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Thread: The effect of phantom grounds on projected images

  1. #11
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    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Just making sure you don't have a voyage on the Kobayashi Maru Mr. Tribble...


    Besides a scanner amp maker might read this and get rid of the ^&%$# flaw!
    Steve
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    great, will recheck my wirings
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    Michel Rietveld @ Utrecht The Netherlands

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  3. #13
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    Resurrecting an older thread here. It's good to keep newer people on the forum informed.

    My question though, is what if you have a dual ch amp that ties the ground together via board traces? Curious if it would be better to break the trace on the board and keep the ground isolated to just the power ground.

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    I never saw this and had thought my tuning sucked and I could never solve it. Bang. What a difference.

    ok this is extreme but what if you used separate power supplies or used a supply twice as large as need current wise. Yea I hear you why free is free. Just learning.

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    Yeah, same here. I could never get rid of the "wobbly" lines on the Laser Media pattern. I always thought it was just because I wasn't using CT scanners or equiv.

    The PSU I am using is plenty big enough. It's specifically designed for A/B audio amps up to 400W, so 4A constant on each rail and easily 9A spikes shouldn't be a problem. Part of the potential problem I see is the ribbon cable that delivers power to the amp. I believe the ribbon cable is composed of 28awg wire.. Granted, it's using two wires each for -/+/G but I don't think that's enough for large jumps and quick current spikes. I suppose I could modify the amp use larger power wires but I am still wondering about the grounds that are linked. I might try breaking the trace on the board that connects the two signal grounds. If it has an adverse effect, I'll just jump them back together.

  6. #16
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    Two separate power supplies can be used, as long as they meet at a common star point. I am sure this is shown in my diagram.

    AHA dual channel amp -- no need to worry about modifying the amp itself. The singular ground of the dual channel amp should -- itself -- not be a problem.

    The main point is -- just don't connect ANYTHING to the "signal ground" (third pin on an input connector). Use only the + and - inputs to the driver (which should be differential receivers).

    Also make sure there are no "phantom" grounds, which might happen if -- for example -- the X-Y scanner block is bolted directly to the projector chassis which is also grounded. It's best to use an insulator (for example "sil pad") under the X-Y mount and nylon screws holding it to the projector housing.

    And lastly -- I'm not sure if I ever did this in my demo video or not. But you can see if there is "cross-talk" between axes by projecting content only on the Y axis and making sure you have exclusively a vertical line, and then do the same with the X axis and see if you have exclusively a horizontal line. HOWEVER -- don't be fooled because, if you've got crappy scanners or out-of-balance mirrors, this too will cause deflection -- particularly while exercising the Y axis (small horizontal "perturbations"...)

  7. #17
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    Quote Originally Posted by Pangolin View Post
    Two separate power supplies can be used, as long as they meet at a common star point. I am sure this is shown in my diagram.

    AHA dual channel amp -- no need to worry about modifying the amp itself. The singular ground of the dual channel amp should -- itself -- not be a problem.

    The main point is -- just don't connect ANYTHING to the "signal ground" (third pin on an input connector). Use only the + and - inputs to the driver (which should be differential receivers).

    Also make sure there are no "phantom" grounds, which might happen if -- for example -- the X-Y scanner block is bolted directly to the projector chassis which is also grounded. It's best to use an insulator (for example "sil pad") under the X-Y mount and nylon screws holding it to the projector housing.

    And lastly -- I'm not sure if I ever did this in my demo video or not. But you can see if there is "cross-talk" between axes by projecting content only on the Y axis and making sure you have exclusively a vertical line, and then do the same with the X axis and see if you have exclusively a horizontal line. HOWEVER -- don't be fooled because, if you've got crappy scanners or out-of-balance mirrors, this too will cause deflection -- particularly while exercising the Y axis (small horizontal "perturbations"...)
    Thanks for the response, Bill. After hooking up and testing, there was no need to modify the amp. The grid patten and LM Test patterns were sharp and not wavy. The one thing I did learn is; in every single one of my setups, ILDA pin 25 *MUST* be hooked to "power" ground at the PSU output, which your diagram shows. I neglected to do this at first and my scanner output was wavy in a way I couldn't really describe. Once I hooked pin 25 to the PSU ground, all was good!

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