SDA design question

Braddles63Braddles63 Posts: 53
edited March 27 in Vintage Speakers

Here is something I've been wondering for a while. Forgive me if it'd been asked before.

Whats the reason for the different driver layouts for SDA models with similar driver complements?

For instance the SDA2 and the CRS.

Same crossover? (if same generation)
Same MW specs?

So why does the CRS have the space between the MW's and the tweeter lower and sort of in-between and the SDA2 have the MW's close together with the Tweeter above? What the pros and cons of each layout?

IS it the cabinet / baffle size that makes a difference to what works best in each case?



  • pitdogg2pitdogg2 Posts: 6,575
    The woofers need a certain amount of cabinet interior space to work correctly. A bigger box they can be closer together. Being wider apart also give a wider sound stage. There is much more to it just don't want to confuse you more.
  • Braddles63Braddles63 Posts: 53
    edited March 27
    So the distance from the edge of the driver to the edge of the cabinet needs to be a certain measurement? That would make sense.
  • pkquatpkquat Posts: 422
    I'm not an expert and I did not stay at a Holiday Inn Express last night. From what I have gathered the spacing between MW's affect the apparent width of the sound stage and also the sweet spot distance relative to the width of the speakers.

    The CRS is "compact". A 10" PR was already determined to work well with two MW's which SDA requires. It may be the same PR as the M10. Box size and dimensions would likely be similar. There may have been the ability to use the same tooling for both. My guess is this layout worked best for a smaller compact speaker.

    The 2B likely shared tooling and PR and/or cabinet similarities with the 12C and LF14.

    Polk liked to share many things between lines. Given the similarity of the base design, 1 tweeter, 2 MW, and a PR, polk likely worked to find a good solution to use the same crossover for both..... or popped them in each design and said "sounds good enough".
  • F1nutF1nut Posts: 37,319
    The distance between the mid-drivers is supposed to be roughly equal to the distance between your ears. I don't recall exactly why the CRS drivers are spaced further apart, but they definitely throw a wider soundstage than other SDA's. Perhaps a little too wide.
  • TennManTennMan Posts: 969
    F1nut wrote: »
    ... but they definitely throw a wider soundstage than other SDA's. Perhaps a little too wide.
    I agree with that. I have a pair of CRS+ and at one time I had a pair of 10Bs. I took the drivers and crossovers from the CRS+ and installed them in the 10Bs to make a pair of SDA 10Bs. I know it is subjective but I preferred the sound of the SDA 10Bs over the sound of my CRS+ because the soundstage wasn't as wide and was more natural sounding to me. The SDA 10Bs sounded a lot like my 2Bs with less bass.

  • Dennis GardnerDennis Gardner Posts: 4,589
    edited March 27
    I think that all the Reference Series models held the wider mid woofer spacing that the standard SDAs did not have. This was done for a few years until the 3.1tl came out with that tight driver complement. I do agree that the CRS imaging seems wider than most any model I've heard, but that could be dur to the way they don't mind being further from the back wall than my other SDAs. The rear PR must help with that.
  • xschopxschop Posts: 516
    Thanks to Kevin I found this thread once again and I can bother F1 once again.
    I read it a few weeks back and its always stuck as the best discussion on SDA CRS driver placement.
    Braddles pic and F1's comment as well as Tennman's experience with his 10B SDA mod piqued my interest in changing the driver placement in my CRS.
    Question is open... What is the Big SDA model that share's the CRS+ crossover and what is its MW center to center spacing?
    Would moving the CRS' MWs to match or a percentage be a worthwhile modification?
    Thanks everyone.


  • F1nutF1nut Posts: 37,319
    The 2B and CRS+ share the same crossover, but use a different stereo driver and of course, different PR's.

    I don't have my 2BTL's handy to measure them right now.

    The spacing of the CRS+ drivers definitely results in a wider soundstage than other SDA speakers, so logic would dictate moving them closer would narrow it. Is the soundstage too wide for you?
  • pkquatpkquat Posts: 422
    The M10, SDA 2B and 1C all appear to have the same spacing between MW's. My 1C's have roughly 0.050" between the baskets in the middle.
  • xschopxschop Posts: 516
    edited May 11
    Its been along time since I've listened to a set of Big SDAs (1C I believe)
    Since Im rebuilding a set of CRS I want to adjust the spacing to optimize soundstage.
    I've always surmised the CRS factory MW spacing was wider due to the tweeter placement geometry and the fact that if the MWs were closer, the tweeter cutouts would make the baffle weak and flexy....

    Oh...Hope you know the E.F. Hutton comment was a compliment.
  • F1nutF1nut Posts: 37,319
    I did and liked it.
  • xschopxschop Posts: 516
    If someone has a set of 2Bs handy to measure the MWs and their relation to the tweeters, would be appreciated.
    Im thinking a compromise between the two geometries are in order.
  • F1nutF1nut Posts: 37,319
    I can get that for you tomorrow.
  • xschopxschop Posts: 516
    Thanks once again F1. Ill post/ask here on this subject from now on.
  • xschopxschop Posts: 516
    Whipping out the Pythagorean equation, the CRS tweeter center is 6.56" from each MW center.
  • Braddles63Braddles63 Posts: 53
    I have a set of 2B's now as well. :)
  • F1nutF1nut Posts: 37,319
    Wouldn't you know.....I was standing in front of them today, actually put my hand on one and completely forgot to take the measurements. Not to worry, I'll be able to get them next week if someone else doesn't first. Sorry about that.
  • txcoastal1txcoastal1 Posts: 7,429
    slackers... :/

    Now what are ya'll trying to do :)
  • F1nutF1nut Posts: 37,319
    I'm just trying to remember what day it is.
  • txcoastal1txcoastal1 Posts: 7,429
    F1nut wrote: »
    I'm just trying to remember what time of day it is.

    Fixed :p

  • gmcmangmcman Posts: 779
    edited May 13
    I checked my 2B's and the MW cone centers look to be 6 5/8" apart.

    The tweeter center is 6 1/4" above the MW centerline.
  • kevinkokevinko Posts: 126
    Would the placement of the PR have any relation to the placement of the drivers?
  • xschopxschop Posts: 516
    edited May 13
    Again the Thag theorem says 7.07" for tweeter to MW centers on the 2Bs... Why the half inch difference between them and the CRS when they share the same crossover?
    Post edited by xschop on
  • F1nutF1nut Posts: 37,319
    Different sized cabinets.
  • txcoastal1txcoastal1 Posts: 7,429
    xschop wrote: »
    Again the Thag theorem says 7.07" for tweeter to MW centers on the 2Bs... Why the half inch difference between the two when they share same crossover?

    Is there any bracing

  • westmassguywestmassguy Posts: 5,561
    I think there's a lot of wiggle room, a Goldilocks zone if you will. I own 2As and CRS+s. Very different spacing of the woofers, yet both provide the interaural crosstalk cancelation. They present slightly different soundstages, but both very effective.
  • xschopxschop Posts: 516
    For the stereo signal, isn't it ideal to have the tweeter as close to MW as possible for the crossover frequencies to mesh?
  • F1nutF1nut Posts: 37,319
    With all due respect, I don't think we are qualified to second guess the genius of Mr. Polk.
  • xschopxschop Posts: 516
    Mr Polk was limited to the yield strength of particle board of that era.
  • SchurkeySchurkey Posts: 1,660
    edited May 13
    Polk has had essentially two midwoofer driver spacings:
    Close together
    Entire SDA 1 line
    Entire SDA 2 line
    SDA SRS 3.1TL

    Entire SDA SRS 1 line
    Entire SDA SRS 2 line

    I suspect that the overriding (but not "only") concern was "What is an acceptable front baffle width given that we are using a certain sized midwoofer driver and need an "outboard" driver(s) to get the SDA effect?" The most-expensive speakers were "allowed" to have a wider baffle, potential customers weren't scared-off by the bigger "presence" of those speakers. It may have been a sales advantage back then.

    By sales volume, I think the close-together front baffle won by a landslide, although I could be wrong. I don't know how well the CRS sold in relation to the regular SDA speakers.

    The CRS (rear) baffle had to be wide enough to accept the 10" bassive radiator and a terminal cup. That in large measure sets the minimum dimensions of the baffle. Assuming a cubic-rectangle box, we've also set the minimum dimensions for the front baffle. From there, it's a matter of form:
    The early CRS could have had either wide or narrow-set midwoofers and horizontally-positioned tweeter(s), and Polk happened to chose "wide-set", or
    The later CRS had wide-set midwoofers and a vertically-positioned tweeter.

    So while the most-expensive speakers were "allowed" to have a wide baffle, the Little Guys "had" to have a wide baffle.

    I have no doubt that Polk knew the wide-set midwoofers would increase the SDA effect. I have to believe that Polk wanted that enhancement on the CRS, since they could have done narrow-set midwoofers if they'd wanted to--all they needed was to position the tweeter horizontally like they did on the early CRS. This is the "acoustic" reason for wide-set midwoofers on the CRS.

    I don't know what the internal structure of a CRS is like. I suppose an alternative theory is that maybe Polk saved the cost of installing a reinforcement panel by wide-setting the drivers to spread the vibrational load closer to the edge of the baffle, rather than having it concentrated in the middle by narrow-setting the midwoofers. This is the "structural" reason for wide-set midwoofers on the CRS.
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