On Upgrading What’s Analog…

By Bryan Geyer

THE POSIT—Recent technology advances make it practical to upgrade select analog functions in many conventional stereo systems. It’s now possible to…
…dramatically extend the range and enhance the linearity of low bass response.
…materially reduce the mass of the main speaker enclosures.
…clarify the midrange response by isolating it from low bass modulation.
…eliminate archaic high-level passive crossovers that reflect only low order filter slopes.
…enable precise line-level active crossovers with full 4th order (-24dB/octave) attenuation.

BACKGROUND—The pursuit of good sound is evolutionary, but the conviction that high fidelity = big speakers persists. Low frequencies have long wavelengths (28 feet at 40 Hz), so big transducers, in big enclosures, have always been considered essential to reproduce, propel, and propagate big bass. Regardless, monstrous monkey coffins that require prominent front-and-center placement look absurd in domestic living rooms, so reasonable compromise is vital. The usual course is: (a) downsize the speakers, or (b) always entertain outside, or (c) cram the system into a “man-cave”, even though the main LR offers superior acoustics (lower Schroeder frequency), or (d) use headphones only. These are odious options; let’s pursue better.

WHAT’S CHANGED—In the mid ’90s, the push to popularize home theater became dependent on finding a practical way to recreate Godzilla’s footfall while squeezing the mass out of the additional speaker enclosures. This paradox looked hopeless until design guru Bob Carver, then at Sunfire, proposed his astonishing new (1997) self-powered “True Subwoofer” as a viable solution. Market acceptance was slow until Sunfire’s patent grip was softened; then the trade piled on. The new subs could pump out pure (low harmonic content) 30Hz sine wave bursts at pressure levels exceeding anything previously envisioned. Today, relatively compact subwoofers with only a single 10" or 12" diameter driver (e.g., https://www.jlaudio.com/collections/home-audio-e-sub/products/e112-gloss-home-audio-e-sub-powered-subwoofers-96279) can produce lower, louder, and cleaner bottom bass (the two bass octaves below 80Hz) than the very best, biggest, and most expensive full range floor-standers ever built by anybody, anywhere.

Tempering this marvel is the reality that these new self-powered subwoofers excel at just one thing: They can handle those two bass octaves better than any conventional woofer. They’re able to do that because they’re expressly designed for a single, solitary mission. They have extremely deep, ultra stiff cones, long and compliant surrounds with extended x-mass capability, and they’re driven by high power class D amps that apply carefully contoured equalization. Unlike the woofers in a costly full range floor-stander, the “sub” doesn’t have to produce any appreciable output above ~ 140Hz. Conversely, a conventional full range floor-stander is designed with the expectation that it must also cover the full low-to-middle bass range, commonly extending to frequencies approaching ~ 800Hz, and do so with good linearity. That’s a stringent additional assignment.

TODAY—If the intent is to achieve optimal fidelity, we need to reconsider how stereo systems should be configured. Clearly, paired stereo subwoofers* should handle the bottom bass octaves. The main stereo loudspeakers can then be reduced in size to reflect the fact that they will no longer have to reproduce those subterranean sounds. Stand-mount main speakers with 6" to 8" drivers have proved absolutely optimum. Excellent bookshelf-size speakers, like Harman’s Revel Performa M126Be (https://www.audioholics.com/bookshelf-speaker-reviews/revel-performa-m126be), or the Revel M106 (https://hometheaterhifi.com/reviews/speaker/bookshelf/revel-performa3-m106-2-way-bookshelf-monitor-loudspeaker-review-part-one/), come to mind, and they confer welcome decor relief. The related subwoofers are best placed near the front corners of the room, at or near floor level, where they’re generally easy to accommodate. There’s simply no need, anymore, for monstrous four to five foot tall main speakers that are packed with multiple 10", 12", or 15" woofers. At least, not unless you’re personally committed to high-end electrostatic-type speakers, such as those championed by Sanders Sound Systems. (In that event, please see https://classicalcandor.blogspot.com/2019/09/on-elephants-in-room.html.)

In addition to the cited decor benefit, separating the low bass from the upper bass presents a compelling aural upside. In a conventional setup, the main woofer and its power amplifier handle both signals, so there’s often some mild modulation blur of the upper bass or lower mids when the score calls for a sudden burst of big bass thunder. Routing that sound directly to the low-pass amplifiers, inside the self-powered subs, means the main speakers will now be unaffected. The main power amplifier will never see those low bass signals when they’re appreciably below the crossover notch. Eliminating this potential midrange modulation is one of the most vital rewards that you can claim when you add subwoofers and install an external active crossover. Some listeners feel that the midrange benefit is even more audible than the subwoofers’ bass extension. Your own impression might be a bit program-dependent in sensing these complementary improvements, but it’s clearly evident when the score fits**.

MODERNIZING CROSSOVERS—A further concern is that conventional full-range speakers impose the need for accurate driver-level crossover networks. Such filters are fussy to design and awkward to assemble. They’re wholly dependent on precisely defined R/L/C components that must pass high current signals to low impedance loads. Tight tolerance passive parts of this unique nature are rare—most have no other fundamental application in the scope of the electronics trade, so they’re often difficult to obtain in the exact values that are needed. The inevitable consequence of this difficulty is compromise, and compromise breeds inaccuracy†. Even when they’re effectively designed and constructed, these high level crossovers often prove inadequate because they’re typically just first order or second order (-6dB or -12dB/octave) filters with gentle attenuation slopes. Full fourth order (-24dB/octave) passive crossover networks would be more suitable, but they become impractical (too complex, physically prohibitive) for high level (low impedance) applications.

The solution to this crossover dilemma is to move that function from its classic position, between the power amplifier outputs and the various driver inputs (where it must process high level signals), to an earlier point, prior to the power amplifier, where the components will see only lower line level signals.This preferred location assures ready access to lots of stable, tight tolerance R/L/C components of virtually any desired value. When combined with active op-amp circuitry it’s then possible to form highly selective Linkwitz-Riley type filters with full 4th order attenuation slopes. The penalty implicit with inserting the crossover in this line-level position is that a separate power amplifier will now be needed to connect each crossover output (high pass and low pass) directly to its designated driver. That means operating in “bi-amplifier” mode, a form of connection that confers useful advantages; e.g., better load (driver) damping; also the ability to independently dictate the signal amplitude sent to each power amplifier.

A fully external (meaning separately enclosed and powered) active crossover controller is invaluable when adding supplementary self-powered subwoofers. The active crossover’s high-pass outputs can feed the main stereo power amplifier + main speaker system, while its low-pass outputs feed the line inputs on the self-powered subs. (The active crossover thereby supersedes any built-in low-pass filtering that might be a part of the sub’s internal circuit, so the subs should then be operated in their selectable “bypass mode”.) This setup facilitates convenient adjustment of the “mains-to-subs” dominance (the relative output levels of the main speakers compared to the subs) from a single, central location. Without this crossover controller you’d need to crawl to each individual subwoofer and separately adjust their input sensitivity controls—and repeat that crawl every time you want to tweak the mains-to-subs settings or alter the relative channel balance.

An external active crossover controller with variable frequency capability will allow the user to position the crossover notch at a point that’s optimum for the main speakers in use. For most subwoofer setups, this will involve a setting that’s somewhere between ~ 76Hz and 100Hz. (Lower crossover frequencies are not advisable, regardless of the main speakers’ perceived bass capability.)

Loudspeaker systems with internal line-level active crossovers + self-powered class D amps are already in common use as near-field monitors; also as desk-top setups. Designs of this nature offering higher power output have also appeared, e.g., the “Kii Three”. Clearly, this trend will accelerate. We’re going to see lots of analog circuitry integrate with the loudspeaker, and some of the speaker makers are likely to merge with those that currently make power amplifiers. Of course, vacuum tube power amps—already a moribund species—will then fade forever. Ditto for big solid-state quasi-class A power amps and heavyweights that resemble engine blocks. Niche products will always exist, but it’s doubtful that those who make them will prosper and survive, so be wary about where you hitch your wagon.

THE PREAMPLIFIER—Refer https://classicalcandor.blogspot.com/2019/12/on-controlling-volume.html. Amen.

Footnotes…
*Paired stereo subwoofers (not a solo “shared bass” subwoofer) are essential for acoustic advantage; refer https://classicalcandor.blogspot.com/2019/03/basics-about-bass_20.html.

**Re. “when the score fits”…is a phrase that brings to mind a subwoofer quirk that you need to know. Many (all?) of the companies making self-powered subs provide the useful ability to select automatic “signal sensing” as a means of activating the sub’s internal power amplifier; i.e., to auto-awaken the sub from its passive sleep state. In sleep mode, the sub stays on, but it dissipates minimal power until it perceives that it’s time to rumble. The sub then goes into active mode, and remains active until there’s no input signal for an appreciable duration, e.g., 20 or 30 minutes. An initial problem is sometimes evident with respect to the maker’s signal-sensing level. Bear in mind that the sub can only hear very low frequency signals because its line-level input path is through a low-pass filter. When there’s persistent heavy bass (as with virtually all pop music today), the sub will awaken quickly. But when the bass content is evident only where Mozart scored it, that sub might not awaken (go active) until you’re a third through an extended concerto. Classical music just doesn’t exhibit the same pounding bass line that persists with pop. Unfortunately, the folks that make subs think classical music is extinct, so they normally set the signal sensing to trigger at a higher (less sensitive) level than what’s optimum for classical format. In many cases the original factory setting is as much as 14dB to 16dB too high (too insensitive). So—when you buy a sub, communicate with your supplier. Have the maker set your subs to auto-activate when The Lark [is barely] Ascending. (I bow to Mr. Vaughan Williams.) If they fail to do this you’ll then need to return your subs to the source, and have a factory guy readjust the trip point (warranty return, maker’s expense). With all of the subs that I’ve seen, this setting is always an internal adjustment. Some subs might now make this tweak externally accessible, but internal-only is more logical because it’s a “set once” consideration, and external access invites misuse. Home-based DIY readjustment is definitely not feasible. The internal class D “plate amp” has complex multi-layer boards + tiny surface-mount (not “through-hole”) components, and that makes this simple task too risky when no schematic is at hand.

Be assured that there’s no downside related to increasing the subs’ activation sensitivity. When your external active crossover controller’s low pass outputs (to the subs’ inputs) cease sending a source signal (for the specified time lapse) to the subs, they’ll automatically revert to their dormant sleep mode, exactly as intended. The only instance in which this might not happen is if your crossover controller suddenly became intolerably noisy—with noise so annoying that you’d be fully absorbed with that fix before noting that the subs didn’t go to sleep.

†It’s only natural that those who own big full-range floor-standing speakers would assume that the internal high level crossovers buried inside their costly purchase are both optimum and accurate. Regardless, their faith is likely misplaced. Accuracy is certain to be deficient in the case of passive networks that have accumulated appreciable use because passive high level crossover components drift. High level operation = high level stress. (Refer paper headed “On Crossing Over,” at https://classicalcandor.blogspot.com/2020/01/on-crossing-over.html.)

BG (April 2021)