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  1. #1

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    Default Help Understanding SDA

    Hey there,

    I have had a pair of SDA SRS 1.2tl for a few weeks now and I am trying to just make sense of the proprietary cable for my own mental health.

    Not having the original interconnect, I was told on this forum that the only cable I need to hook up between speakers is the "pin" cable. Therefore I only have one length of wire connecting the two speakers.

    It is my understanding that at any given moment, the right channel is sending the surround effect signal to the left channel and vice versa.

    How is this possible with just one wire connecting the two speakers? Wouldn't there need to be two wires connecting them?

    I looked at the schematics and it appears that the "blade" connector is connected to ground. Would that mean that connecting the blades together in effect create a common ground scenario that would be dangerous to the amplifier/speakers?

    Thanks so much!!

  2. #2

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    Start with this: http://www.bostonaudiosociety.org/pd...01-07-7304.pdf
    and the article on pg 5, "Adding a Null Switch to Your Stereo."

    Yes, it's an old (1973) article but the difference signal concept is fundamental and timeless. This article explains the basic concept of creating a difference signal from a common ground amplifier, independent from the Polk SDA effect. It also explains some other applications for the difference signal, and that it is not only not dangerous to a common ground amplifier, it allows you to tweak your setup in the ways described in the article.

    You can do the difference signal tests with any (common ground) amp, and regular (non-SDA) speakers. Hook the left and right + terminals from the amp to the respective + on the speakers (leave both the minus terminals on the amp open--they are connected internally to the amp), then run a single wire between the minus terminals on both speakers.

    If you do this with SDA speakers, unplug the SDA cable. The SDA cable essentially does this same thing, in parallel with a normal speaker/amp connection, except it only directs a filtered version of the sound (because of the way the interconnect cable attaches to the crossover), and just to the dimensional drivers. (edit: in later versions of SDA speakers anyway; I think it was different in earlier versions, going to the tweeter array too)

    In the SDA interconnect cable, if you take an ohm meter you'll see the blade-to-blade connection is open (i.e. only the pins have continuity). Now if you look at the schematic (I only have the 2.3TL handy but should be the same idea on the 1.2 I think), and knowing there is only a single wire (which connects the minus side of the low frequency Dimensional drivers) you'll see (if you ignore the filtering of the crossover components) it's the same difference signal connection described in the article.

    The SDA schematic is a bit confusing because it shows the blade connector terminal attaching to the minus low frequency terminal (this connection is made inside the speaker). This has relevance only when you use the AI-1 box so consider this connection as an open circuit when you use a common ground amp and regular interconnect cable because there is no continuity between the blade connectors in the cable.

    By the way, if you want to hear what the difference signal alone sounds like, connect an amplifier as described in the article to regular speakers (or SDA's without the interconnect cable) and listen to something like Pink Floyd's Dark Side of the Moon. All the stuff that normally images in the center is gone (because it's equal and in phase in the left and right, so it subtracts to zero in the difference signal).
    Last edited by Bubinga99; 10-03-2009 at 09:27 AM.

  3. #3

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    Thank you for that info! - I will read the article.

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    IIRC the blade is connected to the neg terminal(ground) and you will need to utilize it if using the AIO-1 non common ground interface,Ben62670, makes a nice aftermarket cable for the p/b SDA series.YMMV.
    JC approves....he told me so. (F-1 nut)

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