Ummm. aha. And, for the record, may we all know who "I" is? Could you please state, for the benefit of the members here on PL, your credentials, including any degrees you hold, any patents you hold, and any accolades you have won as a testamente of your own ability to make such a recommendation?
It's easy to fall into a trap and take things out of context.
With respect to why Cambridge makes the recommendation they do in their manual, and why there are three connections on a scanner amp -- if you have only a single scanner amp, then you might get away by connecting your "signal ground" to the scanner amp's ground. But for everyone here doing two-axis laser graphics and beam shows, you have two amps, not just one, and connecting your "signal ground" to the ground connection of both scanner amps will cause this kind of "ground loop" (I hate that term because it is way over used), and the X amp will experience "ground bounce" when the Y amp undergoes hard acceleration, and vice-versa. The result is crappy images (to some extent), the extent depends on the gauge of the wire, the degree of acceleration, and other factors. You can avoid all of this by making only one single ground connection *somewhere*, and the best place to do that is the common ground connection at the power supply itself.
For the record, I have nothing to prove. If you want to follow my recommendations, it doesn't make me feel any better about myself. And if you don't take my recommendations, it won't make me feel any worst about myself. I am confident in who I am as a person, and confident in my abilities as an engineer. Likewise, there are those who have known me for a very long time and, lets just say my reputation preceds me. For those who don't know me, I have the technical certifications, patents, awards, and other accolades (letters of commendation from heads of state) to prove that, by and large, I know what I am talking about.
Of course, I can always make mistakes (more about this in another post). I myself have made the very same recommendation that Cambridge made years ago (thanks for pointing this out DZ -- I will change my own docs). Today I count those older recommendations as a mistake, and my latest recommendation is to connect the DB25 to the ground of the power supply and NOT directly to the amps. I have my reasons, and in the lab, I can show you or anyone else the difference.
Bottom line -- people (and companies -- BIG ONES) send me scanners to improve. The people here on PL can universally tell you that when they get back their scanners from me, they are wired the way I am saying in these recent posts. And wiring these scanners is the very LAST thing that I do -- so the wiring is not done out of prejudice, or wanting to prove that I am right. The fact is, these systems don't look as good as they do, unless they are wired the way I say. And those are the facts.
In any event, anybody here is free to choose whatever wiring method they want. On a personal level, I don't care. You aren't going to hurt my feelings if you don't follow my recommendations, and aren't going to stroke my ego if you do. Aside from just wanting every laser show to appear as good as it can (for reasons stated in other posts), I really don't care. Especially if you are a competitor!! Definitely don't follow my recommendations !!!
On a hobbyist level, maybe we can just change the statement a bit. The wiring I suggest is "what works for me". I believe it will also work for everyone else, and recently, big projector manufacturers have changed their designs, so they believe me too. You can try it, and see if it works for you too. If it does, then great. If it doesn't, then figure it out for yourself. Really that's the spirit here on PL -- figuring things out for yourself (hopefully, with the help of others), and then sharing your experiences.
As a final note, can I call on some of those for whom I have tuned (and changed the wiring on) their scanners, to post in this forum about what your scanners looked like before, and after they visited Pangolin? Sure, there are many changes done, but the wiring is one of the changes that leads to the pretty pictures .
Best regards,
William Benner