Subwoofer Blending

I had also been considering REL, Velodyne, Thiel, Martin Logan, and other subwoofers to incorporate into my system. While the bulk of Velodyne's subs are probably not in the same league as the other brands that I mentioned, their new Digital Drive series subs are both very musical and very flexible. I am heavily considering a DD-12 to add to my system. I hope to audition it against the Thiel SW1 (which is more than twice the price), and while I do think that the Thiel is likely to be sonically superior, the edge on flexibility and controls has to go to Velodyne.

FWIW, where possible, I am a fan of running your mains full range, and keeping the sub crossover set low, and bring it in as necessary to load the room to your tastes.

Tom.

The very best subwoofer blending (for a mono sub or for stereo subs) comes from using a combination of crossover slopes that were developed from theory (not by me) and first proven in the early eighties in pro-sound installs. The THX company has taken these useful slopes and named it "the THX crossover", but did not invent/develop the concept:

You would apply a 2nd-order (12dB/octave) Butterworth filter to the ProAc speakers and a 4th-order (24dB/octave) Butterworth filter to the subwoofer, at about 60Hz.

Why?
Because at ~60Hz, the ProAc woofer is mechanically rolling off its output at 12dB/per octave. This would be measured best up close to the cone's surface (I realize this is a ported speaker).

Add 12dB/octave electrical rolloff to that acoustic (mechanical) rolloff and you get 24dB/octave rolloff.

A good subwoofer, measured close to its cone, has a nearly dead-flat amplitude response over the proposed 40-80Hz crossover range, before any electronic x-over filtering. Thus, when you give it an electrical 4th-order rolloff at that same 60Hz, you would measure the subwoofer's upper-range rolling off at 24dB/octave, to at least past 150Hz.

So you effectively have a 24db/octave highpass filter on the ProAc's, and a 24dB/octave lowpass filter on the sub.

Which is the only filter combination that results in those two drivers being 360 degrees out of phase -one wavelength- which means the peaks and dips of any shared continuous sine waves line up. It would mean that the sub(s) and the ProAc's are phase coherent.

But they would not be TIME coherent- those two filters make one driver start moving a half of that 60Hz's period (1/120th second) sooner, and the other start a half period later. Which is thus "one period (360 degrees)" of difference. They are "In Phase", yes, but the beginning and ending of a tone burst at 60Hz are not clean. This info is in any filter-theory book.

What also mucks this up from being a perfect blending is the sealed-box sub itself, w/o any EQ, is rolling off at 12dB/oct starting down at, say 25-30Hz. This adds more time delay (phase shift) all the way up past 100 Hz, a delay that also changes with frequency (decreasing with increasing frequency). The max time delay, BTW, is at the 25-30Hz tuning frequency, equal to 1/4 of 1/25th-1/30th second (1/100th to 1/120th second). If this is a ported sub, that 1/4-period delay occurs probably up at 60 Hz, for even more unwanted time-delay through the crossover region. Any EQ in the sub adds more time delay.

This means the sub probably sounds best placed a little closer to the listener (via tape measure) than the ProAc's woofer, because it already is 12-15 inches farther away, acoustically/mechanically, at 60Hz than the ProAc's woofer.

If one could obtain a sub that has its sealed-box resonance down at 10-12Hz, then that "12-15inch closer" spec would not be needed, as the sub's own inherent LF mechanical time delay would be nil up at 40 Hz (though delayed 1/40th to 1/48th of a second down at 10-12Hz). Do consider that, if you have two subs, an "equal" tape measure distance could very well be reached by swinging the tape over to the sidewalls, for the subs' placement.

But that 10-12Hz sealed sub does not exist, so for "even greater perfection" from a conventional sealed sub, one would have to modify the electronic crossover, by adding an "all-pass" lattice, a passive-component network (even more complicated for a ported sub) to it. An all-pass network adds time delays to a particular range of frequencies, without changing the amplitude response, except for the loss at every frequency from going though all those passive parts. It is part of any analog telephone system- the low and high tones of the voice are delayed by the capacitance and inductance of the long copper lines, and the lattice network selectively delays the middle tones, so that all tones arrive at the same time, albeit later.

So try 24dB/octave on the sub at 60Hz or so (somewhere between 50-70Hz) and 12dB/octave on the ProAc's at probably that same frequency, and place the sub a little closer to you than the ProAc's woofers. Crossovers having those slopes can be built for you by Marchand Company and by dbSystems. A "THX" crossover has those slopes, seen in a "THX" home-theater receiver, but is often fixed at 80Hz, for a sealed 5.25" two way, or for a small, ported 6.5" design.

How do you know it is "right", by ear? Run the range with string bass, listening for mumble and stumble. Then apply a steady kick drum (Fleetwood Mac/Dire Straits), which is ~50Hz, and listen to the attack when moving the sub closer/farther than "tape-measure equal". Check with Stereophile Test CD #1, by quickly jumping from track to track, like a series of "burps", so that you hear more of the direct sound, and less of the room's build-up (which fools a an SPL meter, compared to what you hear on the transients of music).

Based on a great deal of experience, this 12/24 x-over at 50-70Hz would be the quickest way to improve the blending. These slopes are different if your upper-range speakers were panel dipoles like Magnaplanars, BTW.

Best regards,
Roy Johnson
Green Mountain Audio

Hi, I have a general comment. More amps for the same money don't mean better sound than the same money spent on a "better" stereo or mono amp. You wont more push from you low watt amps I believe. If the single ended low watt sound is your thing and you wont more steam try what I did, also no cost for tube replacement etc. PASS ALEPH single ended amps probably have the sound you like and real horsepower to drive normal speakers but not very difficult speakers. I have mono 100watt ALEPH mod.2 and love them. Very Rare on used market but can be found ($3,000+used). The 30 watt stereo ALEPH is not the same weight and effortless sound and the 60watt stereo is good but not the = of the 100 watt mono. The 200 watt mono is going to cost a fortune in electric bills.

I believe a subwoofer is a mistake for most systems. I found that most speakers that go to 45hz with a decent 7 or 8in. woofer will produce plenty of base when the powersupply and sources are of high enought quality. Many products don't spend the $ on beefy powersupplies using first class parts. Your room size does effect base response in that a big room needed to have more air moved. 20 by 30ft. room will need need a 10in. driver of 87 to 90 db. with some real muscle before it to play a cello/stringbase conviencingly.

Last I believe you use a PLACETTE preamp. I am interested in them. Do you know anyone in the Chicago, Ill. area who owns one? Do you have the active model or the volume control alone? I am interested to hear more about this preamp. Thanks, in advance, for responding to my questions about the placette. Best wishes, good listening. Peter H.

Thanks for all the input; exactly what I needed. I'm now thinking about bi-amping (one amp for tweeters and another for woofers) as the 2.5s have damn good bass if given enough power. Think I'll start another thread for bi-amping advice. Thanks again, everyone!

I'm definately in the camp for running a speaker like your Proac full range. -- As to your question, however, my experience is that to get a good match, you need cross-over frequency, volume, and continuously variable phase control for a sub. I've never heard the piece myself, but another thread identified the Paradigm X-30 as a good control unit. It has low-pass filter, volume, and phase control for the sub and if you want to try the high pass filter approach for your Proac it has one selectable at 50, 80, and 120 Hz. Good luck.

I don't have a specific answer, but I would strongly suggest demoing subs and picking whichever one sounds more musical, regardless of the sub's ability to provide a high-pass output. REL does not feature a high-pass output, but they are very good subs and should not be struck off your list for this reason.

ACI speaker manufacturer offers a passive high-pass filter to go between the preamp and amp (at line level). This would only work if you connect the sub to the preamp at line level, and would clearly not work if you connect the sub to the power amp at speaker level (REL's favored approach).

Lastly I have heard of people getting a very large non-polarized capacitor and connecting in series with the main speakers. Off the top of my head I think you'd need a non-polarized capacitor, rated to at least 100 volts (to be safe) and with a capacitance of around 200 millifarads. Not sure how easy this is to find.

I think it may be best to run full range .. but if you can get the capacitors cheaply try it out.

Hi Tomryan. Most powered subwoofers usually include both a variable adjustable low pass filter for your sub and a switchable high pass filter with three or so settings for the feed to your mains. I can understand you wanting to lessen the load on your SET amp, but I would try as much as possible to avoid running your 2.5s through the high pass circuitry of the subwoofer's crossover.

I own ProAc Tablette 50 Signatures and Response 1SCs and love the midrange of these monitors. I have never even gotten close to acceptable results running any of my monitors through my sub's high pass filtering. The beautiful midrange of the ProAcs and Sonus Fabers I tried this with were completely lost with this method. It's tough enough retaining the midrange quality of these speakers and not muddying them up running them full range and rolling the sub off at 35-40Hz, so filtering both the mains and the sub at least for me is not acceptable.

I should note that using the method you're considering does work for me in my all NHT home theater system. To my ears they blend pretty seemlessly, but audiophile speakers can be another issue. My ProAcs are much more revealing of musical detail and soundstaging than my NHTs, so it's easier to compromise their qualities if I'm not careful in the setup.

For music I would probably avoid the Velodyne and look for a REL Storm III to match with your 2.5s. Once you get a powered sub you can always experiment with integrating it both ways, but I'm sure you'll prefer running your ProAcs full range. Is buying a second amp like the one you have and biamping a feasible alternative? Good luck!

The Velodyne's have an excellent high pass filter, the REL's don't. I'd suggest you look at the Velodyne SPL line. There's a 8", 10", and 12". The SPL 800 (8") does a great job for audio as long as you're not playing real loud. I agree that filtering your speakers is much better than full range, much less distortion and more dynamic.
Good Luck,
Curt