phase reverse points on the patchbay

Discussion in 'Patchbays' started by godcity, May 8, 2001.

  1. godcity

    godcity Guest

    I've got a Soundcraft Ghost console which doesn't have a phase reverse on the tape returns or line inputs. My tape returns are normalled to the board so I decided to wire up some phase reverse points on the bay to use with the insert points. I can't seem to get it to work.
    Is it because my insert points are unbalanced?
    The only way I seem to be able to flip the phase is to send it to a piece of outboard gear that has a phase reverse switch (like a Drawmer 1969). I've even plugged in two TT cables to the insert points and used aligator clips to try every combination, but there's only one way that signal will pass.

    Please tell me that I'm a dipshit and it's not my equipment. And then after you get done telling me that I'm a dipshit, tell me why.

    thanks
     
  2. anonymous

    anonymous Guests

    Kurt, when you have an "unbalanced" signal (like on a 1/4" connector) you have the "hot" going to 'tip', and the ground to 'sleeve'. By trying to do a "polarity reverse" with an unbalanced connector, you are effectively sending "hot" to "ground"...which leads to a whole lot of nothing.

    In order to reverse the polarity of a unit you will need something to 'flip'. With an XLR connector, you can make 'Pin 2' or 'Pin 3' "hot"...these can be reversed, there by reversing the polarity of the signal (the "hot" now travels on the "cold" pin and vice versa).

    If I may make a couple of suggestions...first, you might consider getting a 'polarity checker' and 'click' each microphone during the recording process. That way what you have on tape will all be in the same polarity. The other suggestion would be to make up a dozen or so 'polarity reverse' XLR cables.

    Rather than using the 'polarity reverse' switch on the console when changing the polarity of a microphone's output I use these cables. The reason being that I'm not sure where the 'switch' is in the signal chain in the desk or on a unit...and a mic pre (even a class A mic pre) will handle the top half of a wave differently than the bottom.

    Most signals, especially vocals, are not symmetrical wave forms and will sound different when hitting the mic pre "upside down".

    Neither here nor there, just a suggestion. In the meanwhile, in order to do 'polarity reverses' in your world, you're going to have to install a "balanced" patchbay. And while you're at it...don't forget to include like a half a dozen "4 hole mults"...they can be a life saver!!

    Best of luck.
     
  3. godcity

    godcity Guest

    ok. then how come the 1969 will flip the phase in an unbalanced insert point when rewiring won't?
     
  4. Ang1970

    Ang1970 Well-Known Member

    Unbalanced = + and G
    Balanced = +, -, and G

    In the unbalanced line, take + and connect it to G, and you eliminate the difference between + and G. That difference between + and G was your signal, and it is now non-existant. That is why signal disappears. (Or, it is likely that the + and - leads on your inserts represent send and receive. Therefore you are plugging an input into an input and an output into an output.)

    In a balanced line, take + and connect it to G, and there is still a difference between - and G. If you connected - to G, you'd still have the difference between + and G. The signal would be the same either way, but with the opposite polarity. That's why you still get a signal when you flip the phase switch on your drawmer.

    Hope that makes sense.
     
  5. godcity

    godcity Guest

    OK. So If I take an empty point on my patch bay and solder the + and gnd of the input and the - and gnd of the output all together, then I solder the - of the input and the + of the output together, will this flip the polarity when using it in conjunction with my insert points?
     
  6. Ang1970

    Ang1970 Well-Known Member

    Umm... the way you describe it, it sounds like you'll just have one big ball of solder dumping everything to ground! lol

    If your board uses a single TRS plug for both send and return on the inserts, you are really dealing with two separate UNbalanced lines (that happen to share the same ground connection.) This makes it impossible to swap the polarity when you plug into another unbalanced in/output. You can swap the polarity when going to or from a balanced i/o by sending the + lead to the - post.

    In the patchbay, you would need 2 open jacks for each polarity invert. The first jack would be wired as usual:
    + = +
    - = -
    G = G

    The leads from the first jack would then be connected to the second jack as follows:
    + > -
    - > +
    G > G

    The same thing can also be achieved by making some inverted patch cables. This is a much more economical alternative, as you free up valuable patch points and can make as many inverted patch cables as you want. Connect the tip lead on one end (+) to the ring terminal on the other end (-), and the ring lead (-) to the tip (+). Sleeve (G) goes to sleeve (G).

    Remember, if you use an invert patch to connect two unbalanced devices, you get nothing, zip, nada, you lose, good day sir. The unbalanced device needs to be connected to something that is balanced in order for it to work.
     
  7. KyleSong

    KyleSong Guest

    While we're on the subject of patch bay features, I want to add that a good 1:1 line transformer like a Jensen JT-11SP-6M is a great thing to place behind the bay with points going in and out from the bay.

    Since no wires actually touch(!) across the transformer, you can patch through it to break ground loops, eliminate DC offset, or interface "non standard" gear.

    When interfacing balanced outputs and unbalanced inputs, it is often preferable to connect the cold (-) lead of the balanced output to the ground leg of the unbalanced input. This way, the signal that was on the cold lead is conserved in the input of the unbalanced gear, and the level remains consistent. Connecting the cold lead to ground on unbalanced inputs is so common, that many studios wire their unbalanced gear into their system this way, for seamless hookup with balanced gear.

    Some transformerless but still "electrically" balanced gear, like my Apogee AD-8000, hates being connected out to an unbalanced input, (like an API 550B for instance), where the cold (-) lead has been shorted to ground on the unbalanced input in the manner I described above. The Apogee wants to see the cold lead left hanging, along with whatever level penalties that entails.

    When one piece of gear won't play like the rest of the crew, you can't keep rewiring! One good answer is to simply use the patch bay and insert the line transformer between them. The transformer balances the input of the unbalanced device, so the interface mismatch is gone.

    A solution to your unbalanced phase questions can be found here too...

    Look: (ignore the periods, they are spacers)

    UNBAL OUTPUT:.....(+)......(G)
    into........................I........ I
    XFRMR INPUT:.......(+).......(-)
    xfrmer....................x........x
    XFRMER OUTPUT:...(+)......(-)
    into........................I.........I
    UNBAL INPUT:........(G)......(+)......<-- Phase reversed!

    The transformer creates a balanced signal at its output. The signal is in a push / pull (audio is AC current) configuration, so either side can be used as the (+) side, since both sides now have signal.

    The reason is that since a transformer requires magnetic transfer for energy to move between its coils, all that gets across the transformer is any signal that can cause magnetism inside of it. Since magnetisim requires motion to create electricity, only the changing part of the signal (which makes the magentic field expand and collapse, which is equivelent to motion through a static field) will get across the coils in the transformer. Any DC offset that you may have picked up will be eliminated, since it has no changing component and will not create a CHANGE in the magnetic field, and therefore it will not create any electricity in the secondary coil.

    This also means that while the signal going into the transformer may have been unbalanced - living on only one wire, with a ground reference on the other - that however the charge got into the coil is ultimately irrelevant. What gets across the coil, is the CHANGE, and since audio is Alternating Current, producing both negative and positive charges, the signal at the output is push / pull, with signal flowing one way, then the other. This is a balanced configuration, so the output of the transformer is always balanced. You could look at either leg of the transformer for the signal, it's just upside down on the cold (-) leg.

    This is why you can selectively choose to connect either leg of a balanced output to the ground side of an unbalanced input, and still expect to find the signal on the other leg of the balanced output, and why you can reverse the phase of a signal after it has come out of a transformer without worrying about which leg has the signal. If you had done that directly with the unbalanced signal, you would have lost all the signal into the ground, and had nothing but ground into your (+) input.

    Conversely, an UNbalanced output is not push / pull, and the information is only on one of the wires, referenced to ground on the other. You can connect it directly to any input, even a balanced one. No matter that all your signal is on one leg - the addition of the two legs (signal and nothing) will still yeild the complete signal.

    To answer your application directly, I'd advise getting at least two transformers for your patch bay for stereo, and then making seperate phase crossing [(+) to (-), (-) to (+), and (G) to (G)] cords or PB points. That way, you can use the transformers and the phase flippers for seperate jobs when appropriate. Rememeber, those phase points and cords will work just fine without needing a transformer in balanced to balanced applications!

    Good Luck!

    Kyle
     
  8. sapplegate

    sapplegate Active Member

    You really need to send your tape returns through a (half-)normalled patchbay. Assuming your tape deck is balanced out, you now have hot, cold and neutral to play with and you can do the phase patch thing easily. Additionally, if you patch outboard here, you get a stronger, and presumably more common-mode noise immune signal going through the outboard. Works for us, we too have a Ghost. RNC's are the only thing I patch in the inserts. Good luck!
     

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