MATING SUBWOOFERS with “Mini-Monitor” Main Speakers

How to select and optimally mate subwoofers with mini-monitor main speakers – When space or decor constraints dictate the use of small “mini-monitor” as your main speakers instead of classic full-range speakers, you will need to know how to mate subwoofers with mini-monitors to supplement the bass in some manner if your goal is full fidelity.

The common way to extend bass coverage in a high quality 2 channel system is to add  (2) subwoofers to your mini-monitors. However, positioning those boxes generally means you may have to hide them in the room corners. Of course, many subwoofers are too massive for small rooms that are already restricted to mini-monitor main speakers, so try to 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.

We must next decide at what crossover frequencies to apply. If you’re using small monitors with ~ 5-inch woofers they’ll exhibit a rapid roll off approaching 80-85 Hz, so select a higher crossover, like 94 Hz. At that frequency you’ll also need to assure that your subwoofer is capable of a near flat output of 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 resonances 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. Anything bigger usually 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.

compact subwoofer
JL-Audio-E-Series

Clearly, you should select the optimum crossover frequency, consistent with 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. You should also filter the high-pass drive to your main speakers to reject signals below the crossover point. This latter filtration is particularly 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-Rileyaligned 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 acceptable!

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. The price at this writing is approximately $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 pass-band (to main power amplifier) and the low pass-band (to self-powered subs). The ±5 dB (in precise ±1 dB steps) front panel level controls are displayed for each passband, on each channel. These controls make it quite easy to trim and shape the respective gain settings as desired thereby optimally accommodating programs of a different genre. In addition, XM66 includes a damping control that permits fine tuning of the response at the crossover notch.   You will now be able to build in a slight (+1 to +2 dB) bump at the immediate hi/lo hinge point, to smooth over any perceived evidence of the passband transition.

merchand XM 66

Aurally blending subwoofers with main speakers can take up an endless amount of time with tweak-and-listen trials which can get utterly 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 ultimately 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 ~100 kΩ, so it’s fully load compatible with almost any preamp ever made.

The same holds true 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 an input impedance of ≥ 10 kΩ. To summarize, the Marchand XM66 crossover unit will integrate quite easily in your system. 

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