Powerful center-channel speaker

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Project by ferstler posted 10-12-2008 02:15 AM 4983 views 0 times favorited 12 comments Add to Favorites Watch

The left and right channel speakers in this system are Allison IC-20 models. They are among the best of their kind ever made and finding a factory-made center speaker that could work with them was impossible. So, I built one of my own, shown in close up, without a grill screen, in the second photo. If you look through the window in the full-room photo you will see my shop out back where the center speaker was made.

The center speaker uses Allison tweeters and midrange drivers, just like the IC-20s to the left and right, but with only two apiece instead of four like the bigger speakers. The woofer is a single Allison IC-20 unit. The IC-20s have two such woofers each. The crossover network, built from scratch by me, is a hybrid design combining values used in several different Allison models over the years. While the center speaker is only half as powerful as the left/right mains, it is more than powerful enough to do the job in this room.

The cabinet top, bottom, and sides of the center system is made of .75-inch pine, stained and coated with polyurethane. The front and back are mdf, painted black. The tweeters and midrange drivers are located in a sealed sub-enclosure, so there is no need for rubberized gasketing around their mount perimeters to seal the woofer enclosure space. The front panel is angled back 4 degrees to allow the vertical array to focus its somewhat narrow vertical dispersion cone at seated ear height across the room. The left and right IC-20 systems are tall enough to not need front-panel tilting. (The 45-degree side-angled-panel design of the IC-20 units, which each have a vertical row of drivers like the single plane of the center speaker, gives them superb horizontal dispersion, and the center unit, due to the design of the Allison drivers, also has wide dispersion.)

All three front systems can go pretty deep into the bass, but the very deepest bass is still shunted to large Velodyne and SVS woofers in the two front corners of the room. A big Rane equalizer fine tunes the response to within 2 db between 90 Hz and 16 kHz. If you look above the big window you will see a black-colored housing that can lower a 4×8 foot front-projection screen. The projector further to the rear of the room, out of the photos. A third photo is posted to show the equipment rack for this installation. Power to the center and system control is via a big Yamaha receiver, although power to the IC-20s comes from a big Carver power amp. The center speaker is biamped, with a 260-watt total feed. The total installation power is in excess of 2000 watts.

I have published over 200 audio magazine articles over the years and authored four books dealing with audio-video and recording quality, as well as helping to write and edit the 2005 edition of The Encyclopedia of Recorded Sound. In all my experience, this 8.2-channel installation is the best I have ever heard, period, with the center unit that I built easily able to run with the big main speakers.

Added two more photos. One is of the new equipment rack. I downsized by removing some processors, the old laserdisc player (it was moved to my smaller system), one of the two DVD players, the Carver power amp, and the crossover network for the second subwoofer. The second subwoofer is gone, too, and put into storage. The downsizing was done for the sake of my wife, who would be utterly at a loss with what to do about servicing the stuff if I kicked off. The wife also replaced the couch and chairs, and the new photo shows them, plus the way the room looks with the drape pulled and the projection screen pulled down. The left-rear part of the room is also now shown. The small TV on top of the shortened equipment rack is a portable that we can move anywhere in the house to view. Use it mainly for late-night TV and newscasts.

Howard Ferstler

12 comments so far

View mtnwild's profile


3474 posts in 3554 days

#1 posted 10-12-2008 04:56 AM

You are wired and set. Nice set up. Great job.

-- mtnwild (Jack), It's not what you see, it's how you see it.

View Bigdogs117's profile


1864 posts in 3648 days

#2 posted 10-12-2008 05:08 AM

Great job! Very nice equipment. Thanks for posting.

-- Rusty

View woodworm's profile


14468 posts in 3617 days

#3 posted 10-12-2008 09:00 AM

Yes! you got the power..

Thanks for the post.

-- masrol, kuala lumpur, MY.

View Callum Kendall's profile

Callum Kendall

1918 posts in 3730 days

#4 posted 10-13-2008 12:17 PM

Great job!

Thanks for the post


-- For wood working podcasts with a twist check out

View Dusty56's profile


11819 posts in 3715 days

#5 posted 10-15-2008 05:53 AM

WOW >>>>Very intense system you have there and thank you for all of the details : )

-- I'm absolutely positive that I couldn't be more uncertain!

View ferstler's profile


342 posts in 3547 days

#6 posted 10-20-2008 01:22 AM

One way I check the voicing of the speakers I have built is to make use of an AudioControl SA-3051 real-time analyzer. The unit includes a 20-second averaging feature and I basically move my measurement microphone slowly over a roughly 1×1×5 foot area near seated ear height at the listening couch in my main AV room while the RTA averages the speaker-generated energy hitting the microphone over that area. (I use this room for evaluating both the speakers I made and also any speakers I reviewed for magazine publication articles.) The energy source used is decorrolated pink noise, which is equal energy per octave. The decorrolation helps to prevent coherent reinforcement in the bass, which allows the unit to better average total power. This technique minimizes hot spots and sanding-wave artifacts and gives a decent idea of the power input from a speaker (or stereo pair of speakers) into the listening room. A multitude of multi-angled and averaged measurements in an anechoic chamber would be better, but the technique I use is actually quite good and correlates decently with some of the anechoic work I have seen with speakers I have reviewed.

Howard Ferstler

View gadgetman's profile


2 posts in 2292 days

#7 posted 03-13-2012 05:04 PM

Nice system indeed!
Though I am partial as I have a similar set-up for my Main Theater system. I just found one of my IC-20 woofers (one of the inverted ones) surround was in sad shape. Yet all others look to be fine with no signs of degradation in near future. Not sure why only one failed. Just wondering if you’ve had any similar issues. .. To predict future I might expect. Searching net for similar issues I found your posting on this site and it caught my interest. I realize this is now years later and I may not get any answers.
If yo get this… If you get this… I’ll expound more a bit more.
I’ve only owned these IC-20’s about 10 +/- or so years. Luckily I did get them still new (though old) in OEM sealed boxes from a guy who had them (and some other smaller Allison’s he had already unloaded) stored for future use … which later was not practical for his needs. I was searching a resonance issue with that speaker and figured maybe a speaker driver was loose. I did find a number of mounting screws amongst both speakers which needed some tweaking. But sadly also found the degraded surround. Just sent it out for rework and hope it is returned to normal. I wonder why Allison didn’t build these as Left Right set of twins with normal and inverted woofers swapped between the two. May not matter but I was just wondering.
Beautiful job on the center build! After some experimenting I found a suitable center with an Aerial CC3 B which I Bi-amp like the IC-20’s. I see you used Allison Fours for sides and rears. Nice! I considered the same but didn’t as I had an extra set of Ones ( for sides) and Threes (for rear) and used the Fours in a secondary system. For the real bottom n main theater I have 3 Velodyne ULD-18’s used as Frnt Lft, Frnt Rt , and in rear LFE ( signals courtesy of Lexicon MC-12) . I bought all 6 Velo ULDs which Sony had at their Manhattan recording studio some years ago when they were remodeling. I gutted them, reinforced the cabinets, and had all cabinets 6 refinished in Mahogany before reassembly. Two are in secondary systems and one is still stored for future use.

View ferstler's profile


342 posts in 3547 days

#8 posted 03-13-2012 09:58 PM

Hey, it is great to find another IC-20 owner. Only about 50 pairs were made, by the way. Mine are over 20 years old and I refoamed the woofers some time back, coating both the insides and outsides of the foam surfaces with liquid butyle rubber (obtained from Roy Allison) to make them last a bit longer than typical. The Allison surrounds (like most others of that era) are urethane foam that has a life of anywhere from 10 to 15 years, depending on the humidity and temperature. I got my surrounds from Simply Speakers, down in Florida. I have also refoamed a number of speakers from local Allison owners who are friends. Odd that one of yours deteriorated faster than the others, but perhaps there were some impurities in that piece.

I have known Roy Allison for decades and learned a lot about speaker design from him. I also have published two books on AV and two more involving recording reviews, and also helped to edit and did the technical writing for the second edition of “The Encyclopedia of Recorded Sound” and also “The Routledge Guide to Music Technology.” Published over 150 magazine articles on audio, too. Done with all of that now.

My own IC-20 models are considerably modified. First, I reconfigured all of the woofers to face outward. The push-pull design is only going to be important if the systems are handling the low bass, and even then the acoustic advantages are minimal. In my case, I now have two home-built tower/cylinder subwoofers in the room, replacing the Velodyne F1800 (which I sold to a friend) and the SVS (which I broke down, selling the TC-Sounds driver it contained). The cylinders were made out of Sonotube concrete post molds. Both Hsu Research and SVS have used similar materials, and the tech rep at SVS helped me calculate the acoustic needs of my home-built subs.

Those new subs are 68 inches tall and a foot in diameter and use Dayton Titanic MKIII 12-inch drivers. In that room, they play plenty loud enough and can get down to 20 Hz with ease. Power is courtesy of a new Crown XLS1000 sterero power amp that puts 350 watts into each sub. (They are ported and actually tuned to 18 Hz.) To really smooth the bass, they are equalized by an ART EQ351 mono equalizer. The three front speakers are equalized by a Rane THX22 (the IC-20s) and an AudioControl C-131 (the center speaker). I check the EQ by means of an AudioControl SA-3051 RTA, which also used with doing speaker testing for magazine product reviews.

Those Velodynes of yours have to be superb. I did review some of the 15- and 12-inch HGS models that came along after the F series, and no subs I ever reviewed had less distortion than the Velodynes. With most music this is no big deal, but with seriously heavy stuff like big pipe-organ presentations the servo feature does make a subtle difference, at least below about 25 Hz.

I did an almost complete rebuild of the IC-20 crossover networks, keeping only the chokes. (Chokes last forever, caps do not, and resistors probably should be changed out periodically, too.) The originals were mounted on removable modules in the back panel, but my new ones are on large boards that are internally mounted on the cabinet bottoms. I prefer to have the components spaced further apart than on the original module, and the original was so compacted with components it was impossible to remove and replace items in it, anyway. Another reason I got rid of the push-pull feature with the systems is that the reverse tubes would not clear the huge, new polypropylene capacitors in the rebuilt networks. (The original network had half the caps poly styles, but the large-value ones were conventional dipolar jobs, which are physically smaller than poly versions; the rebuilt networks have all poly caps.) Poly caps do not sound any better, but they last longer and have greater power handling.

I also rebuilt the networks in the Model Fours, converting the first-order high-pass filters to second order, and then installing polyfuses similar to what are in the IC-20s to protect the drivers from potential problems. Those are old drivers and replacements are not easy to find. I also refinished the Model Four cabinets.

Finally, the center speaker I originallly built (and pictured) had been replaced by a more svelte version. It is made out of cedar (the sides, top, and bottom) and mdf (front and back) with the cedar 1.5 inches thick. The height is the same, as is the vertical MTTM driver array (mids and tweeters), as is the cabinet width, but the 10-inch woofer has been replaced by an 8 incher. That is big enough, considering the 90-Hz crossover point.

The equipment cabinet in the original system was also replaced. It was a cheap, store-bought unit and I replaced it with a model of my own design made mostly out of cedar, with shelves of MDF. I think that I posted some info about this project, the subwoofer project, and the new center speaker project somewhere on this site a while back. I also have a second system in another part of the house and I think I have a posting that deals with its home-built speakers also on this site somewhere. Go take a look.

Finally, to make my wife happy I had the paneling in the room covered with plaster and we painted the room beige. The carpet was also replaced with a beige version, and the bookcases were replaced with much better versions stained mahogany. Nope, I did not build the bookcases. I love to build some things out of wood, but building a seven big bookcases was not something I cared to fool with, especially since my wife was in a hurry to get new bookcases in there and I work slow with woodworking projects. The video projector was also removed (a new HD version would be too expensive for this retired guy, but I at least have a 56-inch set in another system in another part of the house), as was the pull-down screen above the window. The room does look better now. Some photos are attached, and you can even see that Crown amp on the bottom shelf of the new equipment cabinet.

PS: there is an Allison chat group on Yahool that you might be interested in. I think you can learn about it at:

Howard Ferstler

View NBeener's profile


4816 posts in 3201 days

#9 posted 03-13-2012 10:08 PM

This is nothing but a HUGE tease, to me !

I wanna’ come listen !!!!

-- -- Neil

View ferstler's profile


342 posts in 3547 days

#10 posted 03-14-2012 12:51 AM

I am in Tallahassee, Florida. If anybody does want a listen, just phone me to arrange an appointment. I love visitors, and I am home most of the time anyway, being retired.

Incidentally, I mentioned that the cylinder subwoofers were a foot in diameter. They are actually 14 inches in diameter, with a total interior volume of 5 cubic feet. The four-inch ports (exiting at the bottoms) are each 20 inches long. I mentioned that the subs were 12 inches in diameter in my previous post, because I am considering building two more, with the new ones 12 inches in diameter and about 58 inches high. They would be used in my smaller (home-theater) system in another part of the house, replacing a Hsu VTF-3 MKIII that I would then sell (this is a superb sub, but I think using dual cylinder jobs would look better in that room, even if they performed no better), and would make use of 10-inch versions of the Dayton Titanic 12 inchers that I used in the bigger versions. Subs like this are easy to build, and cheap to build, too (well, the outboard amp can be a bit costly, as are the drivers), and something an amateur woodworker can do well with rather simple tools.

The trick is to get my wife to release the money, which she is not about to do for a while, because we (1) still have to pay for the room upgrade and (2) have to replace some savings we spent on a new car for my disabled sister down further south. Money is tight these days. Some photos of the sub details are attached, as is a shot of one of the IC-20s after it was refurbished by me. The room wall in the IC-20 photo had not as yet been plastered over. I think that I posted some project photos of these units elsewhere on this site.

Howard Ferstler

View gadgetman's profile


2 posts in 2292 days

#11 posted 03-15-2012 04:45 PM

Hey Howard:
Thanks for the reply with valuable info/education. I had no idea so few IC-20’s were made. That makes them that more special. I have a tendency to prefer things a bit different, less common, and of low production.
When I took my inverted woofer out, I did notice the (or one of the) crossover board(s) mounted to back between the two woofers. I also noted the bit of routing done to accommodate the inverted woofer edge in back board. Now you have me thinking of swapping the inverted ones outward. But things like that will be when I’m retired… maybe a year or so away. And BTW, yes I recall your name in articles read when younger. Considering your background, owning a set of the same speakers as you makes them even more special too. I also have two NHT based systems… One (in MBR) with 2.5i Frnt, 1.5 Sds & Rrs, Aerial CC3, Center and one ULD-18. Other in exercise room is smaller and older with NHT 2.1’s Frnt, 1.1’s Sds and Rrs, 1.1C (center) and Two VMPS “Original” subs. My Living room system is a mixed bag for sure… Allison Fours front and rear, NHT HDP2 sides, Polk CS10 center, and one UL-18 (makes a good end table as well resting next to recliner for my magazines etc.) In my Detached garage I have a temp set up with 5 old Klipsch KG 2.2’s ( I think that is right #) till I time have to upgrade to 7.1 when I make more room and sturdy mounts/shelf to put my old Cornwall’s up front & high (ceiling is 11’) and angled down and maybe upside down. Or I may do a swap with some spare/stored Allsion Ones (2 sets of) and one set of Threes (w/ beat up cabinet).
I should have mentioned, in my family room main theater it’s much different than yours…(T-1 we call it as I’m now up to 4 added secondary ones) Alll Allisons are mounted high on walls and upside down of course. Coming up with a bracket design to properly support as well as tilt the IC-20s downward (to match the DYI 96” screen angle of ~ 9 degrees) was a bit challenging. The tilt was appropriate since the screen (and speakers) are quite high since screen is above a 40” Sony XBR CRT which also has the Aerial Center on top of it. The smaller and lighter Threes were much easier to mount. The Side Ones are actually just resting … one on the high fireplace mantel and other on top of a high (DYI) stereo component cabinet which for whole house music supplying and is totally separate from all the HT electronics. The room has cathedral ceiling which allowed high mountings… and as I said all Allisons are upside down.
I’d love to keep chatting here but since I’m still with corporate America, I have many things to tend to today so can’t chat much. Do you have a direct email I could further chat (maybe send pix if/when more appropriate) or is that just not done here?

View ferstler's profile


342 posts in 3547 days

#12 posted 03-15-2012 06:09 PM

Hi, Gadgetman,

I can be contacted at:

I correspond with scads of individuals, although not as many as when I was writing regularly. Doing the Encyclopedia editing and writing work also involved me doing short biographical sketches of over 100 audio notables (many of whom I already knew, but also many that I had to link up with for the first time), and so I ended up with a huge correspondence file.

Since “retiring” I have not corresponded with many at all (nothing like being a has been!), but I do love to exchange emails with a few people, and since you seem to be both interested as can be in audio (and video) and endowed with some very good gear and interesting ways to use it, I would be glad to share ideas with you.

That wall supporting the two IC-20s must be a solidly built piece of work.

When you send me an email, make sure the header says something that will keep me from deleting it, unread, as spam. Ninety percent of the stuff that comes in to my address is spam. Geeze, what a world.


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