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.

*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)

No comments:

Post a Comment

Meet the Staff

Meet the Staff
John J. Puccio, Editor, Publisher, Reviewer

Understand, I'm just an everyday guy reacting to something I love. And I've been doing it for a very long time, my appreciation for classical music starting with the musical excerpts on the Big Jon and Sparkie radio show in the early Fifties and the purchase of my first recording, The 101 Strings Play the Classics, around 1956. In the late Sixties I began teaching high school English and Film Studies as well as becoming interested in hi-fi, my audio ambitions graduating me from a pair of AR-3 speakers to the Fulton J's recommended by The Stereophile's J. Gordon Holt. In the early Seventies, I began writing for a number of audio magazines, including Audio Excellence, Audio Forum, The Boston Audio Society Speaker, The American Record Guide, and from 1976 until 2008, The $ensible Sound, for which I served as Classical Music Editor.

Today, I'm retired from teaching and use a pair of bi-amped VMPS RM40s loudspeakers for my listening. In addition to writing the Classical Candor blog, I served as the Movie Review Editor for the Web site Movie Metropolis (formerly DVDTown) from 1997-2013. Music and movies. Life couldn't be better.
Karl W. Nehring, Contributing Reviewer

For more than 20 years I was the editor of The $ensible Sound magazine and a regular contributor to its classical review pages. I would not presume to present myself as some sort of expert on music, but I have a deep love for and appreciation of many types of music, "classical" especially, and have listened to thousands of recordings over the years, many of which still line the walls of my listening room (and occasionally spill onto the furniture and floor, much to the chagrin of my long-suffering wife). I have always taken the approach as a reviewer that what I am trying to do is simply to point out to readers that I have come across a recording that I have found of interest, a recording that I think they might appreciate my having pointed out to them. I suppose that sounds a bit simpleminded, but I know I appreciate reading reviews by others that do the same for me -- point out recordings that I think I might enjoy.

For readers who might be wondering about what kind of system I am using to do my listening, I should probably point out that I do a LOT of music listening and employ a variety of means to do so in a variety of environments, as I would imagine many music lovers also do. Starting at the more grandiose end of the scale, the system in which I do my most serious listening comprises an Arcam CDS50 CSD/SACD CD player, Goldpoint SA4 Passive Preamp, Legacy Audio PowerBloc2 amplifier, and a pair of Legacy Audio Focus SE loudspeakers. I also do a lot of listening while driving in my 2016 Acura RDX with its nice-sounding ELS Studio sound system through which I play CDs (the ones I especially like I rip to the Acura's hard drive so that I can listen to them whenever I want) or stream music through the system using my cell phone. For more casual listening at home when I am not in my listening room, I often stream music through the phone into a Vizio soundbar system that has remarkably nice sound for such a diminutive physical presence. And finally, at the least grandiose end of the scale, I have an Ultimate Ears Wonderboom Bluetooth speaker for those occasions where I am somewhere by myself without a sound system but in desperate need of a musical fix. I just can't imagine life without music and I am humbly grateful for the technology that enables us to enjoy it in so many wonderful ways.
Bryan Geyer, Technical Analyst

I initially embraced classical music in 1954 when I mistuned my car radio and heard the Heifetz recording of Mendelssohn's Violin Concerto. That inspired me to board the new "hi-fi" DIY bandwagon. In 1957 I joined one of the pioneer semiconductor makers and spent the next 32 years marketing transistors and microcircuits to military contractors. Home audio DIY projects remained a personal passion until 1989 when we created our own new photography equipment company. I later (2012) revived my interest in two channel audio when we "downsized" our life and determined that mini-monitors + paired subwoofers were a great way to mate fine music with the space constraints of condo living.

Visitors that view my technical papers on this site may wonder why they appear here, rather than on a site that features audio equipment reviews. My reason is that I tried the latter, and prefer to publish for people who actually want to listen to music; not to equipment. My focus is in describing what's technically beneficial to assure that the sound of the system will accurately replicate the source input signal (i. e. exhibit high accuracy) without inordinate cost and complexity. Conversely, most of the audiophiles of today strive to achieve sound that's euphonic, i.e. be personally satisfying. In essence, audiophiles seek sound that's consistent with their desire; the music is simply a test signal.

William (Bill) Heck, Contributing Reviewer

Among my early childhood memories are those of listening to my mother playing records (some even 78 rpm ones!) of both classical music and jazz tunes. I suppose that her love of music was transmitted genetically, and my interest was sustained by years of playing in rock bands – until I realized that this was no way to make a living. The interest in classical music was rekindled in grad school when the university FM station serving as background music for studying happened to play the Brahms First Symphony. As the work came to an end, it struck me forcibly that this was the most beautiful thing I had ever heard, and from that point on, I never looked back. This revelation was to the detriment of my studies, as I subsequently spent way too much time simply listening, but music has remained a significant part of my life. These days, although I still can tell a trumpet from a bassoon and a quarter note from a treble clef, I have to admit that I remain a nonexpert. But I do love music in general and classical music in particular, and I enjoy sharing both information and opinions about it.

The audiophile bug bit about the same time that I returned to that classical music. I’ve gone through plenty of equipment, brands from Audio Research to Yamaha, and the best of it has opened new audio insights. Along the way, I reviewed components, and occasionally recordings, for The $ensible Sound magazine. Recently I’ve rebuilt--I prefer to say reinvigorated--my audio system, with a Sangean FM HD tuner and (for the moment) an ancient Toshiba multi-format disk player serving as a transport, both feeding a NAD C 658 streaming preamp/DAC, which in turn connects to a Legacy Powerbloc2 amplifier driving my trusty Waveform Mach Solo speakers, supplemented by a Hsu Research ULS 15 Mk II subwoofer.

Mission Statement

It is the goal of Classical Candor to promote the enjoyment of classical music. Other forms of music come and go--minuets, waltzes, ragtime, blues, jazz, bebop, country-western, rock-'n'-roll, heavy metal, rap, and the rest--but classical music has been around for hundreds of years and will continue to be around for hundreds more. It's no accident that every major city in the world has one or more symphony orchestras.

When I was young, I heard it said that only intellectuals could appreciate classical music, that it required dedicated concentration to appreciate. Nonsense. I'm no intellectual, and I've always loved classical music. Anyone who's ever seen and enjoyed Disney's Fantasia or a Looney Tunes cartoon playing Rossini's William Tell Overture or Liszt's Hungarian Rhapsody No. 2 can attest to the power and joy of classical music, and that's just about everybody.

So, if Classical Candor can expand one's awareness of classical music and bring more joy to one's life, more power to it. It's done its job. --John J. Puccio

Contact Information

Readers with polite, courteous, helpful letters may send them to classicalcandor@gmail.com

Readers with impolite, discourteous, bitchy, whining, complaining, nasty, mean-spirited, unhelpful letters may send them to classicalcandor@recycle.bin.

"Their Master's Voice" by Michael Sowa

"Their Master's Voice" by Michael Sowa