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  • #1

    bridged PA line out

    I have a 50w bridged power amp - so neither side of the speaker is at ground. I would like to add a line-out.
    The first idea is a use a small transformer as shown below.


    Click image for larger version Name: l out.jpg Views: 240 Size: 95.5 KB ID: 966893

    My question is am I overthinking this? Is there a simpler option? Am I overlooking something?
    It simulates fine, but I have not built or tested it yet.

    The transformer circuit brings several nice features.
    - it can drive a balanced TRS line into hi-z or 600 ohm load
    - it can drive an unbalanced TS line
    - it can have the sleeve grounded or lifted

    A few parts are shown for simulation purposes only. R107 simulates a balanced load. A short across R105 simulates an unbalanced TS connection.

    Comments and suggestions are welcome.




    “If you have integrity, nothing else matters. If you don't have integrity, nothing else matters.”
    -Alan K. Simpson, U.S. Senator, Wyoming, 1979-97

    Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

    https://sites.google.com/site/stringsandfrets/
    Tags: None
  • #2
    My thinking is that a line out potential divider can be connected to either speaker output.
    ie take a single ended sample off a balanced signal.
    My band:- http://www.youtube.com/user/RedwingBand
    • Likes 4

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    • #3
      Originally posted by pdf64 View Post
      My thinking is that a line out potential divider can be connected to either speaker output.
      ie take a single ended sample off a balanced signal.
      Good point. Thanks. In theory that should work and would be simpler. I'll try it to see if there's any difference.
      I only hesitate since the bridged amps are not exactly linear (what amp is - especially at higher output levels?), which means the difference signal at the spkr is not an exact copy of either amp output.
      I'm a bit paranoid that this may be the case, but again, maybe I'm over thinking it since I have not tried it.

      “If you have integrity, nothing else matters. If you don't have integrity, nothing else matters.”
      -Alan K. Simpson, U.S. Senator, Wyoming, 1979-97

      Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

      https://sites.google.com/site/stringsandfrets/

      Comment

      • #4
        Considering that any line out circuit will include a series resistance that is very substantially higher than the load I wouldn't expect any problems just tacking a line out to the output jack as usual. How much trouble could be introduced by a circuit that's at least a hundred times greater in impedance than the load?
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment

        • #5
          Originally posted by Chuck H View Post
          Considering that any line out circuit will include a series resistance that is very substantially higher than the load I wouldn't expect any problems just tacking a line out to the output jack as usual. How much trouble could be introduced by a circuit that's at least a hundred times greater in impedance than the load?
          You are right. I'm not worried about impacting the spkr signal. I'm just trying to get as good a copy of that as possible. The spkr sees a difference signal, which is not ref'd to ground. Thus the transformer approach. I'm mostly concerned with how the transformer will interface with long cables, external mixers, amps, etc. Is there something in how I've done it that will cause problems or should be done differently?
          “If you have integrity, nothing else matters. If you don't have integrity, nothing else matters.”
          -Alan K. Simpson, U.S. Senator, Wyoming, 1979-97

          Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

          https://sites.google.com/site/stringsandfrets/

          Comment

          • #6
            I guess I don't entirely understand the schematic above. But I don't see anything wrong with it from a safety perspective.?. A plain line out would be used as Pete described, from a single speaker + terminal, and would not have the advantage of running balanced to a TRS cable. I don't know how useful that might be for you so this is strictly your consideration. I haven't been hampered by not having a balanced line out but it must come up because they are offered on plenty of gear. I was only pointing out that there's probably no need to complicate things unless you do need the extra features.
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "If you're not interested in opinions and the experience of others, why even start a thread?
            You can't just expect consent." Helmholtz
            • Likes 1

            Comment

            • #7
              This wouldn't happen to be a class D power amp would it ? That's a whole different animal.
              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
              REMEMBER: Everybody knows that smokin' ain't allowed in school !

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              • #8
                Originally posted by loudthud View Post
                This wouldn't happen to be a class D power amp would it ? That's a whole different animal.
                No - the power amp is much like the one in this amp...
                https://sites.google.com/site/string.../solo-100-v4-3

                That does raise an interesting question though... what if it were a bridged class D amp? What special consideration is needed?



                “If you have integrity, nothing else matters. If you don't have integrity, nothing else matters.”
                -Alan K. Simpson, U.S. Senator, Wyoming, 1979-97

                Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

                https://sites.google.com/site/stringsandfrets/

                Comment

                • #9
                  I'm not totally up to speed on class D power amps. I recently purchased a Soldano SLO mini 30. It uses a class D power amp with a balanced output. The connector for the speaker is the standard phone plug and there is 12VDC on what would normally be the grounded shell of the speaker cable. Looking at the data sheet for the class D chip, they want you to use R/C filters on the output to measure the THD. They don't do a whole lot to filter the switching frequencies from the audio.

                  Read about it here: https://www.ssguitar.com/index.php?topic=5155.0

                  It's been too hot to work at my shop for over a month, but it looks like there is a break in the weather this week. I'll see what I can learn about these chips.
                  WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                  REMEMBER: Everybody knows that smokin' ain't allowed in school !
                  • Likes 1

                  Comment

                  • #10
                    Originally posted by loudthud View Post
                    I'm not totally up to speed on class D power amps. I recently purchased a Soldano SLO mini 30. It uses a class D power amp with a balanced output. The connector for the speaker is the standard phone plug and there is 12VDC on what would normally be the grounded shell of the speaker cable. Looking at the data sheet for the class D chip, they want you to use R/C filters on the output to measure the THD. They don't do a whole lot to filter the switching frequencies from the audio.

                    Read about it here: https://www.ssguitar.com/index.php?topic=5155.0

                    It's been too hot to work at my shop for over a month, but it looks like there is a break in the weather this week. I'll see what I can learn about these chips.
                    Wow - the demo sounds pretty cool for a $249 amp.
                    DC at the output - yikes - but at that price point every component matters.
                    Thanks for sharing that.
                    “If you have integrity, nothing else matters. If you don't have integrity, nothing else matters.”
                    -Alan K. Simpson, U.S. Senator, Wyoming, 1979-97

                    Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

                    https://sites.google.com/site/stringsandfrets/

                    Comment

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