... I also went too far and had to add a few extra blocks to mount the speaker into. Would've been great to have a CNC mill to do a proper job of it, but then the work piece wasn't flat to start with...

    Apologies for the horrible photography on that one - needed the flash to show up the details. You might also notice a bit of a crack along the bottom - whilst chiseling (with poor technique!) I broke the whole narrow jaw off the bottom right. It glued back ok, but not great. The whole thing looks better from the front

    The little black circle on the right side is from the router chuck pressing in and burning the face of it (I did some routing from the front side). It's also visible around the bottom left corner. The wood's got plenty of cracks and other things, but I'm going to be charitable and call that "character"! With the speaker in, it should look a bit like this:

    I plan to mount the electronics in the back, again by routing out a space (but not going through to the front). The front will still look like this when it's done, and the controls are currently planned to go on top, on the right, like so.

    I'm planning to use a screen from an old Nokia E63 (I've got 2 of them lying around). I'm also sure there'll be a volume knob, which may double as a power switch. It'll probably be continuous rotation, since I'm planning to often control this remotely from a phone/other device. I'm unsure what other buttons will be there and what they'll look like.

    Once the glue dries, I'll be mounting the speaker and giving it a test run. I'm still working on all aspects of the electronics (have started on some...) so you'll read about those in due course. I guess there may also be some tweaking of the sound once the speaker's mounted and I know how it sounds. Hopefully the next update comes a bit sooner!

  • Measuring speaker parameters

    Jon Dansie • 08/27/2014 at 03:23 • 0 comments

    I spent a while looking at speaker drivers with no real guide to what I was looking for. I wanted something cheap, but good, with good bass and all those other things people like me think they want. I was getting nowhere, and taking a long time to get there. One day I was out walking and saw a discarded TV on the side of the road, and thought "well those speakers are free, and if they sound bad I've got an excuse!" So back home to grab a screwdriver, and I was the proud owner of a pair of ~5x2.5 inch 8 ohm speakers.

    For those interested, the host TV was a:

    Unfortunately, the generous TV donor didn't leave any spec sheets on the side of the road. (I know, right!?!) A bit of googling found me a similar pair that were once for sale in Brazil, with the useful added tidbit that they were 10 Watt speakers. I also went back and grabbed some of the TV's boards to harvest capacitors, and in the hope that I'd find a part number for the audio IC, but no luck there. So it looked like I would be measuring things myself.

    I'll spare you the details of all the things I tried and failed at, and skip to the one that worked - Speaker workshop using Claudio Negro's cables - http://www.claudionegro.com/.

    I won't repeat the process - you can read that on Claudio's site. Interestingly, though I followed all the steps, my passive component tester never worked - it just said everything was a 620 ohm resistor... But I pushed ahead and seemed to get ok looking results. (good enough for my needs, anyway)

    The top curve is the smoothed out estimate, and the bottom one is the actual measurement. I've cut off the low frequencies on the measurement because they were just noise. The Thiele Small parameters are estimated at:

    Ignore everything between Max excursion and Volume of Sealed Box in the right hand column - these are values you're meant to supply from the data sheet... I had to take a bit of a punt measuring piston area too, so it might be wrong. But close enough for a first speaker project...

    Having done a course on basic speaker design, I was expecting to grab some numbers here, put them in a sub-woofer calculator and get a box volume. However, that process is for speaker drivers with a total Q well below 1, and here I find myself up around 1.4! So it's back to the drawing board I guess. Apparently open baffles are good for high Q drivers? And come to think of it, the TV was probably "open" backed with its ventilation holes. But does open baffle suit my requirements for portability?

    Might be time to focus on the electronics while I mull over the design...