John Reekie
Technologicality, at work and play

The HifiZine active loudspeaker project – a proposal

Please note: this proposal for an 8″ two-way active loudspeaker project has been superseded by the following articles:

• The Mini Convertible, a 6″ two-way active loudspeaker project
• The Convertible active loudspeaker – revisited

Introduction

I’ve wondered for a while if it might make sense to publish some “HifiZine projects” – fully-documented DIY projects that supplement the many tutorials in the Technical and DIY section here on HifiZine. For me, any such project would most likely be in the area of DSP-based active loudspeakers, with a documented construction guide and a provided DSP configuration file. The idea would be that those tackling their first project would be able to build a DSP-based active speaker with low risk. Then, once they had learned the basics of DSP and speaker design, they could proceed to refine and tweak the project as they wished.

This idea hadn’t solidified into anything concrete until recently. There were many questions. For example, the speaker configuration: should it be a 2-way? A 3-way? Integrated subs? MTM? Open-baffle, boxed, or a hybrid? And which drivers? The choices are almost endless, so some firm selection criteria would need to be laid down before even starting. And the electronics…. with the need to buy multiple boxes or cards from different sources, I didn’t see a clear way to crystallize an easily-obtainable, “universal” solution. Finally, the whole lot had to be reasonably affordable but still offer great performance for the money.

The catalyst to resolve the dilemma came somewhat unexpectedly. miniDSP recently announced a two-way plate amp that incorporates their eponymous DSP functionality together with a B&O ICEPower module. While I generally prefer separate amps because of the flexibility to “reuse and recombine” into different projects, this new miniDSP plate amp does solve all of the problems of the electronics for this type of project: easy to build (nothing to build at all, in fact), easy to source, and affordable.

The plate amp would constrain the speaker to be a two-way. Constraints are good: they reduce the size of the decision space and enable forward progress to a solution. The two-way requirement also counts out open baffle speakers – while it’s certainly possible to make a two-way OB (and I’m working on one with the Involve/ER Audio mini panels), it’s trickier to get a good result and any such speaker will be of interest to a smaller niche of listeners/DIYers.

If it’s going to be a boxed speaker, though, I’d prefer a “uniform directivity” design – that is, a smoothly-varying off-axis response without the off-axis tweeter “flare” that is common to two-way designs. That immediately suggests a waveguide or horn on the tweeter. While waveguides used with compression drivers have gotten a lot of attention in recent years in the Econowave project, I didn’t feel that a digital Econowave clone would be suitable as a HifiZine project. (If you’re interested, though, check out “gainphile”s nicely-documented DSP Econowave.)

Tweeter

I decided that my preference would be an efficient dome or planar tweeter with a waveguide. The Vapor Audio Aurora is an example of such a speaker. The waveguide serves to limit dispersion of the tweeter at the lower end of its range and increase efficiency – or conversely, reduce distortion for the same output level.

But, designing my own waveguide – never mind machining it into the baffle like that – is not something I’m prepared to tackle. It would also work against the idea of a project based on readily-sourced components. I needed an off-the-shelf solution! Fortunately, one came to light, the Monacor DT-300 tweeter and matching WG-300 waveguide. I found a couple of favorable comments online, detailed measurements by Kimmo Saunista here, and use in a project by Troels Gravesen here (although modified, as he often does). Kimmo’s measurements indicate good distortion performance and a possible crossover point somewhere around 1.7 kHz.

Other tweeter/waveguide options

Since writing this proposal, I’ve been keeping an eye out for other tweeter options for this project. Here are some:

• Pellegrene Acoustics has waveguides for a number of tweeters, including the Vifa XT25, SB Acoustics SB29, and Transducer Labs NC29.
• Creative Sound Solutions has the Planar2 horn-loaded ribbon. There are no published measurements, but it does look a lot like the driver used in the Soundfield Audio Monitor 2 (click link for measured off-axis response).
• John Krutke reports a good result with the SB29RDCN-C000-4 in a modified PE 8″ waveguide (search for “February 5, 2010”). The downside here of course is the need to modify the waveguide.

Woofer

An 8″ woofer seems most likely to match the directivity of the tweeter/waveguide combination at the crossover frequency. I worked through the online catalogs of Madisound and Parts Express to find a readily-available 8″ woofer that meets the following criteria, roughly in decreasing order of priority:

1. Smooth response through the midrange i.e. suitable for a 2-way
2. Sufficient bass extension and output to be usable without a subwoofer
3. Reasonably high sensitivity to match the tweeter as closely as possible
4. Reasonable price
5. 4 ohm impedance to get the most power from the ICE module

In the end, I chose the Seas U22REX/P-SL, which matches all of the criteria except the last. At around $150, it’s a bit more expensive than I would have liked for this project, but relative to total project cost, not excessively so. This fairly new driver is the lower midrange in the Linkwitz LX521 but its parameters and response curve also make it well-suited as the woofer in a two-way. The (nominal) 8-ohm impedance means that the amp module will deliver only (a nominal) 60W into the woofer, but the sensitivity of the driver makes up for it: peak output levels are adequate at 108 dB (at 1 m), or around 105-6 dB once baffle step compensation is included. At these levels, all of the drivers I looked at are excursion limited below 100 Hz or so anyway.

The final piece of the puzzle is the cabinet design. The woofer is flexible enough to be used in either a sealed or ported enclosure. I decided to do both. The main speaker module will be a heavily-braced monitor of around 265mm wide, 320mm deep, and 440mm tall (10.5×12.5×17.5″). This is a sealed box with the plate amp mounted on it and an internal volume of around 20 liters. In this configuration, the speaker has an f3 of around 80 Hz, making it perfect for crossing over to a subwoofer or for use in an HT system. (And, of course, with DSP you can use a Linkwitz transform to make it any f3 and Q that you want, within power and excursion limits.)

The base of the monitor will be removable so that it can be replaced by a stand that turns the speaker into a ported enclosure. This opens up the interesting possibility of having different alignments by swapping out stands. An alignment fairly close to the standard QB3 will be best for rock music; a lower tuning frequency will give an underdamped alignment for a tighter bass; and a larger enclosure with an EBS (extended bass shelf) alignment but lower power handling will likely be favoured by classical music lovers. (Scale below goes from 20 Hz to 200 Hz).

Summary

The total cost of this project starts at around $1000 per pair, plus shipping costs, plus whatever you want to spend on cabinet materials and finishing. So realistically let’s say around $1200-$1400 per pair:

• miniDSP plate amp x 2: $550
• Seas U22REX/P-SL x 2: $296
• Monacor DT-300 and WG-300 x2: $100-$150
• Wood, wire, and finishing: $50+

The result should be a speaker with good dynamics, the characteristic clarity of active speakers, and very good top-to-bottom performance without the typical compromises of a two-way.

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