Mating Subwoofers with “Mini-Monitor” Main Speakers in a 2 Channel Stereo (not “home theater”) System…
When space or decor constraints dictate the use of small “mini-monitor” main speakers instead of classic full-range speakers, you will need to supplement the bass in some manner if your goal is full fidelity. The common way to extend bass coverage is to add subwoofers, but positioning those boxes (yes, you’ll need a pair) generally means they’ll have to hide in the front room corners. Of course, many subwoofers are too massive for small rooms that are already restricted to mini-monitor main speakers, so limit your search to subwoofers that will fit the space available. Avoid ported reflex and passive radiator (“drone cone”) models. Sealed-box subwoofers assure more linear bass response, and sealed subs can be more easily phase-matched with your main speakers at the crossover point.
Next, decide what crossover frequency to apply. If you use small monitors with ~ 5 inch woofers they’ll exhibit rapid falloff approaching 80-85Hz, so select a higher crossover, like 94Hz. 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 you might have to pick a lower crossover point. Choose a logical compromise, but don’t consider anything below 80 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 hinge.
The smallest subwoofer that I can personally recommend is JL Audio’s E-Sub e110. It sums to about 1.8 cubic feet, and weighs 53 pounds. Any bigger tends to get impractical, so check the linear dimensions of the e110 and use that as your limit guideline. The JL Audio E-Sub e110 is capable of relatively flat output over the 30 Hz - 130 Hz range, and it’s solidly built; it’s a fine small subwoofer.
Clearly, you should select the optimum 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 this is to keep that low bass energy out of the main speakers’ power amplifier.
The modern and most effective way to assure accurate 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 far simpler filtering, often just for the low-pass (high-cut) stage, and many are not full 4th order filters. Some subwoofers also include rudimentary high-pass (low-cut) filtration too, but only with simple first or second order (6 dB, 12 dB/octave) attenuation slopes, and that’s just not adequate.
Effectively accomplishing a clean, complementary crossover transition is of vital importance, and a Linkwitz-Riley aligned 4th order active filter is the optimum solution—but don’t despair if your preferred subwoofer omits this feature.
Why not? Well, because the best way to utilize such 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 option. 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, 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/lo 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 wavefront 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 crossover’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 10 kΩ to 20 kΩ, 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 generally not a problem. The XM66 is normally furnished with gold-plated RCA-type in/out connectors; balanced 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 input impedance of ≥ 10 kΩ. In sum, the Marchand XM66 crossover unit is easily integrated.