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KIH #90 – Offshore bass accounts

  • Work smart, not hard. It could be the same thing. But today is about when it’s definitely not.

    Enter the precision active high-pass filter preceding your main amp and speakers. That sends the now eliminated low bass to an active subwoofer at exactly the same just inverted attenuation profile.

    In short, we shift the heavy labor of low bass to a proper brute like a 500-1’000-watt class D amp of extreme damping which is built into an active subwoofer where it direct-connects to a beast-mode woofer specialized in just the first two octaves of 20Hz-80Hz. Beast mode typically means a beefy surround for large excursions and a super-stiff thick and heavy membrane that can withstand large strokes without flexing or breaking up. Beast mode also means that such a driver could never meet a tweeter at 2kHz. It’s why you won’t find it in a 2-way monitor.

    But now this two-way monitor can be smaller and cheaper. Without low bass, it goes on vacation. If we high-passed our monitor at 60Hz with a typical 4th-order Linkwitz-Riley filter defined by its -6dB point, already a published -3dB point of 88Hz can apply. That’s a real baby monitor. Most will be good for ~50Hz – 3dB.

    By sending our low bass to an offshore subwoofer account, the voice coils of our monitor’s mid/woofer no longer pay taxes on it with heat. Bass that presents at a driver which can’t actually reproduce it extracts that tax payment regardless. Death and taxes. Voice coil heat drives up voice coil impedance. That means more resistance. Hello, dynamic compression.

    In a two-way monitor, the woofer is actually a mid/woofer. It produces bass and midrange on the same cone until it meets the tweeters around typically 2’000Hz. Whatever happens to that cone’s impedance reflects across its entire bandwidth. Chop off the most demanding bass which it couldn’t do in the first place. Lighten up some bass which it could do but still would work harder on than higher up. Presto: more dynamic range across its bandwidth, instantly!

    Because bass from a small mid/woofer demands big excursions to compensate for lack of cone surface, eliminating the need for big excursions reduces distortion. Chopping off low bass benefits dynamic range and micro resolution; instantly!

    But there’s more. The same applies to the amp which drives our monitors. This was brought home beautifully in a recent review of Menno van der Veen’s Trans-SE10. It’s a small ~€1’700 10-watt KT88 single-ended amp. I compared its performance in filter mode to running it conventionally with a full-range signal down to 20Hz.

    In filter mode, the sound of that small amp came unreasonably close to that of 200-watt transistor monos. It didn’t sound the same but wasn’t far off – certainly nowhere near what the price discrepancy would have suggested. When the active high-pass filter in front of it disappeared and low bass suddenly reappeared, the amp sounded clearly thicker and slower, showed lower separation and less micro detail. And now dynamic compression was a factor. For most intents and purposes, it hadn’t really before. Making the amp work a lot harder clearly didn’t sound as good!

    A friend of mine asked me a while back what if anything he could still do to improve his system. Knowing most of the system, I pitched my icOn 4Pro SE autoformer passive with custom hi/lo-pass filters. Dan runs Cube Audio Nenuphar so ~93dB 10” widebanders in a quarter-wave box augmented by dual Voxativ RiPole subwoofers fronted by a low-power custom David Berning tube amp. Because my hifi betting average with Dan was good, he trusted me again on this recommendation and ordered his unit with a fixed 50Hz filter so 10Hz higher than mine.

    “10% improvement” he enthused over the phone to indicate that the difference proved unexpectedly significant. “Nenuphar breathes more easily, resolution and microdynamics clearly improved, the Berning now cruises and bass is truly spectacular.” Words to that effect confirmed my own findings with sound|kaos Vox3awf monitors and a Dynaudio 2 x 9.5” active subwoofer.

    That’s our map for today’s terrain. Now we’re ready for the intended takeaway. Drum roll, please.

    Want to explore the world of low-power valve amps without the typical limitations? Copy our recipe. It creates ideal/idealized conditions for such amps to give their very best. They can prance around like show ponies with ribbons in their tails. They don’t become lumbering draft horses that pull a beer-barrel cart like beasts of burden. What’s more, your small amp can now be a modest sort. It needn’t cost so much. You’re no longer stressing its output transformer. In this scenario, all the immodest costly burdens have been shifted onto the active subwoofer. And hello, at €1’390 for the Dynaudio 18S I bought for this very purpose, your bass machine needn’t be gigantic or super dear either.

    All you need for the recipe to come off is a sub with a high-pass option on its analog output/s. If that high pass is fixed, it tends to be 80Hz. You will most likely fix the sub’s internal low pass to the same frequency. That’s it. If the sub has flexible high-pass values, they will likely start at 50Hz to perhaps extend as far as 200Hz. Now you get to decide where to set this sub/monitor hinge to get the most benefits for the speakers’ dynamic expression and resolution. If you want to filter at the most precise level, there are external analog or digital solutions.

    But today isn’t really about such details. Today is just about a very basic concept. It’s about how to get low-power amps to sound their very best. It’s all about making them work smart not hard; cruising not bruising; permanent vacation, not dreary 9-5. That’s it. Once that concept settles in, you’re ready to consider the small print of latency in DSP subs, where to best set up a mono sub and more. But all journeys start with a first step. Today that’s about the easily demonstrated benefits which a 2.1 system of monitors+sub has if you exploit filtering the main speakers through an active high pass which now also removes low-bass duties from their amp.

    This installment focused on the amp but obviously, the same applies to the speakers. They can get smaller and cheaper too without giving up bandwidth, SPL or performance quality. Work smart not hard. If you apply this concept strategically, you’ll have gains in size (smaller), price (lower) and performance (higher). Suddenly even lesser single-ended 45, 50, 2A3 and 300B amps become pedigreed contestants. They just need ideal conditions to thrive. Now you know how to create those. This simply goes beyond the old advice of just high-sensitivity speakers. Remember Dan’s Nenuphar. They and the elite amp which drove them still sounded better when the hard labor of bass was shifted to elsewhere rather than let the widebanders run wide open.

    Just imagine how much truer this becomes once the players involved get less costly and ambitious. You can enjoy loud undistorted playback with real low bass from smaller speakers and a smaller amp just by cleverly routing their signal through a quality but still compact subwoofer’s filter.

    Written by Srajan Ebaen

    Srajan is the owner and publisher of 6moons. He used to play clarinet at the conservatory. Later he worked in audio retail, then marketing for three different hifi manufacturers. Writing about hifi and music came next, then launching his own mag. Today he lives with his wife Ivette and Nori and Chai the Bengal cats in a very small village on Ireland’s west coast, between the holy mountain Croagh Patrick and the Atlantic ocean of Clew Bay in County Mayo’s Westport area. Srajan derives his income from the ad revenues of 6moons but contributes to Darko.Audio pro bono.

    Letters to the editor – Weeks #21 & #22, 2021

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