TRS-80
TRS-80
The Repository->DIY->TRS-80
Dayton RS180S-8 + Vifa D25AG
40-20,000 Hz
Impedance Minima: 7.5 Ohms (safe for any amp)
8"W x 11"D x 34"H
Cost for drivers and XO parts: less than $220
Bill of Materials available TRS-80 BOM.doc.
I designed and built these for a friend of mine who wanted a new set of mains speakers. In a scenario similar to the Classix, what started as an odd combination of drivers turned out to be such a pleasing, exciting speaker that I'd like to share it with the DIY community. It's hard to say if there would be anyone who would build the TRS-80, but I'll put this out there for any brave souls who want to try something off the beaten path of the normal all-aluminum 2-ways, and also would like a surprisingly high-fidelity, classy-looking speaker on a miserly budget.
Also, I'm pulling down the design for the VIC-20, which used the Scan-Speak 8545K + Vifa D25AG because this one simply blows it out of the water, especially for the price.
(the photo shows them in their 75% finished state at InDIYana 2009)
Enclosure Design
The dimensions and overall look for these speakers are "borrowed out of admiration" from Totem Acoustics. They have a small footprint, and when tastefully finished, can pass for upscale furniture. (read: high Spouse Approval Factor) A lot of people are surprised when they see these in person, having thought they looked much larger in pictures. But try it yourself; grab a yardstick, they're two inches shorter than that!
Anyway, let's talk about how to build these things. First off, I learned the hard way that cabinets with really long walls like this need a lot of bracing or they will resonate quite a bit. I used two "shelf/windowpane" braces tying all sides together, along with two small horizontal braces behind the drivers. And that is the bare minimum of bracing needed; just enough to bring the "knuckle-rap test" up to a pitch I thought was adequate. Bracing aside, there isn't really any sort of trickery to these cabinets. They use 3/4" MDF all the way around, and you should be able to build them with one 4'x8' sheet (I think. don't quote me on that, though). The drivers are flush-mounted, as would be par for something elegant like this. As far as finishing goes, I think personally I think these would look best with a pretty veneer (so that's the plan, anyway). If you're looking for inspiration, just look at what Totem does, and run with it. (Though don't ask me where to get those spikes they use. I have no idea. You'll have to be creative on that one.)
Now, you will notice that the diagram calls for a "removable shelf" at 22.75" inches down. This is optional, and I leave this up to a builder. The idea is that, depending on how you like your bass, you can tailor the internal volume of the cabinet to taste.
Shelf in place - The internal volume of the cabinet is 22 Liters, which offers a very good tradeoff for this driver in terms of bass extension versus transient response. The cabinet is tuned to 43 Hz and gives an F3 of 41 Hz, which is very nice for most listening applications. The vent should be 2" dia x 3" long. (This does not include a flare)
Shelf removed - The internal volume of the cabinet is 30 Liters, which is the textbook "optimal box size" for the RS180. This will offer pretty much the most bass extension you'll be able to squeeze out of this driver; if the box got any bigger, the low bass would get especially "flabby." The cabinet is tuned to 41 Hz and gives an F3 of 37 Hz. The vent should be 2" dia x 2.5" long. (This does not include a flare)
As you can see, I put the vent about a foot off the ground, and the crossover mounts to the bottom of the cabinet. I lined the cabinet with egg crate foam, but did not use any stuffing.
Crossover Design
The crossover for the TRS-80 is about as simple as I can get away with, using these drivers. The High Pass filter is an ordinary 2nd order electrical filter with resistors in series to pad and contour the treble response. My experience with this tweeter is that, the fewer crossover parts you use on it, the better it sounds.
The Low Pass filter is a careful balancing act between SERIOUS cone resonance suppression and EQ. The woofer and tweeters both have 4LR acoustic slopes, and cross just slightly above 2000 Hz.
The Dayton RS180 is not too commonly used in 2-ways; a few popular exceptions would be the Modula and Natalie P by Jon Marsh and the Cryolites by Lou C. Instead, lately you seem to find it holding down the bass in 3-ways. The reason being--and I can tell you from experience--is that it is a tough driver to cross up high, and I would not recommend it for a 2-way to a beginner or even intermediate crossover designer. However, if one insists on using it in a 2-way (or midrange in a 3-way), the generally-held praxis for the RS180 is to cross it low (like 1600 Hz or below), and to use steep filters. This is because the cone resonance of the RS180 is not only ungodly loud, but it also falls into an area where our ears are extremely sensitive. While first working with this driver, I hadn't suppressed the peak enough; I found the speakers to be very sibilant and "hissy." I futzed with the tweeter's filter for quite a while, not able to fix the problem. Then, while I had the tweeters disconnected, I clearly heard the extra sibilance: it was coming from the woofer! I thought I had done an adequate job of filtering that mess out, but apparently not!
So you're thinking: if one is supposed to cross the RS180 so low, why did Paul cross it higher? Well, the higher you can cross a woofer, the more interesting choices of tweeters one can pair it up with. Yes, the RS180 can play higher (just as the very costly driver it emulates, the Seas Excel W18, can), but it's a careful balance. I use a quasi-3rd order electrical filter, combined with an elliptical filter to snub the breakup by 50+ dB, as well as a contour filter to soften the knee of the roll off. All mashed into 5 parts! The end result, however, sounds very natural. Take a look at the response graph. I don't see any more cone resonance, do you?
Listening Impressions
This is a little strange. The TRS-80 offer both exactly what you would and would not expect from an all-aluminum speaker. First, the level of detail these things can deliver continues to surprise me. I feel weird saying this, but these are the most detailed speakers I've designed or built so far (as of January 2009, anyway). I'm hearing tiny nuances and bits in recordings I have never heard before on any other speakers. But what's equally as surprising about these speakers is that they are not "overly revealing" or fatiguing, as metal-cone drivers sometimes get a reputation for. They actually have a very pleasing, robust sound that should keep most ears very happy!
This is the predicted on-axis response, which is somewhat misleading. Do not be alarmed by the dip at 3 KHz. Once you move slightly off-axis, the treble measures ruler-flat. The speaker also has a very subtle midrange dip which helps make this all-metal speaker easier to listen to for long periods of time. (System measurements coming soon)
The TRS-80 is a pretty "insensitive" speaker. I'd rate it probably around 82 dB @ 1 Watt, so it's probably best suited for solid-state amplifiers. My only complaint about the TRS-80 is that you cannot dump loads of power into these woofers. According to sims, you should be able to get to 95 dB @ 1 meter, after which point the RS180 will [probably] exceed xmax. I tried this myself with an SPL meter, and while one woofer started "popping" against the back plate once the volume crested 95 dB, the other one was hearty and still ready for more punishment. So I cannot say exactly how anyone else's would perform; I seem to have gotten the worst and best-case scenarios. In an ideal world, I'd like more headroom, but we're talking about "painful" SPL levels here, and for a small or medium-sized room, these speakers will really fill the space up nicely. In a large room, they'll would still sound very good... but they will not be able to deliver chest-pounding SPL, unless you pair them with a sub. My $.02, for big rooms, if you want a big sound, build bigger speakers (hint: you'll need woofers larger than 7")
Final Thoughts
If the Classix had a more sophisticated brother, it would be the TRS-80. Classix was a small, stout fellow. He probably went to Southern Illinois University, did a bunch of partying; but he was a fun and reliable guy, and was generally liked by everyone he met. The TRS-80, on the other hand, was taller than his brother, a bit more stoic-looking. He went to University of Chicago and spent his time studying. He was more articulate, more refined than his brother. But the two still had familial similarities: they were both made of a strange combination of drivers, and both offer an extremely enjoyable listening experience that would satisfy most any music lover.
Oh, and for those who wondered why on earth I would name a speaker after a crappy old computer, well it's sort of a stupid in-joke I have with myself. For some reason, I decided to name all my tall 2-way designs after old computers (I even refer to these enclosures as "The Commodores"). A lot of my fellow speaker geeks suggested I do a design called the TRS-80 (aka "Trash-80"), and after looking at the pairing of these two drivers, I knew I had the design to fit the name!
by Paul Carmody | last updated April 21, 2009
