An Interview with Mark Kovach of Miracle Audio

LB:      Because the speakers are not full-range, you offer two models of accompanying wall-mounted subwoofer (RoseAnn and RoxAnne), and a subwoofer amp.  How do you integrate the mains and the subwoofer?

MK:     Larry, as you mentioned, both the RoseAnn and RoxAnne subwoofers are wall mounted, either in or on wall.  Most hi-end speakers sold today aren’t full range either (despite being hawked as such) and a simple FFT analysis will bear out that myth.

The implementation of a sub in hi-end audio has been a bone of contention since the inception of a higher audio “awareness” mostly because of the claim that the lower frequencies can be heard apart from the rest of the audio spectrum in the main stereo speakers. This is certainly true for an incorrect implementation, but it’s also true that many “Full range” speakers do a less than stellar job of integrating all frequencies, so I never understood the complaint about subs from the hi-end community even when they are correctly installed.  I guess the problem even today is that too few hi-end systems incorporating subs have them installed correctly and that slants the overall opinion against subs in general.

One big problem in sub implementation is utilizing those that contain a built-in amplifier.  I don’t understand how someone can have isolation devices for their CD player, preamp, tuner, amp etc., and then have a powered sub.  To me this is problematic times three.  First, you’re subjecting your sub amp to constant stress from the back wave of an up to18 inch driver that occupies the same box.  Even if the box is rigid and resonating below the lowest frequency, it is resonating and constantly shaking your amp like it’s a martini for James Bond.

Second, you’re subjecting your amp to a very large and powerful magnetic field generated by the speaker magnet.  I can’t begin to imagine how this field influences the amp’s circuitry, and not in a good way, especially if the circuit contains inductors or metal can components.

Third, because the powered sub needs power, you must contend with both audio cables and AC (power) cables in close proximity to each other. This is potentially a recipe for hum and buzz.

So to implement a sub, begin by purchasing a non-powered (passive) sub.  Both Miracle Audio subs are passive.

Most powered subs also include with the power amp circuitry, a crossover (filter).  Some are simple low-pass and some include both high- and low-pass filters.  To properly utilize the Hi/Lo filters, you must route your main stereo signal through the sub on its way to the main L/R speakers.  This is another path towards integration disaster, if the quality of the filters is less than stellar, and I’m sad to say that most are, as are the amps included in most powered subs.

The next step in proper sub integration is a high quality filter, most definitely a low pass, but you may need a combination low/high-pass (more on this later) type, and you also must get yourself a good quality power amp.  It ceases to amaze me as to how many times I’ve seen a second rate amp driving a good passive sub in an otherwise well thought-out system.  Now I’m not saying that the sub amp needs to be equal in quality to the main stereo amp, but it should be able to supply enough current quickly and efficiently to the sub.  Full frequency response is not necessary as it will most likely be supplying audio to no higher than 2 kHz including overtones that are greatly attenuated even in a fundamental 110 Hz bass wave, which by the way is probably the absolute highest crossover Fc (cutoff frequency) at which you should have your low-pass filter set.  I prefer more in the range of 40-70 Hz, but some speakers such as electrostats, ribbons and satellites aren’t capable of lower frequencies and must be crossed-over at 200 Hz, and in some rare cases even higher.  These speakers are so fast at producing non bass frequencies that they require a special crossover that can delay their output until the less quick subs can catch up to them for proper time alignment and integration.

Miracle Audio’s Sub-Ultima is a reasonably priced 200W amp designed to drive a passive sub, and includes a built in low-pass crossover.

Now to adjust everything into one homogenous sound, there are two possibilities.

The first is to use just the low pass filter.  The advantage is that you route your non-bass audio directly to your main stereo amp and speakers without the signal first passing through another piece of equipment; the high-pass filter.

It’s good to know the realistic low-end full frequency response for you main speakers. Many specs say 20 Hz +/- some dB level but this won’t help much. You are looking for the lowest frequency that is equal in volume to all other frequencies being produced by your speaker.

I find this frequency by using SpectraPlus (http://www.spectraplus.com) a very sophisticated FFT analysis program (and more) in combination with a calibrated microphone, noise generator (included in SpectraPlus) and computer running at 3 GHz.  This set up is also a fool-proof way to seamlessly integrate your sub.  I highly recommend it but if your budget is tight, there are many other programs and mics available that should work fine.  I would spend money on an analysis rig before I’d buy new interconnects, speaker cables or power cords.

Using the FFT graph, you can see where your main speakers are bass lacking and how much Sub you need to fill in the missing frequencies.

A less accurate alternative is a calibration and test CD that can provide test tones from 20 kHz down to 20 Hz in ½ octave increments.  You can use your ears to determine when the sound seems to get softer (i.e., lowers in volume) but the more accurate way would be to use a decibel meter in conjunction with your test CD.  Many apps are available for smart phones that can do this function.  Although I’m skeptical regarding the quality of the microphones on these phones, it’s probably better than using your ears.

Once you know the roll-off of the main speaker, you can set the low-pass Fc on the filter to that value, say 50 Hz.  Even though my speakers do not roll off at a steep slope, I prefer the slope of the Low pass filter to be set steeply at 24 dB /octave or even 48 dB /octave if possible.

Set a comfortable listening level for your main speakers, run the noise generator or a CD with lots of bass, like a church organ, and bring up the volume of the sub amp until the bass frequencies are filled in on the FFT graph, or you like the amount of bass in the mix.

On many good filters including the one on the Miracle Audio Sub-Ultima amp, in addition to Level (Volume) and Fc, there are adjustments for Damping and Phase (delay) that can really mess you up and create havoc in your integration, or they can really tweak your system into a beautifully homogenous sound.  This is for another time and place, but I wouldn’t even touch these without an FFT program as a check.

Now if that’s not enough variables, let’s add a high-pass filter to the main speakers.  Although you must pass your precious sound through another piece of equipment, there is a tremendous advantage to this configuration, assuming that your filter is of high enough quality for the advantage to outweigh the disadvantage of having to use another piece in the loop.  Let me expand upon this.

All frequencies that cannot be reproduced by your speakers will be wasted — mostly as heat — and can cause distortion and other undesirable side effects.  This is because many speaker manufacturers, either to save money or through some overly optimistic evaluation of their product, don’t limit the low end of their speakers, thus wasting energy at the power amp.  By using a high-pass filter to send only those frequencies that your speaker can reproduce, you make both your speakers and power amp more efficient.  You may be pleasantly surprised at how much more natural (less forced) your system sounds when it is configured thusly, especially if your power amp is a low wattage valve type.

To test this, simply run your system with high-pass filter set to the proper Fc (lowest frequency it can reproduce) with the sub volume at zero.  As you switch from filter to bypass, you should not lose any bass, and may really like what’s going on in the mids and highs.  If not, the high-pass filter may not be of sufficient quality to warrant it’s use, and you may be better off using only the low-pass filter.  If you like what you hear, then add the sub back into the mix.  Some further tweaking may be needed.  If you are using a Miracle Audio Divinitive linestage and decide you want to upgrade to a three-speaker system with sub, an optional analog high-pass filter and sub amp line driver can be installed into the unit.

The above technique is how we incorporate all Miracle Audio on-wall speakers with our on-wall subs, and it can be a good starting point for integrating any three way system, although it becomes somewhat more complicated and time consuming as you move your speakers off the wall and onto the floor.  I must again strongly suggest a FFT analysis set-up to best accomplish this.