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MiniDSP and Amp help - static/whine

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  • #16
    Neil and Mike, thanks for your help.

    Mike, the hum only occurs when the second amp's speaker outputs are plugged into the MiniDSP. Both amps powered, only one amp's speaker outs connected has no hum. But as soon as I plug the speaker out from the other amp in, the whine starts.

    Neil, I read the provided datasheet and also took the heatsink off of one amp to take a look, and all I know is I don't have the capability to sync the clocks as shown. The soldering/desoldering is far too small for me to even attempt.

    I may just have to switch these over to passive crossovers or wait to buy a 4 channel amp. The active setup was really so simple to implement in terms of the crossover, this is a bit frustrating.

    I appreciate the input guys, thanks.

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    • #17
      Originally posted by scottvalentin View Post
      Neil and Mike, thanks for your help.

      Mike, the hum only occurs when the second amp's speaker outputs are plugged into the MiniDSP.
      A little confusing there ... Only when the DSP outputs are plugged into the second amp? OR, Only when the speakers are plugged into the second amp outputs?

      Does your laptop brick have a thee prong ac plug (e.g, a grounded plug)?

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      • #18
        Originally posted by Millstonemike View Post
        A little confusing there ... Only when the DSP outputs are plugged into the second amp? OR, Only when the speakers are plugged into the second amp outputs? Does your laptop brick have a thee prong ac plug (e.g, a grounded plug)?
        Yes three pronged plug. For clarity, there's no whine with both amps powered and output 1+2 of minidsp plugged in from amp1. As soon as I plug in output 3 or 4 (which are from amp2) I get the whine. Or with 3+4 only it's ok, but plug in 1 or 2 and it starts to whine. Sent from my D6603 using Tapatalk

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        • #19
          Originally posted by scottvalentin View Post
          Yes three pronged plug. For clarity, there's no whine with both amps powered and output 1+2 of minidsp plugged in from amp1. As soon as I plug in output 3 or 4 (which are from amp2) I get the whine. Or with 3+4 only it's ok, but plug in 1 or 2 and it starts to whine. Sent from my D6603 using Tapatalk
          If you have an AC ground adapter laying around try it insuring the ground is not connected. Many switch-mode power supplies connect their minus DC out to the AC ground prong. That test should eliminate ground loop as an issue. But it does sound like there is amp-to-amp interference as well. You may have two issues since going to 1 PS helped somewhat.

          Did you try Neil's suggestion to place a metal barrier between the amps? Almost anything can be used to test (e.g., baking sheet). or physically move the amps farther apart to see if that affects the whine.

          Click image for larger version

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          • #20
            I don't have such an adaptor, would another way to just be to remove the ground connector from the AC Mains in on the amps? I will give the metal barrier a try. Thanks

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            • #21
              No change using a large iron pot to cover one amp - whine didn't change pitch or volume. Removing the ground at the amp input just made the whine very loud (so ground still plugged in the wall, but not connected to the amps).

              Neil mentioned a star ground that would occur after the MiniDSP outs - how would I wire that?

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              • #22
                Wife, "Where you running off to with my skillet?" Uhm .... dear ... it's to make my stereo sound better. Scott(?) gets one of those weird kind of looks like when you're talking to your dog, he cocks his head sideways, and you think he understands you.

                Does the blue light stay lit continuously? IIRC, under normal operation it should blink, but remain solid blue when programming. A black paint stick, nail polish, or some other easily attainable small applicator will fix that annoying blinky light.
                http://techtalk.parts-express.com/fo...khanspires-but
                http://techtalk.parts-express.com/fo...pico-neo-build
                http://techtalk.parts-express.com/fo...ensation-build

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                • #23
                  Originally posted by scottvalentin View Post
                  No change using a large iron pot to cover one amp - whine didn't change pitch or volume. Removing the ground at the amp input just made the whine very loud (so ground still plugged in the wall, but not connected to the amps).

                  Neil mentioned a star ground that would occur after the MiniDSP outs - how would I wire that?
                  "Removing the ground at the amp input" What did you mean? Did you remove the negative DC connection to the amps power terminal?

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                  • #24
                    Scott,

                    Sorry, here are some ramblings as I have no solid suggestions for you to try. But I can tell you that I use two of those same exact amp boards, a MiniDSP 2x4 (model that comes in a nice case), two 19.3v laptop bricks, and one 5vdc wall wort to power the MiniDSP. My prototype has similar spacing between the amp boards and dsp as yours. My prototype is dead silent; no hiss, no hum, and no whine. The wall wort is only 2 prong. Both laptop bricks are 3 prong and I have not cut any of those grounds. I have no star ground. The 2 wire output of each laptop brick feeds the power input terminals of its amp board. I simply made 4 rca plugs that run both + and - from each DSP output to the amp boards (L and R on each amp board share the same -).

                    The only thing I can see that I did different than your setup is that I added a 20 uF film cap in series with each tweeter output + to give some pop and low freq protection for the tweeters.

                    Also, I use one of those TDA amp boards for the two woofers, and I use one TDA amp board for the two tweeters. Maybe if you are using one amp board per side you could try doing what I have working. Or also add some tweeter protection caps... hope any of this helps.
                    Craig

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                    • #25
                      Craig,
                      Thanks for chiming in - I did recall you had the same setup and was hoping to come up with something to try.

                      I will go back to the two bricks and try two tweeters on one amp and 2 woofers on the other - easy enough to move the RCA's around. Thank you for your suggestions.

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                      • #26
                        I think you already had the same setup as Craig at one time; two amp PSs and the mini-DSP powered from a wall wart. So something may be going on.

                        One thing that caught my eye in the pictures was the RCA cables. Non seem to be shielded (or a least not the last several inches using the connectors/pigtails that came with the amp). I might try rotating the the mini-DSP 90o to face the amps and tighten up/shorten/separate all the signal cables from anything else. You might try testing both amps with the RCAs unplugged from the mini-DSP and their plus/minus pins shorted to each other. If the whine goes away. it's upstream from the amps, if not, it's in the amps.

                        The RCA feeds into the mini-DSP "seem" like shielded cables. If not, they are running right across the speaker outs. Again, cable runs may be an issue.

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                        • #27
                          Ok thanks Mike I will test that out too as I'm going to rewire the rcas between the amps and the minidsp.

                          I will report back in a day or two!

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                          • #28
                            Scott,
                            as much of a pain as it is to deal with all of these issues... this thread will be a great source of valuable information for lots of other folks who will end up having a similar issue in the future, thanks for sticking with it. Hope you guys get it figured out.

                            TomZ
                            *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
                            *Part 2 *Gluing multiple curved laminations of HDF *Cello's Speaker Project Page

                            *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

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                            • #29
                              Ok all, I believe I have solved the issue! No more whine!

                              I was re-jigging the board by adding some RCA plugs on the amp input side (from the MiniDSP) rather than the previous hard-wired version (photo 3 for reference).

                              I wired the plugs on both amps as I had before, with the red, white, and ground wires connected between the amp plug/wire that came with it and the plug (see first pic). I had moved the plugs to either side hoping to create some distance. I hooked it up to one speaker and sure enough, still some hum/whine (very high pitched). I had added an extra wire to take the ground from one set of plugs to the other side, and connected that, still hum. Tried moving the inputs around, and doing two tweeters to one amp, and two woofers to the other, still hum.

                              As I was fiddling, the small ground wire from one of the amps disconnected from the plug ground - see photo 2. HUM GONE! So I touched that side to the wire and hum on, took it off, hum off.

                              Conclusion:
                              For whatever reason, only 1 ground between the amp input and the miniDSP out pairs is connected. This seems to have solved the very high pitched whine. It now seems to work! Thanks all for sticking with this and helping me out!

                              I will post some pics of the speakers and cabinets soon - I have a write-up underway.

                              Scott

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                              • #30
                                Oh, and good luck and safe travels to all those headed to MWAF - one day I will fly down there, one day. A bit far from Calgary, Alberta (Canada).

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