I'll be honest: when I designed the TRS-80, I had a feeling that not many people would build it, or even notice it. It was that darned Vifa D25AG tweeter; a very nice-sounding tweeter, but one that has been on the market so long that it's ready for its own episode of "I love the '90s!" The obvious question becomes: could we take the idea behind the TRS-80 and translate it into a similar design? One that would use the same cabinet, lovely bass extension, achieve as good-or better-sound quality from a sexier, more modern-looking tweeter? Yes, and we will call it "Amiga."
The Amiga is a boutique HiFi killer. Although the components used here are relatively inexpensive, when properly executed will sound and look like they cost several times that much.
The cabinet dimensions and overall look of the speaker were borrowed from a well-known audiophile loudspeaker company. At 34" tall and 8" wide, they have an understated-yet-classy appearance. With a quality finish, they would easily pass for fine furniture. The Amiga should be constructed with 3/4 stock (MDF or Baltic Birch plywood), and absolutely must be braced inside the cabinet. I would recommend the use of at least one "windowpane" brace, as well as some standard "shelf" braces tying the sides of the enclosure together. I made a detailed cutaway diagram in both Imperial and Metric.
The interior walls of the enclosure need to be lined with some sort of convoluted foam or BlackHole (honestly, I use mattress topper from Wal Mart). There is also a wad of stuffing resting in the bottom 1/4 portion of the cabinet.
The cabinet is about 30 Liters internally, and is tuned to 39 Hz. This gives an F3 of 34 Hz (ooh, yeah!). The vent is 2" diameter and is 3.5" inches long.
You could build these sealed into a cabinet of 8-11 liters, if you wanted to pair them up with a sub.
The crossover for the Amiga is very similar to the TRS-80. It uses a 3rd order electrical filter with a notch on the woofer, and a 2nd order electrical plus an L-pad on the tweeter. The crossover point is at 2000 Hz, and the slopes are 4th order-ish. I won't go into too much detail, but I will reiterate that the Dayton RS180 is a difficult driver to use in a two-way. It has a hefty amount of cone breakup, which will introduce all sorts of lower treble garbage and sibilance unless it's squashed down by 50+ dB. Once properly attenuated and shaped, the RS180 is a joy to listen to; but in the wrong hands, they will easily produce a speaker that sounds tinny, hissy, and barky, and will leave you wanting for some cheap big-box brand speakers.
The Peerless / Vifa DX25 tweeter was a new one for me when I started this design. It had been toyed with around the DIY community, but mostly it had been passed-over for more-popular tweeters. Its reputation was basically: "It's high sensitivity, and has a pretty low Fs, and it sounds OK I guess." The first few days of working with the tweeter, I was thinking the same thing. It wasn't terrible, but it had this sort of nasally-yet-breathy hash it would inject into the music. For a while, I thought maybe this was just an artifact of a cheap tweeter diaphragm, but then I realized it was caused by an excitation in the rather large impedance spike around the tweeter's resonant frequency--exactly the same problem that plagues its sibling, the XT25.
The solution was simple: forget the low Fs, cross higher and out of the way of the Fs,and use some sort of Impedance Compensation (in the case of the Amiga, resistor R2 is helping accomplish this, though sometimes a full-blown LCR shunt filter is needed). Once I solved the Fs spike issue, the DX25 were actually very pleasant tweeters to listen to. They do their job, don't bring any attention to themselves, and don't introduce any odd-order harmonic distortion as cheap tweeters often do. Their Frequency Response is sort of lumpy, but this actually doesn't seem to pose any audible problems (ie: noticeable dips or peaks) when used in a final speaker system.
Note: check your tweeter
At some point in the last few years, Vifa/Peerless/Tymphany (whatever their nom de plume is) changed suppliers of the dome material. The version I originally designed the Amigas with was the DX25TG09-04, whereas the current model is the DX25TG59-04. I have been asked many times if this would make a difference in the design. I didn't have one of the newer versions on-hand to measure, but looking at the differences on their respective spec sheets, it seemed as though the tweeters were within 1-2 dB of one another, so I let it go. A few months ago, I had an opportunity to measure a new pair of these tweeters in the design, and find out once and for all if it was true. It turns out that the spec sheets are precisely true, and the new ("59") tweeters are 2 dB hotter than the originals ("09"). Will they drop into the original crossover? Yes. Will the treble be a bit bright? To some ears, yes it might.
There is a fix, though, and it's super-easy. Simply swap resistor R2 (4.7 Ohms in the original crossover) for the updated value of 2.4 Ohms. (Or, if for some reason you cannot access and/or modify the crossover, you can accomplish the same result electronically by putting another 4.7 Ohm resistor across the + and - leads of the tweeter--essentially creating a parallel resistor pair with a total of 2.35 Ohms)
So, to clarify, if your tweeter is labeled DX25TG09-04, use this crossover.
If your tweeter is labeled DX25TG59-04, the crossover shown on the page you're reading right now is the correct one.
I'm out of verbosity. Basically, if you like the Overnight Sensations, you'll like this. The tonal balance is similar; the voicing is slightly relaxed to make for long listening sessions. However, the quality of the drivers will definitely showcase fine details in a recording. The imaging puts on a good "dog and pony show" like a quality monopole speaker should, with a very tight center image, and sounds seeming to appear around and behind the speakers. The bass extension should definitely surprise; with an F3 of 34 Hz, there's a good chance you can leave the subwoofer turned off. The overall sound is big; it's a 2-way that pretends to be a 3-way. The drawbacks of this design are that the speaker is not very sensitive (about 83-84 dB, after baffle step loss), and the RS180 only has so much excursion. My own experience is that the RS180 runs out of xmax somewhere around 96-98 dB, depending on the music, and modeling confirms this.
But gosh, for a HiFi boutique sound on a pawn shop budget, the Amiga is hard to beat. These are speakers I would have wanted to own, back before I was sucked into the world of DIY. I can just see myself: auditioning them at several HiFi dealers, then scouring the net for weeks before finding a good deal, which still would have been several times more than the cost of DIY!
Builds by other DIYers
These two were built by Ante and Mario, who live in Croatia. They are both beautifully crafted and show great attention to detail. Ante did his in black, while Mario chose red.
[tombond] showed off his finishing skills in this stunning build.
This build was done by HTGuide member [Anderton]. The baffles are solid eucalyptus, and the rest of the cabinet is veneered in Flame Birch. He gives additional details here. Who wouldn't want a pair of these in their living room?
Special points if that font makes you wistful.
I'm sure both girls are avid computer historians. ;-)
by Paul Carmody | this page was last updated December 19, 2020