SDA-CRS/CRS+ Something Different

westmassguy
westmassguy Posts: 6,850
in DIY, Mods & Tweaks
Ever since I acquired my CRS+s, I've been amazed at what these little boxes can do. I had
done an initial, cost constrained re-cap on the crossovers, and all the usual upgrades we do. After acquiring my 2As, and doing all the upgrades on them as well, it occurred to me that one upgrade was not really possible with the CRSs, Damping. From the factory, all they came with, was a wad of Dacron on the side where the crossover is located, and that's it. Nearly all the other SDAs, Monitors, and RTAs had rolls of Dacron behind the woofers, extending all the way to the top of the cabinets in most cases. As part of the cabinet upgrades, we remove the Dacron, and install strips of BH5, No Rez, or Sonic Barrier on the rear wall, and re-install and re-position the Dacron. You can't do that with the CRSs. The best you can do is cut and fit small pieces on BH5 et al. and stuff the Dacron back in one side. Not the ideal solution. I had been tossing around a few scenarios for adding additional damping material to mimic what we do with all the other vintage Polks.

My 1st thought was cutting and fitting BH5 on the back wall, adding a doughnut shaped piece of BH5 to the back of the Passive Radiator, then stretching and tacking a double layer of Bonded Dacron directly behind the woofers. The problem with that is the possibility of over damping the enclosure and the fact that we never install Dacron anywhere near the Passive Radiator. My second idea was to create some type of baffle that would mount behind each woofer. I could attach BH5 to the fronts, which would face the back of the woofers, and it would give me a gap where Dacron Batting could be inserted. The baffles would have to be large enough to actually do something, i.e. catch some of the mid frequency back waves from the woofers, but also be small enough not to block, interfere or constrain the larger sound and pressure waves. The intent after all, is to clean up the midrange handled by the woofers, without sucking the life out of the music.

In the interim, my friend Darryl @voltz had acquired a pair of CRS+s, and asked if I would do his crossovers for him. I agreed, and tossed the idea of experimenting with the damping, being a trooper, he said sure. I had planned on rebuilding my crossovers at the same time. We had agreed to do Sonicaps, Mills Resistors, TL Modification, and replace both Lo-Pass inductors with Jantzens. I'd also be supplying Erse Inductors for the Sub Bass Drive, Cardas Binding Posts, and reworking the Interconnect with SpeakONs on his. Darryl supplied me with the same Flat Oval Cable he would be using for the speakers. An 800VA Dreadnaught would also be constructed.

The Crossovers were straight forward. The spool inductors were eliminated; new mounting plates were made, and secured with stainless hex head cap screws and nylon standoffs. The new Jantzen, and Erse Inductors were unwound to the exact values needed. All components were attached with 3M VHB tape, and Nylon Wire ties. SpeakON connectors and Cardas posts were installed on the Binding Post Cups. Darryl wanted the deluxe Dreadnaught, with Copper Shielding, Dynamat, and cast SpeakONs.

For Damping, I decided to go with the baffles. I cut 7" X 7" pieces of 3/16" Masonite. These were glued and bolted to 5" X 5" steel angles. The Masonite was sealed with lacquer, and BH5 applied to the face. I purchased some 1" Bonded Dacron, and cut two 19" X 19" sections, and folded them in half.
Do to circumstances; I was unable to complete my own Crossovers, so I decided to install Darryl’s Crossovers, one set of dampeners and the Dacron in my cabinets for a test run. This would allow me to hear the difference between the stock, and new damping treatment.

I installed the crossovers, and the new Damping Baffles in the left speaker about 5" back from the woofer openings (I believe Darryl eventually placed them 6" back). The Bonded Dacron was installed in the left cabinet, and the stock Dacron in the right. I connected the new Dreadnaught, and fired everything up. I let them breath for a while before playing some familiar tracks. The Stereo Image was dead center, a good sign. I immediately noticed a smoother sound quality from the left speaker. The right speaker sounded as if it were shouting during certain passages. Keep in mind, my woofers were original, and my RDO-198-1 Tweeters had already been broken in. Obviously all the new components needed a good deal of burn-in, but overall, I was very pleased with the improvements. Top to bottom, the left sounded very well balanced, while the right speaker sounded off, more forward in comparison. When compared to my 2As, the left speaker was very similar, while the right, not so much. Although my concern for over-damping the enclosure was well founded, I believe they sound just about right. You can experiment with the alternatives to BH5. No Rez, and Sonic Barrier, may well react differently. I know Sonic Barrier has 5 different thicknesses available, so you can play with the amount of damping. My goal was to duplicate as closely as possible, the additional damping we install in other upgraded vintage Polk cabinets. I believe that goal was met.

There is one caveat; severe space constraints. If you're using the original Spool Inductors as mounts for the Crossovers, Tony's @gimpod boards, or custom Air Core Inductors, the baffles may not fit without some creative rearranging.

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Home Theater/2 Channel:
Front: SDA-2ATL forum.polkaudio.com/discussion/143984/my-2as-finally-finished-almost/p1
Center: Custom Built forum.polkaudio.com/discussion/150760/my-center-channel-project/p1
Surrounds & Rears: Custom Built forum.polkaudio.com/discussion/151647/my-surround-project/p1
Sonicaps, Mills, RDO-194s-198s, Dynamat, Hurricane Nuts, Blackhole5
Pioneer Elite VSX-72TXV, Carver PM-600, SVS PB2-Plus Subwoofer

dhsspeakerservice.com/
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