2006 Center Channel Speaker Project

(by an anonymous Klipsch Heritage Fan)

We needed a center channel that could keep up with our four Klipsch Cornwall II speakers. We were using one of the old surrounds I built back in 2004, each of which has two 5 inch woofers and a silk dome tweeter. Good little speakers but too small to keep up with the mighty Cornwalls. I wanted something that would match the Cornwalls as closely as possible, but we do not have the space to accommodate another Cornwall as a center channel. We looked at a conventional center channel design with a horn midrange and tweeter flanked by two woofers. We would have flown this above the monitor on a shelf attached to the wall studs. However, we decided to build the center channel into the monitor stand. The monitor stand had four glass shelves. The notches for the shelves can be seen in the picture below. This meant separating the components of the speaker into two sides, one for bass and one for midrange and treble.

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In brief, here are the components: Two Klipsch K1077 8 inch woofers. These have 'bucking' magnets which are essential to operate near a CRT monitor like ours. The woofers operate in a vented box tuned to 32 Hz. The box padding is actually viscoelastic "memory" foam which has a slower rebound than conventional foam and is much denser. This was an experiment. I'm still withholding judgement on the outcome.

An Atlas PD-5VH midrange driver on a Klipsch K401 horn. This is the same driver and horn combination used in the Klipschorn, which is the top of Klipsch's line.

A Crites CT-125 tweeter. This uses an Eminence APT-50 driver on a custom horn. This is much better than the K77/T35 tweeter Klipsch still uses in the Klipschorn, and the CT-125 is sold as a drop-in replacement. I find it at least as good as the K79 tweeters Klipsch put in the Cornwalls. One curious note: Eminence's spec sheet, in a tiny note, says that the red terminal is actually negative. I reversed the leads and it made a small and inconsistent difference, so I left them in "correct' polarity as per Eminence.

The crossover is based on the Klipsch Type A, with the addition of an inductor on the midrange output to roll it off above 5000 Hz. The crossovers are 400 and 5000 Hz. The crossover sits inside the midrange/tweeter module, under the midrange horn.

The stand has ribs that cannot be removed-they are supporting 125 lbs of monitor. So both modules have to rise above the lower ribs. I made seven blocks, four for the bass module and three for the mid/treble, to raise them just above the stand ribs. The mid/treble module has that very long horn and driver. It has to be angled slightly to allow the horn and driver to stick out the back. The very low position of the stand would mean that the midrange and treble would be aimed at the ankles of the listeners. I had Tim sit on the loveseat and hold a string tied to the back of the stand, holding it up to his ear. I stood a piece of cardboard upright on the floor, against the string, and marked the angle. This turned out to be 10 degrees, which became the tilt of the mid/tweeter baffle. It points right at the occupants of the loveseat.

Before I made the modules, I brought the drivers near the monitor to see if their magnets would interfere with the CRT's scanning. The woofers didn't, and the midrange driver is not only self-shielded but far away from the front of the CRT. The tweeter is not shielded, and so that is why it is below the midrange horn.

I got the grille fabric from Duracrest. They have been Klipsch's supplier for many years. This is very close in appearance to the grille cloth of the front Cornwalls. We chose one large grille to hide the notched "picture frame' molding at the front of the stand. We didn't want to explain to everyone why those notches were there.

We had a piece of Sintra cut for the grille panel. Sintra is a PVC foam material that is much more rigid than the office-type foam core material. It is commonly used for outdoor signs. My sign man, bless him, not only cut the sheet to fit but laminated it with black vinyl film at no extra charge. We still had to cut two 'windows' in the sheet to let sound out. In retrospect, the 1/4 inch Sintra was too high tech. We should have used 1/4 inch plywood. Sintra in this thickness can be cut with a utility knife if you are feeling very strong and prepared to use up several blades. It cannot be cut with power saws because it melts. Maybe a variable speed saber saw might have cut it without melting the edges. I wore out my hand and arm making the window cuts and Tim had to finish the final cut-throughs. We scuffed the glossy black vinyl to reduce amount shown through the grille cloth. I went over the cut edges of the 'windows' with a black Sharpie so the white material will not show through the grille.

We attached the grille cloth to the frame with Beacon Adhesive Fabri-Tack, which is sold in hobby stores. I have used this to make several costumes. It dries clear and fast. Much less trouble and fumes than traditional contact cement. The frame is wedged between the stand's glass top and the floor.

 In our first listening test, the midrange and tweeter were too loud compared to the woofers, which was not surprising. The Klipsch Type A crossover was designed for the Klipschorn which has a bass horn woofer, much more efficient than the vented design here. We moved the midrange and tweeter lugs to the next lower taps on the autotransformer and all was well. The autotransformer is on the right top side of the crossover picture. It matches the output of the midrange and tweeter is 1 dB steps merely by changing two connections.

 After we installed the center channel, we used our Yamaha receiver's YPAO self-measuring system to optimize response of the entire home theater. The YPAO uses a small microphone at the listening position and emits a series of test tones through the speakers, adjusting level and frequency response to flatten the output of the system. We ran this twice and the center was just too loud. We used the manual mode and reduced the center volume down to the same level as the flanking Cornwalls, giving the best results. After listening to several pieces, it seemed that the overall system sound was too tubby in the 7 channel stereo mode. Part of this is due to the Cornwalls, which are known to have excess output around 80 Hz. In the 7 channel stereo mode, there are four 15 inch and two 8 inch woofers loading the room, which can overwhelm it. The receiver allows us to parametrically equalize any of the channels. We reduced the center channel's output in the 63 Hz band by 1.5 dB (Q =1) and this cured most of the tubbiness.

 The result is a better 'curtain of sound' and more natural speech on video programs. There is no sense of strain at very high volumes, which we rarely use. This system can easily overwhelm our 16 x 25 listening room with a vaulted ceiling and could be louder still, if we wished.

Acknowledgments:

Crites Audio: http://search.ebay.com/_W0QQsassZbobcritesQQhtZ-1

Information on Cintra:

Apex, Jr (woofer supplier): http://www.apexjr.com/

Atlas Sound: http://www.atlassound.com/

Eminence:

Paul W. Klipsch

1904-2002