Ruminations on a DIY Powered Desktop Speaker System

[TKoP]

Just one of those things, thinking of what I might look to buy if I were to try and build an relatively inexpensive powered speaker system for my bedroom.

Since I wanted to avoid work as much as possible, i jumped on Madisound's website to look at some full range speakers that I thought might have the response I was looking for and were also relatively inexpensive.

The one I cam across that looked like it might fit the bill was the Aurasound NS3-193-4A1 3" Full Range driver. Looks like it has an f3 of ~85hz, which is way better than I would have expected, a pretty flat response graph as well. And it was on sale at $13.50/driver.

For power, i found a 40w (2x20w) mini class D plate amp on Parts Express that also had Bluetooth. This was going to cost more at $36 for the unit, but better than other options i saw there.

The recommended (sealed) box size was kind of vague at 2-3 liters (which should be 122in^3 to 183 in^3), but in either case, would be small enough to fit on my night stand. The idea is to connect it to my Echo Dot that is already in there.

Anybody have any thoughts? I'm not sure what T/S parameters might be ideal for a project such as this, so the drivers picked could be total crap for all I know.

 

[Flint]

Aurasound makes great fullrange drivers and that one looks pretty nice. With a 20 watt max input you can get about 93dB SPL at one meter with one speaker, and approximately 96dB SPL with two.

The bass will not be amazing, but it will be clean and clear in the main listening range without serious deficiency in the bass.

The amp is interesting, but note that you need to completely seal the cabinet to get decent bass, so you might want to consider a sub enclosure to separate the speaker's cabinet from the amp's, or a separate box altogether.

The enclosure needs to be sealed completely tight for this to have any reasonably natural bass, and I'd go with an effective 2.25 Liter enclosure stuffed full with Acoustistuff batting material. Since it won't play loud, you shouldn't need extra bracing for that small of an enclosure, so 1/2 inch MDF would be fine as well - though it would be suspenders and a belt to use 3/4" MDF and dampen the inner walls with mineral-filled rubber mats.

I like this project! I want to see it happen.

 

[TKoP]

Well, that might make it easier then if you don't think the plate amp would be a good idea as I've already got one of these hanging around somewhere. Or maybe get something like this and get a radio built in.

I'd probably then just solder the speaker wire to the driver (or maybe get some quick connects) and run the speaker out of the box with no binding post or terminal cup. I'd caulk it pretty hard on the inside of the box to keep it airtight. The little amp by Dayton I have uses those pinching speaker wire holders. I THINK i have some stuffing at home, so would just need to get the MDF -- i'd go for the 1/2" to keep it smaller. I also think I have some foam tape as well.

Am I missing anything?

 

[TKoP]

The enclosure needs to be sealed completely tight for this to have any reasonably natural bass, and I'd go with an effective 2.25 Liter enclosure stuffed full with Acoustistuff batting material.

How much Asoustistuff would I need? I never really figured out what was too little, too much or just right...

 

[Flint]

With a small enclosure, start with half filled, then after time of listening, try full stuffed. Then experiment to your tastes.

 

[TKoP]

ok, still got this one in my brain.

• I have a small Dayton Amp already that will drive the speakers, so that's one issue i can check off.
• I also planned to just run the wire directly through the enclosure and caulk the inside to keep it airtight.
• I think i even have some acoustastuff from a previous project I can use as well.

One issue to address is what material to use for the enclosure. The idea is that this is going to be primarily on the nightstand next to my bed, so thinner is better. It should also be good aesthetically too. The normal 3/4" MDF seems like way overkill. Is there some obvious choice I'm not thinking of? And maybe i'm overthinking it -- this is just a desktop system, I'm not really going to use this for anything critical.

Another thought i had was to maybe do a baffle step correction circuit. The Re of the driver is 7.6 ohms and I'm looking at a baffle width of, say, 4.5 inches. From my quick reading, i couldn't tell what db attentuation i should use for the circuit. But let's go with 3db. If that's the case, i would wire the circuit in parallel with an inductor of .5mh and a resistor of 3.1ohms. Hoping i got the math right on that one. Also, given the application, would it be worth it? And should a Zobel network get included as well?

 

[Flint]

Here's what I recommend...

Material... 1/2" MDF could work fine without braces or dampening, or use cabinet grade 1/2" Baltic or Russian birch plywood with basic bracing and moderate dampening. With extensive bracing thinner materials, even plastic, would work.

Crossovers & passive filters... make them external, either openly mounted to the rear of the speaker or installed inline at the amplifier. Then you can play around and experiment with the many options promoted on the DIY sites.

 

[Deerhunter]

So what's happening with this little project?

 

[TKoP]

I still have to pull the trigger... too much of an accountant -- research, cost benefit analysis, time constraints and other stoopid stuff. I'm likely to do it soon though.

The aesthetics are a big thing since it's going to be so prominent. I like what Flint is doing with his woofer boxes -- i may try to figure out how to do that rather than skin them with some veneer like I've done with my other two 'real' projects.

 

[Flint]

So... here's the process...

I "sealed" the MDF with several spray coatings of Shellac with sanding between each coat. Once the Shellac was no longer obviously sucking into the edge of the MDF, I considered it sealed. Then I used a can of stain + polyurethane with a brush and just painted the surfaces, dragging the brush from edge to edge in the direction I'd want a wood grain to appear. After three to four applications of the stain + polyurethane, I let it dry, then used lacquer in one coat on just the edges as a perfect 45 degree angle to make the edges black and blend into the stain + poly on the center are of the panel. Then I lightly sanded with plastic finishing pads to rub off any rough areas - as much as felt right, then coated with multiple thin layers of spray on polyurethane.

That's it.

 

[TKoP]

"That's it" is what he says...

But, now I know and can give it a try.

 

[TKoP]

I still have to pull the trigger...

Bang

 

[TKoP]

1/2" MDF with the cut lines for the various pieces

Pieces cut, now cutting out the hole for the driver

Checking to confirm it fits.. it didn't because of the tabs.

 

[TKoP]

This was assembled to confirm the cuts were correct... fortunately, they were.

The tabs were an issue, so I needed to cutout a portion of the MDF so they'd fit.

To cut these pieces, I used my router.

I was originally going to try and only cut down a little bit...

The results of the first pass...

This time, I was able to get it in.

 

[TKoP]

Here's a shot of the driver from the back

Here is a shot with the cutouts and the notch required in the circle itself so the driver would fit.

 

[TKoP]

This is some veneer a cabinet maker friend of mine was able to get for me. The woods are from left to right, Tiger Wood - Eucalyptus - Teak

Just another shot of them...

 

[TKoP]

Started the assembly process. I both glued and nailed the boxes together.

Fully assembled boxes

Here's a close up of one of the joints. Not great -- but then, i'm not a carpenter. I also new I'd be sanding these down, so wasn't concerned.

Here's the same corner after sanding it down.

Next step, sealing the seams on the interior to make it as close to airtight as possible.

Kind of hard to get in there through the little driver hole that's only 3" wide... gets a little messy.

 

[TKoP]

Time to skin the boxes...

First step is to put on a few coats of contact cement as the MDF absorbs an awful lot of it.

After 2 coats, one more coat then attach the veneer.

Use a bendable utility knife like this and hold it against the box then slice the excess off.

Sometimes, it will take a few passes but, here you can see it's almost all the way cut.

It's hard to see here, but the corner had a piece that was chipped during the cutting process.

I plugged the missing piece with some other piece. It wasn't perfect, but it wasn't on a front part, so I was "ok" with it.

A better shot of the repair made.

And here are the boxes fully skinned.

 

[TKoP]

Now for the stain. I was thinking I'd go with a slightly colored stain at first, but when I was at the store, nothing grabbed me, so I went with "Natural". Glad I did.

Here's a side by side with one stained and one not.

And here's a shot of both fully stained. I think i only applied one coat.

 

[TKoP]

Just doing the internal hook up wiring.

First tab soldered.

Both tabs soldered... it's ugly, but again, I'm no pro and it should be fine.

Just to be sure the solder job wasn't a total failure, ran a test to confirm. 8ohms of resistance on an 8 ohm speaker...

The second one got to 7.7ohms. I don't know that my meter is all that accurate, nor the leads.