Optimizing Subwoofer Integration, Part 1

Welcome Classical Candor's newest contributor, Tech Analyst Bryan Geyer, whose article on "Optimizing Subwoofer Integration" should prove fascinating and informative to anyone interested in the subject of sound reproduction.

Mating subwoofers with "mini-monitor" main speakers in a 2-channel stereo (not "home theater") system

How to select and optimally blend small subwoofers with mini-monitors: When space or decor preference dictates the use of small "mini-monitor" main speakers (instead of big full-range speakers) you'll need to supplement the bass if your goal is full fidelity. The best way to do that is to add a pair of small self-powered subwoofers--and the best place to "hide" them is in the front corners of the room, flanked outside the main mini-monitors. This location generally offers acceptable cosmetic compromise, and it assures that both subwoofers are effectively positioned to partially cancel the inevitable modal reflections that muddy the low bass in home listening rooms.

The smallest self-powered subwoofer that I find acceptable is JL Audio's E-Sub e110. These subs are fully-sealed, and sum to about 1.8 cubic feet each; weight = 53 lbs. Bigger gets impractical, so check the linear dimensions of the e110 and use that as your guideline.

The JL Audio E-Sub e110 is capable of virtually flat output over the 30 Hz to 130 Hz range, and it's solidly built; it's a fine small subwoofer. Fully-sealed subs are inherently less fussy to position and orient than ported reflex or passive radiator type subs, and sealed subs are naturally easier to phase-sync with the output from your main speakers at the listening position. This latter benefit will materially simplify final tuning.

Next, decide what crossover frequency to apply. If you use small monitors with ~ 5 inch woofers they'll exhibit rapid falloff approaching 85 Hz, so select a higher crossover, like 94-96 Hz. At that frequency you'll also need to assure that your subwoofer is capable of near flat output up to a half-octave higher, e.g. to 130 Hz. If your subwoofer can't reach that high (many don't) you might have to pick a lower crossover point. Choose a compromise, but don't consider anything below 84 Hz. A lower crossover is not appropriate for mini-monitors of any cone diameter, and going lower always invites more room-related modal trash--disruptive resonance best kept below the crossover point.

Clearly, you should select a crossover frequency consistent what your main speakers can handle. You'll want to filter the low-pass drive, to the subwoofers, to reject frequencies above your crossover point. And you should also filter the high-pass drive, to your main speakers, to reject signals below the crossover point. This latter filtration is especially vital. You don't want to route power-hungry low-bass signals to mini-monitors that can't handle "heavy lifting," and the cleanest way to do so is to keep that low bass energy out of the main speakers' power amplifier.

The most effective way to assure optimum crossover is by means of a Linkwitz-Riley aligned 4th order (24 dB/octave) active crossover. That function is already self-contained in some of the premium high-end subwoofers (including the E-Sub e110). Lesser subwoofers generally provide simpler filtering, often just for the low-pass stage, and many of those are not full 4th order filters. Some subwoofers also include rudimentary high-pass filters too, but only with simple first or second order (6 dB, 12 dB/octave) attenuation slopes, and that's just not adequate. A clean, complementary crossover transition is of vital importance, and a Linkwitz-Riley aligned 4th order active filter is the best solution--but don't despair if your preferred subwoofer omits this important feature.

Why not? Well, because the best way to utilize such a crossover is to implement it externally, as a separate control box that's positioned with all of your other command functions. This will allow you to manage the subwoofer/main speaker blend from a single, central location. If this function stays buried inside each subwoofer, you'd then have to crawl to each separate unit to individually adjust the subwoofer/main speaker acoustic ratio. An external electronic crossover control eliminates that odious task. When this function is external, the subwoofers' internal crossovers should then be switched to their "bypass mode," rendering those internal filters non-functional. The crossover frequency and sub-to-main mix will then be set at the new external electronic crossover control.

Marchand Electronics, of Rochester, NY, offers a professional grade stereo electronic crossover, model XM66, that's ideal; refer http://www.marchandelec.com/xm66.html. The price at this writing is $850. It can be set, by the user, for any desired crossover frequency, and it provides a full 4th order (24-dB/octave) Linkwitz-Riley aligned slope for both the high passband (to main power amplifier) and the low passband (to self-powered subs), with ±5 dB (in precise ±1 dB steps) front panel level controls for each passband, on each channel. These controls make it quite easy to trim and shape the respective gain settings as desired to optimally accommodate programs of different genre. In addition, the XM66 includes a damping control that permits fine tuning of the response at the crossing notch. This makes it possible to build in a gentle (+1 to +2 dB) bump at the immediate hi/low hinge point to smooth over any perceived evidence of the passband transition.

Aurally blending subwoofers with main speakers by means of endless tweak-and-listen trials can get tiresome. There are more direct and precise ways to accomplish this critical final step; request my white paper headed "On Optimizing Subwoofer Gain & Phase Angle." This sheet describes how to accurately set the subwoofer's internal input gain and phase angle controls to assure that a phase coincident bass wave front of optimum amplitude is delivered at the designated listening position.

An external electronic crossover control should be inserted into the audio system at a point that follows the main preamp (or follows the master volume control if using a "passive preamp") and precedes the main power amplifier. The Marchand XM66 controller's input impedance is ~100kΩ, so it's fully load compatible with any preamp ever made. Ditto for any "passive preamp" that utilizes a stereo volume control of 10kΩ to 20kΩ, with no need for a unity gain buffer to load the passive stage when it can mate with the XM66 inputs via ≤ 2 meters of audio cable. That length restriction is normally not a problem. The standard XM66 is normally furnished with gold-plated unbalanced RCA-type input and output jacks; XLR-type connector sockets are available at additional cost. The XM66 output impedance is quite low, so it can couple to any power amplifier that exhibits an input impedance of ≥ 10kΩ. In sum, the Marchand XM66 crossover controller is easily integrated.

BG (December 2018)