The current trend in home audio seems to be "small, convenient, and wireless." People still want music (which is a good thing), but they want a speaker they can fit on an end table, and be able to cue up songs from their phone or computer. Oh, and at least some people want it to sound good! Commercial "wireless speaker" products are a dime a dozen right now, offering a wide variety of sound quality and price points. But what options are there for DIYers?

The Isetta mimics the size and form factor of some of the more popular "wireless speakers" and "table radios" available, but it competes with the best of them in terms of sound quality and output--at a fraction of the cost.

Enclosure Design

I studied the size and shape of popular wireless speakers when designing this, and it comes pretty close, give or take a few inches here and there. The full-range drivers each get just under 1 Liter. The full-rangers are sealed, and extend down to probably 110 Hz where they will gently roll-off, which should make for a smooth transition to the woofer.

The woofer enclosure is approximately 6 Liters (after accounting for port tubes and whatnot), and is what I would consider the bare minimum size for this woofer. However, it does let the sucker play way down low, with a -3 dB point of 35 Hz. The port is 1.5" in diameter, and 10.5" long. The port is mounted on the side of the enclosure, because that's pretty much the only place it would fit. I recommend a flared port if you can get/make one to alleviate chuffing under high SPL. Also, be very sure you seal every bit of the enclosure and around the vent. This is a lot of moving air in a small space, and any slivers where air can escape will immediately chuff when playing any kind of bass.

I designed the Isetta to use 1/2" MDF or Plywood. Could you use thicker stock? Of course, but I think the point is kind of moot on something this small. Plus, one of the primary goals of the project is to be small, so I conserve space however I can.

The full-range enclosures are stuffed with a moderate amount of polyfill, and the woofer enclosure has no fill or lining inside the enclosure.

I hope the drawing isn't too confusing. In case it isn't obvious, the lines on the top indicate the walls on the inside of the box for the sealed mid enclosures. Sometimes I have a hard time communicating more complicated enclosure designs. But honestly getting all these pieces to fit in the smallest space possible becomes a 3D puzzle of sorts.

Driver Selection

The woofer used in the Isetta is the inimitable Tang-Band W5-1138SMF. When it comes to the choices we DIYers have for small, cheap woofers that can play deep in a tiny space with lots of excursion, well, this is all we've had for the last 15 years. The woofer is very low sensitivity, but that isn't really a problem for us, due to the fact that this design relies on a 2.1 amp, so it can draw as much juice as it needs. The woofer is well-suited for this project, and pumps out a mind-bending amount of deep bass for what is essentially a shoebox-sized enclosure.

The original version of the Isetta used the Fountek FE85, which was a fantastic little performer for the money. Unfortunately, that driver was discontinued, so a search for a replacement was on. I went with the HiVi B3N because it still hits the same price target as the Fountek, and also is an exact fit for the woofer hole! Seriously there were a few other pretty nice full rangers I could have gone with, but they're all just plain bigger--which then makes the enclosure bigger, and that was defeating the purpose of the project.

On the plus side, The HiVi B3N are some darn nice-looking woofers. On the negative side, they require a few more parts in the filter; more on that below.


There's good news and bad news when it comes to the electronics of the Isetta.

Bad news first: the Isetta requires a 2.1 amp.

The good news is that there are lots and lots of choices these days, and they are very reasonably priced. A quick search online of "TPA3116 2.1 amp" gives several pages of results, ranging anywhere from $40-$100.

My personal pick right now is the Kinter K3118. Just a really good amp for this type of project. It comes with a beefy power supply (which some 2.1 amps skimp on), so it has a fair amount of wattage to drive the sub. It puts out a clean sound with a very low noise floor, and the Bluetooth implementation is terrific. I like that it has a manual switch to use Bluetooth mode. What I mean is that a lot of Bluetooth-enabled amps these days sort of "auto-switch" when your device auto-connects. This "auto-switch" causes more problems than it solves in my opinion. I'm not saying you have to use this amp. If the Kinter isn't available by you, or you want to try something else, it shouldn't be too hard to find a similar, suitable 2.1 amp.

The other challenge I'm leaving you with the amp is that they come in a ridiculous array of sizes and enclosures. Whether you want the amp to be integrated into the speaker or mounted outboard is up to you and the amp you choose. There is just too much variety in the amp market for me to dictate how to do it one way or the other. The advantage of mounting the amp internally is obvious in that it hides any messy wires and makes for cleaner looks. On the other hand, some of the 2.1 amps out there have some really nice integrated buttons and features that might be nice to have access to externally. So I leave the choice up to you. This is DIY, after all.

Filter design

I had to really wrestle with the B3N to get it to sound pleasing. See, like a lot of full-rangers, when you just play the B3Ns by themselves, they fool your ears into thinking they can handle a full frequency spectrum from 100 Hz - 20,000 Hz--at least for the first few minutes. When you first fire them up and listen a bit you think, "Wow! This is amazing! This little bugger can do it all! Why do I even bother with tweeters?" But then after a few minutes your ears start to feel fatigued. That seeming high treble is actually just ringing in the cone. Then you start to notice that everything just kind of sounds very forward, especially in the midrange.

So the first thing I did when making a filter was to contour the mids a bit, aka: a "Baffle Step Compensation" filter (R1+L1). This helped bring the soundstage back to the speaker, instead of being so forward and shouty. But I still found that there was a lot of leftover sibilance in the 8000+ range; this was that cone ringing I mentioned earlier. That required a notch filter (R2+L2+C2). Ahh, much better! There is still a bit of a peak around 16,000 Hz, but honestly most people can't hear that high, and even if they do, it's just going to "add air" to the overall presentation.

Ironically, after putting this filter together, I looked over at how John "Zaph" Krutke handled the driver in his popular B3N single-driver design, and it was nearly identical--at least in overall topology. Mine uses a slightly different contour in the mids, and achieved the same basic notch around 8000 Hz using some different parts. In his writeup, he says this minimum 5-parts filter is "required" for this driver, and he's right. The Frequency Response graph below shows the output before (orange) and after the filter (black).

The 100 uF cap (C1) is a simple high-pass filter, to try to prevent the driver from playing the way-down bass stuff, in the event your 2.1 amp doesn't have a high-pass filter for the stereo outputs (it probably doesn't).

While I don't have a specific Bill of Materials for you, I'd say follow these general guidelines:

  • use an Iron-core inductor for L1, and save yourself a bunch of money

  • L2 you'll need air core, but it can be 19-20 AWG

  • C1 use an electrolytic capacitor

  • C2 you can use electrolytic or polypropylene capacitor

  • Resistors use 10W wirewound

If you manage to find a pair of the Fountek FE85 and want to go in that direction, the filter design for those is available here. Everything else about the speaker (enclosure, amplification, etc) is exactly the same as the HiVi version.

Listening Impressions

Because of the unique capabilities of the W5-1138 sub, the Isetta can easily fill a moderate-sized living room with music. With a good amp, it can probably even rattle the china cabinet a bit.

This is actually the most-listened to speaker in the house. I keep a Google Chromecast Music plugged into it, and can call up my whole music collection on my phone using Plex. My wife loves it because she can blast Pandora while she does stuff around the house.

Due to the nature of the placement of this speaker and its intended use, it's often listened to off-axis. Now, since we're dealing with a 3" driver, the high end does roll off a bit when you move off-axis. But again, because the listener is usually doing other things, they don't notice at all. However, sometimes I'll just sit on the couch for some quick listening sessions in-between other activities and am consistently impressed with how natural and spacious recordings sound when listening on-axis. It's a nice way to "lose myself in the music" for a few minutes when it's not conducive to go and fire up my bigger 2-channel setups.

Final Thoughts

The proliferation of "wireless speakers" on the commercial market probably leaves DIYers feeling disadvantaged. So many of these speakers get to have the luxury of Digital Signal Processing (DSP) to help them sound better and bigger than they really are. It really is hard to compete with some of these, with the drivers and amps DIYers have to work with. However, I think the Isetta breaks that barrier, and can definitely compete with the commercial "wireless speaker" crowd, and in the end is far more rewarding. Now, you can finally go to your local electronics store and look at the wireless speakers and think to yourself, "I could build that!"

by Paul Carmody | this page was last updated March 15, 2022