Merrill DCA 4 And The Zigmahornet
Ah VSAC… land of the true audio nerds like me for those us for whom buying gear isn't enough. We need to build it, tinker with it, and understand the fundamentals of why it works. Being a devoted audio nerd, I made my way up to VSAC in May of 2008 to cover the show for Enjoy the Music.com (see report here) and exercise my nerdiness and perhaps find a couple things to review (like these Zigmahornets, and the Occam Audio MTM4. Indeed, if you're reading this, you've likely started on the path to true Nerdism but fear not, you don't need a 12 step program. DIY has the lovely characteristic of being able to enjoy things for what they are, rather than what they're not. Something that a person builds will tend to be viewed in a rather more positive light, if only for the characteristics that make it unique.
And, unique is what the Zigmahornets are. The Zigmahornet is the recommended enclosure for the Merrill DCA4, a small (4.5") full range driver as seen above. They are quite affordable at $65 a pair, well built, and well suited for the cabinet in question.
It is an acoustic labyrinth, AKA unstuffed transmission line, or quarter-wave resonator. This enclosure type tends to offer extra bass extension and some reduction in cone excursion vs. a sealed box, and a wider operating bandwidth from the vent than a bass-reflex. On the downside, there are issues with resonances above the intended frequency range (see the ripples on the impedance curve near the bottom of this article), and the contributions to the overall output are delayed relative to the frontwave. This is known as group delay and it has variable audibility based upon frequency, level, and listener.
The tall, slender cabinet has some plusses and minuses. On the upside, there is a low amount of edge diffraction from this cabinet type. Between this, the tiny panel width, and the point-source speaker, these are destined to be imaging champions (and indeed, they are). On the downside, such a slender baffle means that the transition to 4pi radiation (aka. Baffle Step) takes place at quite a high frequency, which can thin out the bass frequencies up into the midrange. Some may wish to modify the front baffle with extensions to help this, or place the speakers close to a wall. More on these issues later.
The construction is straightforward woodwork, and the result is extremely elegant. The only part that might give some problems is the angled top section. They're all 45 degrees, though, so if you can work a saw (you could manage these with a circular saw, table saw, or even a handsaw), you can build them.
They're gorgeous with an extremely high WAF (Wife Approval Factor). They were built with the recommended 0.5" ply, and I painted them rather than wood finish.
There is a small amount of felt on the inside walls near the driver, and no bracing, per the plans. I did a 0.5" roundover on all edges, though in this application, it should be acoustically irrelevant (the driver is highly directional lower in frequency than a ½" roundover is effective). Speakers are hardwired with some high quality 2 conductor copper speaker cable.
On the downside, they're very prone to tipping. Even with a 1.75" base plate, these tall lightweights are very tippy on my carpet, even with outrigger spikes. Those of you with little kids, beware. I have had to do many touch-ups on my paintjob. Indeed, it reached critical mass with a knockover cracking one of the pipe sections off the base.
Fortunately, it re-glued fairly easily. I was forced to go the route of making additional ‘sub bases' for them after all finishing had initially been completed, which was a lot of work, but did give me the opportunity to practice the veneering I'll be doing on my next full-range project.
First System "The Big Rig"
On the plus side, at modest levels, they're very good at imaging, and with simple music, they're fantastic. Upper midrange dynamics are superb, with cymbal crashes and the like portrayed excellently. Bass extension is very light, and treble extension is ‘just enough', but I'm a man who likes my frequency extremes, which is not a strength of single-driver speakers. The midrange is pleasant, clear, and tonally complete.
For the negatives, they're very limited in their ability to play loud, especially in the bass. When you ask prolonged moderately high levels of them, they are very fatiguing. They obviously need more help. Even with the sub kicking along, they're not able to come anywhere near the levels they'll need to through the midbass. This room is probably the bulk of the issue, as my main listening is done in a big room with extremely little reinforcement as it is open to other rooms.
I cannot recommend these to anyone with a large room, or those who enjoy "live" levels even in a smaller room. For those users, if they want full-range goodness, I'd recommend they go to a 6.5" or 8" full-range, and/or a horn-loaded cabinet rather than a quarterwave design. Another solution is to use a less sensitive, higher Xmax driver, though more Xmax isn't always the end-all solution (or even desirable from a driver design perspective) but it certainly would have prevented the distortion characteristics I noted at higher levels.
Second System, Smaller Room
At first, even in this smaller space, there was insufficient bass, at any playback level. So, a tiny subwoofer was brought into the mix. Opposed 8" woofers and a 600W plate amp is plenty to do some very good reasonably deep bass, given the small room and level limitations. The first iteration of the setup had this situated between the Zigmahornets, which were only about 3' apart and me typically listening from 4-5 feet, so a fairly nearfield setup. Once I broke away from nearfield, the mid-bass thinned out. Accordingly, the best setup seemed to require some boundary reinforcement. By getting them closer to the TV, I was able to use the screen as a baffle extension to fill the gap between the sub and the upper bass. If you wanted to use a larger front baffle, that could help, or some people use an inductor-resistor in parallel (in series with the driver. 3 ohms and .8mH would be decent starting values for this speaker) to correct for "baffle step" loss in a single driver speaker.
The advantages of single drivers are readily apparent in this configuration, with a clarity and coherency that are very impressive. As in the "big rig", the lower treble/upper mids are superb, without any of the harshness that was audible in the big system. This can be attributed to the lack of output requirements in this sort of arrangement. Less cone motion equates to less distortion. The SEX amp really played well with these, making very enjoyable music, but there was still a sense of output limitation. This I attributed to the mere watt and a half available from the 6DN7 wanting just a little more than the 90 or so dB the Zigmas had to offer. The imaging and soundstaging was world-class.
Some experimentation with other amplifiers improved some things, but ultimately, the Bottlehead SEX amp was the best match, limitations and all. These are not ‘big' speakers, these are intimate speakers. They're not for bassheads, they're for reasonable levels and requirements. And in that task, they excel. Small ensembles, jazz, rock, other music are well served, even without a subwoofer. Add a sub to the mix, and things get more flexible, but these aren't party speakers, even with a good sub. It is important to note, however, that these will work with a wide variety of amps. They are on the forgiving side, and can do with budget oriented amplification, unless you mean to use an outright "bad" amplifier. The high(ish) sensitivity and easy load (see below) make them an easy match for most amps.
Their strengths are many. Human voices in particular do nearly everything right, with no obvious tonal imbalance. The soundstage is wide and deep, the slender columns easy to tweak placement for wherever I'm sitting. The entire frequency range is clean and smooth, without noticeable rough top end (common to many full-range designs). With a sub in place, the bass is well balanced and smooth. Dynamics are not excellent, but sufficient. Instrument placement and soundstage are stable, easy to interpret, and really don't get much better than these speakers reproduce. I can listen to music all day long through the Zigmahornets, which I can't say of many larger full-range driver loudspeakers.
How time flies. During that time, Dave Merrill, the man who designed the DCA4 driver has passed on. The driver is still in production and available at DIYparadise.com. The plan mockups are available there too, as drawn by Dave Dlugos of Planet10hifi. The price has also increased to approximately $100 USD (they're shipping out of Singapore, now).