A Progress Report (Audio Tech Talk)

 by Bryan Geyer

Today, at a time when so many of us have experienced the audible benefit of properly implemented subwoofers, it seems archaic to assign a single driver as the sole bass source in a serious full-range loudspeaker. The spread from the bottom of the low bass passband (20Hz to 80Hz) to the top of the mid-bass passband (80Hz to 400Hz) is simply too wide for one transducer to handle, and the logical way to address that limitation is to split the 20Hz to 400Hz span, and apply separate power amplifiers to drive separate loudspeakers. Each of the assigned drivers can then be optimized as needed, with stiff, long-throw, piston-like air pumps for the ultra-low bass, and flatter, faster woofers for the middle bass. Implicit differences in efficiency would be of no concern because, with each leg driven by a captive amplifier, the respective levels can easily be balanced—or not—as desired. Overall bass levels could readily be adjusted to compensate for prevailing acoustics, as well as for the whim of the listener.

Giant single-driver bass design was initially popularized at the 1939-’40 New York World’s Fair, but we can do better today, and make everything look less cluttered. Compact class D power amplifiers, utilizing multi-layer boards and surface-mount components, can now be buried inside the loudspeaker enclosure. More efficient (also more precise, also less expensive) high impedance analog crossover* and equalization networks can be tailored to extract peak performance from a designated driver, and then blend smoothly with the ensuing upper bass and treble stage. Of course, these same desirable assets can also be implemented by digital means, and that cost-effective alternative is very popular today, despite the need to introduce yet another cycle of analog-to-digital and digital-to-analog conversion. This further complexity is avoided with the all-analog approach, so I personally favor pure analog simplicity, where less is more. But my personal bias is warped by the fact that I can’t test and verify what’s happening when the digital processing is implemented. Indeed, some of the digital simulation jargon seems artificially contrived**, but I’m an “old school” technician in every sense. I don’t own the kind of instruments (or the smarts) needed to appraise digital manipulation of an analog signal.

So who’s at the forefront of this improvement trend? Who is leading the charge to provide electronically augmented loudspeakers? Well, one niche that quickly embraced the integration concept is the mini-monitor makers. They commonly integrate customized power amplifiers with their speakers, but forego the lowest bass. That’s implicit with desktop expectations. The companies that produce hi-end full range loudspeaker systems can best mirror these improvements. Some already offer floorstanders with integrated subwoofers and multiple internal amplifiers. Some utilize digital signal processing (DSP); others might stick with classic analog design. Staffer Bill Heck has previously reported on his upscale speaker system from Legacy Audio; he uses it with his Wavelet 2 digital processor and says the sound is sensational. (So is the the price.) Editor Karl Nehring also favors hi-end Legacy Audio products. It’s obvious that this trend is active and evolving. More entrants should follow, maybe with models for modest budgets and smaller rooms. 

Stay tuned—check reliable reviewers, e.g., sites like Audio Science Review (https://www.audiosciencereview.com/forum/index.php) and Audioholics (https://www.audioholics.com/) — and stash away some savings; Nirvana may be near!

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*A Linkwitz–Riley filter ( https://en.wikipedia.org/wiki/Linkwitz–Riley_filter) that provides -24dB/octave attenuation is not particularly complex or costly, but tight precision is essential to ensure that the critical - 6dB down locus coincides precisely for both (low-pass and high-pass) of the filter sections, and that it is positioned at the desired crossover frequency. In practice, this is best achieved by “cherry-picking” the critical R/C components, so production gets messy when accuracy is vital. Digital control apps will tend to artificially obscure such production inaccuracies, but the output will reflect the full extent of any error.

**A DSP control application that I once monitored responded to my inputs by simulating graphic bar charts to mimic the impact of each command, just as if reporting a test result. Pure pseudo-science.

Addendum (by KN):

Bryan raises some important issues here that are well worth consideration. Indeed, it is a really tall order to ask a single driver to handle the frequency spectrum ranging all the way from deep bass up through the midrange – something’s gotta give. As he points out, one viable alternative approach is a good pair of two-way speakers augmented by one or better yet a pair of carefully matched subwoofers. There are many subwoofer manufacturers who offer models with not only built-in amplification and control settings, but also apps that allow tailoring of the system setting through your mobile phone or tablet.

 

Bill Heck and I have taken another route, opting for large multi-way loudspeaker systems that assign the lowest frequencies to what are in effect built-in subwoofer systems (a pair of 10” drivers crossed over at 180Hz in each channel of Bill’s system, a pair of 12” woofers crossed over at 120Hz in mine). To clarify, neither Bill’s Legacy Signature SEs nor my Focus SEs employ any internal digital amplification or crossover circuitry; they are both standard passive speakers with normal analog crossovers. Bill does employ the Wavelet 2 DAC /Preamp / Processor; however, that is a device entirely separate from the speaker. Moreover, it can be used with virtually any speaker, not just those from Legacy Audio. You can learn more about the Wavelet 2 in Bill’s review, which was in two parts: Part 1 is here and Part 2 is here.

 

Finally, I’d like to mention that there are several loudspeaker manufacturers that are employing advances in both digital amplification and signal processing to offer loudspeaker systems with amazing capabilities. One such example is the British manufacturer KEF, who offers several speaker models that are not only powered, but also incorporate Bluetooth, wi-fi, streaming services, etc. For example, the KEF LS60 Wireless (pictured) is triamped (100 watts Class AB for the tweeter, 100 watts Class D for the midrange, 500 watts for the woofers). It incorporates a 0.75” tweeter coincidentally mounted inside a 4” midrange driver, plus four 5.25” woofers (two on each side of the cabinet). The speakers can be connected together wirelessly or wired for higher-resolution connectivity. Each speaker has an RCA socket for connect an external powered subwoofer. There is an available KEF app that can be used for all manner of control and tailoring for frequency response, room settings, listener profiles. etc. You can also control the LS60s with Roon or Apple AirPlay – it’s a whole new world, folks. 

I’m still amazed that I can walk into my listening room, sit down in my listening chair, open my iPad, pull up Amazon Music or Qobuz, and control my NAD C658 streamer/preamp right from where I sit and listen to hi-res music through my stereo system with so little fuss. Or that I can prop my feet up in the recliner in my living room, turn on the TV, grab the Roku remote, and watch a performance by the Frankfurt Symphony Orchestra or Belle and Sebastian whenever I feel the urge. Yep, it’s a whole new world…