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Acoustic a20 bad balanced out

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  • Acoustic a20 bad balanced out

    I can't find a schematic. I emailed acoustic, and waiting for reply.

    It has surface mount devices. The output has a pot for level of xlr out.

    At the lowest level on the pot it has very low output. As I turn the pot up, it gets quieter until no sound. All the way up, no sound.

    It has an op amp with 0720 GZ522 on it. Is this just an 072?

    Pin 4 has -14v
    Pin 8 has +14v
    Fine, But,
    Pins 2,7,6 also have -14v

    All three positions on the pot have -14v also.

    Does this mean the op amp is bad? The resistors and capacities around it don't show open. The pot shows correct resistance with movement.


  • #2
    If you have supply on pins other than 4 & 8, the op amp is likely bad. If I suspect a bad op amp, I usually remove it and check the pads on the board to see if the offending voltage has been removed. If it has, the op amp is bad. If the voltage is still there, the problem could be elsewhere. From your description, the chip is likely a TL072.
    "Yeah, well, you know, that's just, like, your opinion, man."


    • #3
      Thanks! That was my thought, but before I went and desoldered a surface mount chip. I wanted reasurance I wasn't missing something. I will see what happens.

      Also, acoustic said they would send me the schematic, if available. That's nice of them.

      I'll let you all know.



      • #4
        The op amp was bad. It's back to life for now.

        Thanks for the tip!


        • #5
          That balanced out was probably connected to a phantom powered mixer input and the +48V killed the Op Amp.
          Some Balanced Out circuits are protected from that, but not all; for example SWR suggests "balanced output must not be connected to phantom powered inputs", easier said than done, because usually mixers have *one* switch which applies phantom power to *all* inputs at the same time, you canīt select which to, what in my view is a defect in design concept.

          EDIT: when/if you receive that schematic, please post it here for future reference.
          Juan Manuel Fahey


          • #6
            Thanks for the phantom power thought. I was trying to think of what would kill it.

            Now you've got me thinking of my SWR amp. It's an early one before Fender. I'm sure it's been connected to phantom at some point. I've been playing it love for years. I will check the schematic.

            Thanks again!

            Oh, I was already planning to share the schematic if I get it.


            • #7
              I checked my SWR schematic and sure enough, xlr connected directly to the op amp and ground. I must have gotten lucky all these years. Looks like DI or 1/4" connections from now on.


              • #8
                Op Amp outputs can safely swing between +/-15V or whatever they are fed, but not beyond. (just check datasheets)
                Connecting/disconnecting +48V phantom ... or even +24V used by some cheap mixers *will* cause a 48 or 24V peak, even if capacitor coupled (most are, a few are not) but in general you have series resistors which will limit current somewhat plus peaks must be brief, a fraction of a second, thatīs probably why many survive.
                Personally I donīt like abuse if it can be avoided, and a very popular protection used in any "external access" connector, where a user might connect a speaker by mistake (itīs quite common) is to add a couple reverse polarity diodes from Op Amp out (or In) pin to +/-15V rails.

                Usually such diodes are reverse biased so "they are not there" but if a peak is higher than +15V or lower than -15V, such peak is safely clamped and absorbed by the supply .
                Of course, current is limited by a series resistor ... which generally is already present.
                Juan Manuel Fahey


                • #9
                  Can you think of an amp, I could look up the schematic for, that has a protected balanced out? One with an op amp, not transformer.



                  • #10
                    Protect any direct circuit you like by adding a pair of caps to the two active lines.
                    Education is what you're left with after you have forgotten what you have learned.


                    • #11
                      I was thinking that, but thought maybe there was more to it. It seems for the cost of 2 caps,. Everyone would do it.



                      • #12
                        Coulda, shoulda, woulda...

                        Perhaps they didn't feel the need.
                        Education is what you're left with after you have forgotten what you have learned.


                        • #13
                          SWR already has caps in series with XLR outputs to avoid DC seeping in, and they point the + terminal "outside" because they expect a positive one, typically 48V (so caps should at lest stand 50V and even better 63V) and in general this is enough, but the act of connecting +48V sends a pulse through them.
                          Caps do not pass steady DC but they happily allow pulses through.

                          So to improve protection, itīs good to add a couple clamping diodes *per Op Amp pin* connected to the outside world.

                          Standard connection is from protected pin to +15 and - 15 through reverse biased/polarized diodes, so under normal conditions "they are not there" but dump into the power rails any excess voltage.

                          In this Peavey mixer schematic you can see protective diodes at each XLR pin connected to some Op Amp pin, with series limiting resistors.
                          These are inputs but same can be done at XLR outputs.
                          I would.

                          Capacitors and resistors are already there, so we only need 2 cheap diodes per pin (#2 and #3 in an XLR) tack soldered to pads between Op Amp inputs or outputs and nearby +/-15V rails feeding Op Amps. A few mm away at most

                          Click image for larger version

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                          Copy what they did with: D101/102/103/104 .
                          They also protected Line input with D105/106

                          I have seen Peavey and Crate also protecting Loop send and receive jacks, the same way and for the minuscule cost and complication, it pays to do so with XLR outputs.
                          Juan Manuel Fahey


                          • #14
                            Look what the cat brought: Roland protecting their XLR outputs.

                            Working principle: as explained above.

                            Practical implementation:

                            Click image for larger version

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                            They use a dual diode package instead of 2 x 1N4148 .
                            Juan Manuel Fahey


                            • #15