Planning a Project With SDA-CRS+ guts...

markamerica

Hi folks!

I'm working on a project, and would like some advice, tips, or suggestions from the esteemed members of this forum. One of the things I've been playing around with is the idea of a center channel speaker to go with my various SDAs. Along the way, what I've decided is that from a timbre-matching point of view, almost every recommendation seems to suggest using a single CRS+ speaker. As some have observed, recently including Darqueknight here, the problem is that their proportions make it difficult to physically place them in their factory configuration. They simply are too big. What I've decided to do is to create something different, and I've begun planning it out, have acquired many or most of the parts, and am going to see how well it works. As I build it, I will take pictures so that I can provide a more complete build post. While I'm yet to build the first bit, and before I've committed to the way it will be built, I thought I'd ask for opinions, advice, and suggestions. I'll be happy for any, including those of the "don't bother" variety. Here's what I'm planning:

My television currently sits on a low flat rack. Its top, on which the TV rests, is removable glass over an aluminum tubular frame, and is approximately 24x40, although it is curved on the front edge, going from roughly 24" at the center to 21-1/2" at the sides.

Much has changed since this picture was taken, including the TV and the SDAs pictured would no longer be in the picture, and there is different hardware generally. The point is to show you WHERE my project is intended to land.

My goal is that whatever I build will not add more than 9 inches in height to the placement of the television. What I am intending at present will involve using the drivers and the tweeters from a pair of (1986) SDA CRS+ arrayed in a single horizontal line (the mids are MW6510s and the tweets are sl2000s). The left and the right will be on opposing sides of the curve of the front, so they will be on slightly different planes. I was left with a bit of a conundrum with respect to the passives, as there are a number of ways I could do this. What I am going to try first is to place a line of 4 6.5" passives across the back, and two on the sides(near the rear.) The cabinet will be divided left from right, and the way I'm building the center section, I will leave room for a crossover network and I will remotely mount the various inductors apart from the 0.4mHs in the tweeter networks. The 1.75mH and 1.5mH inductors for each speaker will be placed in various locations in the center space. The two 5.6mH inductors will be placed in a pocket on either side. The general layout will look like this:

My initial intentions with the crossover network are to essentially build the crossovers of two CRS+ speakers on a single board. I intend also to place a switch with which I can flip between stereo mode(utilizing the SDA circuit) when I have only the TV on, or in center channel mode if I'm using the HT preamp. In fact, I've thought that what I might do is to use a 12vdc triggered relay to make that switchover. If the HT preamp powers on, the 12vdc trigger will fire, and click, we're in center channel mode. Otherwise, it remains in SDA mode and that's the extent of it.

Cosmetically, I will wait to apply veneer&cloth until after I decide the final arrangement. In any case, I will be going for an SDA-like appearance, much in the style of the SRS's. I'll be doing some of the initial mock-up this or next weekend. In terms of the guts, I've gotten some less expensive capacitors from VR3, who has been very helpful. I'm getting some 5.6mH inductors from him for this project, as well as the other upgrades I'm slowly going through. In the end, I don't want to have spent a ton of cash on this until I have some indication that my "sda-in-a-box" will sound ok.

Okay, well, that's the current project here. Fire away...LOL

Mark

westmassguy

Looks very interesting. I too, like many other here faced that dilemma, finding a center to match the timbre of my SDA-2ATLs. I went a different route, and used a Monitor 10 crossover as my starting point. You can read about it here: http://www.polkaudio.com/forums/showthread.php?150760-My-Center-Channel-Project
My two concerns, would be the dual tweeters, and adding enough 6.5" PRs to tune the woofers properly.

nspindel

I'm in the "don't bother" camp, to be honest. Back in the days when I used my sda's as fronts for home theater, I used a Polk CS-350LS as the center channel. I thought the sound matched up just fine. I've seen them sell on this forum in the $100 range.

markamerica

Westmassguy,

I thought about the PR issue, but I'm willing to give it a go. It's good fun to experiment with anyway. Sometimes, you get lucky and even if it doesn't work out, you learn something... I checked out your build project, and that's pretty nifty. My cabinet will wind up being roughly twice as deep from the looks of it. I also made notes of your approach to the tweeters issue. One thing about the passives is that the way I'm building it, I will have room for two more, although the placement will be iffy on those last two. The volume in either side of the cabinet should be roughly in the ballpark of a crs+, maybe a little more, or at least that was the idea. The difference being that the CRS used a 10 inch pr. Problem is, I don't want it down-facing, and so the only way to stay in this general design envelope is to use smaller prs. I'm hoping that I can tune them, but I know that's going to be a challenge, which is part of the whole purpose anyway, right? Don't be surprised if I turn to you for ideas and suggestions as this project moves along. Building the cabinet begins this weekend. That ought to keep me busy until Thanksgiving at least. I'm about to find out how far my skills with a router have fallen off....LOL Anyway, before I go too crazy making it pretty, it's going to need to sound right, or why bother?

nspindel, Yeah, I know. I've thought about that approach too. It may be where I end up if I don't like how this goes, but since I've spent a little bit of coin now to accumulate the parts, I'll give it a try.

westmassguy

Mounting PRs horizontally is a no-no. They're not designed for that. A ported design might be easier to tune, than multiple PRs.
I've not been able to find T/S parameters on Polks 6.5" PRs, so you may have to go with another manufacturer.

drumminman

I'm in another camp - following your project with interest and hoping you succeed, but without knowledge/experience to contribute much meaningful insight.

I will say that from what I've read glass shelving is generally considered to degrade sound because it rings/microvibrates. Could a new entertainment rack or new shelving be in consideration?

Mr. Bubbles

I commend your enthusiasm and hope your final project works well for your needs and ears.

The biggest concern i would have is the placement of the multiple tweets as you show in your drawing. This layout is essentially two separate speakers side by side and NOT a center. This will sound like two speakers laid side by side and NOT a center. In a center speaker the sound should seem to originate from the center and progress outward. I would recommend either a single tweeter with as close to center driver spacing as possible, or maybe a progressive point source as in the larger SDA's. This is by far the biggest issue I see with your proposed layout.

next would be the passive situation as mentioned earlier. This is easily enough fixed by simply going ported instead of passive. It will however effect the enclosure dimensions.

Good luck with the project. And that is a lot of gear on those racks. I'm guessing multiple systems since i spot at least 3 EQ's.

markamerica

westmassguy wrote: »

Mounting PRs horizontally is a no-no. They're not designed for that. A ported design might be easier to tune, than multiple PRs.

So I've read. A ported design might be easier, but what fun is that?

westmassguy wrote: »

I've not been able to find T/S parameters on Polks 6.5" PRs, so you may have to go with another manufacturer.

I've not found the specs either. Spent a half day googling and came up empty. I even looked around for PRs with similar build characteristics hoping to get a "ballpark" idea, but really nothing. May be winging it a bit here.

westmassguy

markamerica wrote: »

So I've read. A ported design might be easier, but what fun is that?
I've not found the specs either. Spent a half day googling and came up empty. I even looked around for PRs with similar build characteristics hoping to get a "ballpark" idea, but really nothing. May be winging it a bit here.

Winging it is where I live most of the time. Good luck. Parts Express has some PRs, but nothing close to the 6.5" unfortunately. They jump from 3.5" to 8" with nothing in between.

markamerica

Drumminman,

Thanks. I have actually considered replacing the glass with more traditional shelving. All the glass does rest on rubber insulators, and I don't notice a problem, although I had an empty shelf at one time, and I could hear it. I suspect with no weight on it, the glass shelve's own weight wasn't enough to settle in the rubber. If the glass itself is resonating, I haven't noticed. That said, I'm not opposed to something new/different, but one project at a time.

markamerica

Mr. Bubbles, thank you for the response!

Mr. Bubbles wrote: »

I commend your enthusiasm and hope your final project works well for your needs and ears.

The biggest concern i would have is the placement of the multiple tweets as you show in your drawing. This layout is essentially two separate speakers side by side and NOT a center. This will sound like two speakers laid side by side and NOT a center. In a center speaker the sound should seem to originate from the center and progress outward. I would recommend either a single tweeter with as close to center driver spacing as possible, or maybe a progressive point source as in the larger SDA's. This is by far the biggest issue I see with your proposed layout...

Yes, I see your point. What I was trying to accomplish was to be a compromise, so that it could effectively work both ways. What you've managed to explain to this dim bulb is that a compromise isn't really possible, at least not in this layout.

Hmmm. Back to the drawing board.

Mr. Bubbles wrote: »

next would be the passive situation as mentioned earlier. This is easily enough fixed by simply going ported instead of passive. It will however effect the enclosure dimensions..

Yes, ports seem to be the consensus workaround. Trouble is, I know less than I should about ports, except that everything I ever owned that had ports managed to be invaded at some point by one or more of the little stinging scorpions we have around here.

Mr. Bubbles wrote: »

Good luck with the project. And that is a lot of gear on those racks. I'm guessing multiple systems since i spot at least 3 EQ's.

Oh, there was a lot of gear. That picture is several years old. Many things rearranged since then. There were multiples of several things then... Point was to show the tv stand/rack...

markamerica

I found some on madisound. There were two different 6.5s. One by accuton, one by SEAS(173mm), might work. Saw in your build thread that you used a single tweeter as described by Mr. Bubbles. Hmmm. Lots to chew on here.

Thanks!

Mark

markamerica

After considering the advice of forum members, I've ditched the idea of the complex triggered business to switch between an SDA array in a single box and a dedicated Center channel speaker. Instead, I am building it as a dedicated center channel speaker I've also decided to use a different model driver. I have a few options, but what I've provisionally decided upon is four MW6501s and two SL2000s. I made up a testing crossover board for this purpose on a sheet of Delrin. I tested the general sound of the drivers with this network to get an idea as to whether I should proceed at all. I was shocked at how good it sounded despite cobbling together a 4.4uf cap for the top tweeter out of a string of larger value caps in series and running it in unmounted, open air form.

I feel like the general idea has merit enough to proceed. I ordered the caps from Erse, though I had to settle for a pair of 2.2uFs rather than a single 4.4.

As far as my notional build, I am going to use both channels of a Soundcraftsmen A5002 both playing the same mono center signal. One channel will power the upper tweeter and the two inner MWs, while the other channel will drive the lower tweeter and the outboard MWs. In essence then, it's actually two separate crossover networks on a single board, but intended to play the same signal.

I am using Mundorf Mox resistors, and as I mentioned, I am using Erse Caps. For the inductors, I am using three standard Polk 0.4mH coils, and a pair of stock Polk 1.55mH inductors from the crossovers of a pair of Monitor 7s I picked up on Ebay a few years ago.

The 1.55mH inductors will be mounted off-board inside the empty space in the middle chamber(see diagram in first post)

It is not likely that I would pay for silk-screening, but I have designed a board I will send off to ExpressPCB if I finish building the cabinet, mock it up, and still like what I hear.

One of the things I considered i that since the minimum number of boards you can order from them is 2, I decided that I would make the front and back layers identical so that I could sandwich two boards together to give me twice as much copper. I used 1/4" traces everywhere, and I may still make some adjustments, but I am using Molex Sr Fit plugs and PCB headers with gold contacts for my wiring harness. If you're unfamiliar with the SR fit, here's a picture:

The PCB will itself mount on the back of an aluminum plate that will be reminiscent of the x-over cover plate on the front of the SRSs, with the exception that my binding posts will penetrate that panel and will have nuts sandwiching the board.

The drivers will be placed in parallel pairs. Ive decided to put the spots om the board for the rde050 polyswitches, and for the bypass caps(750pF) for the tweeters, although I'm unlikely to use the latter.

Here are the schematic and pcb layout screen shots.

As you can see from the schematic, the tweeter networks are actually the two halves of the SRS2/SDA1B tweeter network, separated and teamed with the design of the Monitor 10 Low frequency network.

Ignore the connector numbering. The board layout and schemqtic don't agree. On the board layout, P1 sends leads to one side of the remotely mounted 1.55mH inductors, and those return to P2 and P3 in the bottom position. P2 and P3 sends leads to the + side of the midwoofers, but the top position on them send leads to the + sides of the Tweeters. P4 and P5 bring the leads back from the MWs and the Tweeters.

Additionally, here's the mocked-up board I made for testing. Please forgive and ignore the convoluted series of capacitors in the middle of the board, strung together to make 4.4uF(12.2,30.7,31.65,12.32 in series = 4.4uF.) Sometimes, a fella has to make do with whatever he has handy.

Anyway, that's an update on where this project stands. Still soliciting opinions, comments, critiques, and general suggestions/ideas.

Thanks!

Mark

F1nut

Me? I would have built a new rack/cabinet that a modified single CRS+ could sit in just under the TV with an open back to allow the PR to breath.

markamerica

F1Nut,

I realize that may have been more practical, and certainly simpler, but that would be so not me... I like building things, tinkering with things, and figuring things out. That, and it gave me an excuse to work out some things I intend to apply to my other SDAs as I upgrade their crossovers. For instance, today, I actually received my SR Fit headers and plugs from Mouser, and I printed out the PCB layout to see if my pad placement matched, since I had to create those components. I pushed the header down though the paper, and yep, perfect fit, so at least I knew I could read a mechanical drawing...LOL Point is, I intend to use those on other projects, and this was a good chance to integrate them in a project. I look at this as a learning experience and practice for much more expensive projects, like the SRSs or the 1.2TLs.

Thanks!

F1nut

I'm sorry, but some tweaks/ideas in this hobby make sense and some don't. This one doesn't, but good luck anyway.

markamerica

F1nut wrote: »

I'm sorry, but some tweaks/ideas in this hobby make sense and some don't. This one doesn't, but good luck anyway.

Well F1Nut, that's the beauty of this hobby: Not everything that makes sense to you will make sense to me, and vice versa.

Again, thanks for your opnion.

Mark

markamerica

DSkip,

Make it where? Are forum members doing something hereabouts in January?

Mark

markamerica

DSkip,

Found the get-together thread. I may indeed be able to make that. Thanks for pointing it out. I mostly stick in the Vintage speakers section, so I might never have seen it otherwise. Thanks!

markamerica

DSkip wrote: »

I love when Jesse disapproves of something. Straight to the point with no filler.

BTW, I don't mind F1's undiluted opinions, nor those of anybody else. I just figure that's part of the deal, and besides, I have some of my own. ;-)

Face

You may experience comb filtering with the centers in that configuration. If you're fixated on two centers, one above and one below the TV may work. But a single CRS with the SDA signal grounded would be the most coherent. As for the passive radiator, since there is no magnet to center the cone, they must be mounted vertically. If you have the CRS woofer's T&S parameters, you could calculate the optimum port size for their enclosure. Sealed is another option as maximum bass extension shouldn't be the priority for a center channel.

markamerica

Here are a few pictures of where I stand.

The veneer remains to be done, the wiring needs to be done, the permanent crossover is yet to be built, but here, I was placing my new gaskets and checking the fit after applying melamine. Here's one front view:

markamerica

Rear view shows unfinished panel behind which crossover components will be installed, and where binding posts will ultimately be attached, along with four passives:

westmassguy

So you're the one that bought up all the 6.5" PRs....JK...
Looks really nice. Little concerned about those stacked tweeters being too hot on axis. You might have to pad them down a bit.

PolkieMan

Very nice Mark, BTW everything is bigger in Texas, right :-)

drumminman

markamerica wrote: »

I realize that may have been more practical, and certainly simpler, but that would be so not me... I like building things, tinkering with things, and figuring things out.

Sometimes ya gotta do what ya gotta do :cool:

Nice workmanship Mark. Look forward to your listening impressions.

markamerica

westmassguy wrote: »

So you're the one that bought up all the 6.5" PRs....JK...
Looks really nice. Little concerned about those stacked tweeters being too hot on axis. You might have to pad them down a bit.

Yeah, that was me buying up 6.5 passives like they were going out of style. As for the tweets, I started out with 3.3ohm resistors up front, but found they were too muted. I went back to 2.7ohm, and I think in the end, I will be happier there, though I may play with some values in between. It does bass remarkably well.

markamerica

PolkieMan wrote: »

Very nice Mark, BTW everything is bigger in Texas, right :-)

I don't know about everything, despite the hype, but in this case...

Thanks!

westmassguy

markamerica wrote: »

Yeah, that was me buying up 6.5 passives like they were going out of style. As for the tweets, I started out with 3.3ohm resistors up front, but found they were too muted. I went back to 2.7ohm, and I think in the end, I will be happier there, though I may play with some values in between. It does bass remarkably well.

Awesome, enjoy it!

markamerica

drumminman wrote: »

Sometimes ya gotta do what ya gotta do :cool:

Nice workmanship Mark. Look forward to your listening impressions.

Yessir, I'm nothing if not persistent. I've actually surprised myself a bit. I spent several hours listening to it tuning the passives and playing with resistance in the tweeter circuits, as per my discussions with WestMassGuy, and I think before all is said and done, I may have a whale of a center channel speaker. Played briefly in that mode with the SRSs, it seemed a good fit. Thanks!

markamerica

Update: Obviously, image attachments seemed not to have made it over with the new version of the forum. To remind folks what I was doing, I was in the process of building a center channel that had the same general qualities as the SRSs I use as my main speakers. At first, I toyed with making it a good deal more complex than it needed to be, but the wisdom of some of our members prevailed upon me to make some substantial mods to my original plans. In the end, the speaker would use four MW-6502 drivers, two SL-2000s, with the outer pairing of MWs on the same channel as the upper tweeter, and the inner pair on the same channel as the lower tweeter. In addition, I decided to use 6.5 inch passives collected on Ebay...a whole lot of them. In any event, the box (still awaiting a veneer) looked like this:





It was not too long after this that my wife and I departed for our little vacation, where she promptly had a heart attack. Fortunately, after a couple of stents, she is doing much better, but she had a long rehab process and I inherited all the chores around the farm she could no longer do. It has just been in the last few weeks that I could begin to re-engage on hobbies and other non-essential things that have been on pause since last December.

I finally got around to ordering the board I designed for this. It's a simple affair, and I used Mox resistors and Erse Capacitors, along with some snazzy inductors I ordered from Trey. Not pictured, because they're not mounted on this board, are the two 1.55mh inductor coils. Those are actually original Polk pieces de-wound from slightly higher value pieces from a pair of crossovers picked up on ebay. They are wound on plastic binding post cups, which when turned over, present a very good mounting base.

I have now used Duracoat to finish the aluminum handles and back plate. I used inexpensive binding posts from ebay, that I will later upgrade if/when I'm happy with the speaker's performance. Here are pictures of the finished plate and the crossover mounted on its inside face:





That's where I stand at present. This weekend, time permitting, I will wire it up and audition it minus veneer finish and grill cloth. If I decide I like its sound, I will order veneer (probably a red oak) and complete its cosmetics.

Thanks to all!

Mark