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MiniDSP and Amp help - static/whine

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  • MiniDSP and Amp help - static/whine

    Hello all,
    I am having difficulty getting a pair of bookshelf speakers set up with MiniDSP and REW.

    My setup is 2 TDA amp boards (2X50w) powered by laptop bricks, the MiniDSP kit, and that is fed by a Behringer UCA220 from my laptop.

    The amps and the MiniDSP all work great if I only use two output channels (1 and 2 or 3 and 4) ONLY. If I try to run the pair of speakers using all 4 output channels of the MiniDSP, I get a whine type of noise. Not hum or static, but this high pitched noise.

    Again, either amp board works separately on any of the outputs, it only happens when I try to run a third and 4th output. So only when both amps are feeding speakers do I get this high-pitched whine. Some pics of the temporary setup attached.

    Can anyone help me troubleshoot this problem? I was really stoked to hear my speakers as a pair for the first time, and I guess that will have to wait until I sort this out.

    Thanks
    The speakers ready to go The amps and MiniDSP Close up of the MiniDSP

  • #2
    My setup is very similar; usb dac>preamp>mini dsp>one 4x100 t-amp (mid/tweet) and one 2x400 (woofers) t-amp. It's quiet. But when substituting a Carver AV 705x for amp duties, the only way to get rid of the hum is by grounding the amp chassis to the minidsp chassis. Have you tried that?
    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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    • #3
      Try powering the MiniDSP with a power supply of its own, instead of USB.

      Comment


      • #4
        I have not done that - how would I accomplish that? The amps have a ground at the AC main in, and a ground on the RCA input plug - which one of those do I take to the miniDSP board? Do I just attach it to say a mounting screw on the miniDSP board? Thanks for the suggestion.

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        • #5
          Originally posted by scottvalentin View Post
          I have not done that - how would I accomplish that? The amps have a ground at the AC main in, and a ground on the RCA input plug - which one of those do I take to the miniDSP board? Do I just attach it to say a mounting screw on the miniDSP board? Thanks for the suggestion.
          There is a barrel pin power connector on the miniDSP that you can plug a separate power supply to. The connector is a common size, so you should have something lying around that'll fit.

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          • #6
            Originally posted by scottvalentin View Post
            I have not done that - how would I accomplish that? The amps have a ground at the AC main in, and a ground on the RCA input plug - which one of those do I take to the miniDSP board? Do I just attach it to say a mounting screw on the miniDSP board? Thanks for the suggestion.
            Not sure if you were asking me or bcdemz. If you're wanting to check if it's a ground issue, just touch a wire from a screw on the amp chassis to a screw on the minidsp chassis.

            Also, when powering the minidsp from the laptop, I can not have the laptop plugged into wall power; lots of noise. With the laptop on its own battery power it's quiet.

            You can also use a buck (sp?) convertor to get the correct voltage from the smps to power the mini. They're just a couple bucks on ebay.
            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

            Comment


            • #7
              Thanks for the suggestions I will try those in a couple hours after kids are in bed.

              Comment


              • #8
                Update on this:
                I tried the 5v supply for the minidsp I bought from PE instead of USB for power and that didn't fix it.

                I tried running a wire from each amp heatsink screw to the minidsp screws, no change.

                I ran a wire between the two grounds of the amplifier AC input terminals and that got rid of 90% of the screech/whine. I will be looking to rewire those tomorrow and hopefully just fiddling with those terminals will get me all the way there.

                Thanks for the help so far, will update tomorrow.

                Comment


                • #9
                  Well, I re-wired the boards and added the jumper from the ground of each amp, and I'd say its 85% improved. I can listen to music now without really hearing the whine, but it's still there. Any further ideas on how to iron it out?

                  Comment


                  • #10
                    Scott,

                    I've experienced issues with some of the small amp boards like you're experiencing.

                    Have you tried to unhook the inputs from one amp while both are powered to see if that corrects things?

                    I have no idea if this would work or not, but since one of the common connections between the two amps is the signal input, would a ground loop isolator between the two amps do any good? So feed input to one amp, then run the signal through the isolator, then to the second amp. If you have one around, it might be worth a try. Something like this: https://www.parts-express.com/boss-b...lter--265-4041

                    Hopefully Mike will chime in and shed some light, he's pretty good with this stuff. Whereas I'm just guessing. I really have no idea sorry to say, and some of the 'fixes' I've stumbled upon for stuff like this have been accidental. I have one of those amps in my "Sound Traveler" portable luggage stereo system and sometimes if the plug isn't inserted just right, I get a nasty hum. The plug is in tight, it just hums until I turn the cable a bit. Tried other cables, no difference. Who knows?

                    Hope you 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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                    • #11
                      Originally posted by scottvalentin View Post
                      ... My setup is 2 TDA amp boards (2X50w) powered by laptop bricks.
                      I see brickS as in two. Are using using separate laptop bricks to power each amp? Try running both amps off the same PS / brick. The switching power supplies inside are not isolated (unlike old transformer based PS). You probably don't need but one brick's amp capacity to run that set up.

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                      • #12
                        Thanks Tom and Mike - yes I am running one brick per amp board. I know it will work with all the inputs plugged in, it only whines/screeches when the second brick is plugged in. Running off of one brick is definitely more convenient as well - trying to move this little setup is very difficult with all the speaker wires plus the two laptop bricks!

                        I will report back!

                        Scott

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                        • #13
                          Mike I switched to one brick and this improved things a little more but it seems very sensitive to moving the ac mains input cables. If I moved them around the whine got louder or quieter even though I think the connection at the input is ok (not loose). Any further suggestions?

                          The Dell laptop brick has a white (live) a black (ground or I am connecting it to ground) and a green very thin wire. I read that the green is a Dell connector ID but is it possible this is a ground wire?

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                          • #14
                            Originally posted by scottvalentin View Post
                            Mike I switched to one brick and this improved things a little more but it seems very sensitive to moving the ac mains input cables. If I moved them around the whine got louder or quieter even though I think the connection at the input is ok (not loose). Any further suggestions?

                            The Dell laptop brick has a white (live) a black (ground or I am connecting it to ground) and a green very thin wire. I read that the green is a Dell connector ID but is it possible this is a ground wire?
                            That green wire is connect to a serial ROM like device in the brick. The laptop reads the device via the green wire to identify Dell PSs. You can ground the green wire to black just to eliminate that as a cause.

                            (1) You may be picking up radio interference from the PS switching circuit. Can you test with a 12 V battery? Or, try moving the PS brick as far away from the modules as possible and route the power cable away from all electronics so that only the PS plug is close to the modules.

                            (2) You may be picking up harmonics between the two amps switching frequencies Do you get the hum using only channels 1 & 2 but both amps powered (i.e., power both amps but only connect RC cables to channels 1 & 2)?

                            Comment


                            • #15
                              Originally posted by scottvalentin View Post
                              Any further ideas on how to iron it out?
                              ​The whine is almost certainly caused by the difference in switching frequencies between the two amps. Each amp is switching at approx. 400KHz, and it is common for the difference frequency to be in the audio range--hence the whine.

                              ​The best solution is to force the two amps to switch at the same frequency. Figure 32 of the TDA7492 datasheet shows how to synchronize the clocks. Even better, buy the 4-channel version of the amp to have this circuitry already implemented.

                              ​There can be three sources of the "whine". One is magnetic coupling. This is coupling from the output filter into magnetic leads. Since some wire uses nickel coating or iron-on-copper, it can be easy for the output filter from one set of amps to induce a small voltage into the other. You can test for magnetic coupling by placing an iron object between the boards (there's a cute "iron it out" pun in there).

                              Another possibility is that the switching frequencies are getting coupled through the power supply. But since you are using two laptop power supplies, that shouldn't be an issue.

                              The final possibility is improper grounding. You might have too many grounds, and there is a difference current flowing along the ground. Ideally, the amps should only share a ground at the output of the miniDSP. This would create a "star" ground with the miniDSP output being at the center of the star. However, it is sometimes impossible to have a star connection when devices are grounded through other equipment, and you end up with what some people call ground loops. You may have to do some experimenting. Also, it might help to run the miniDSP from a dedicated USB charger, so that it is isolated from other ground paths. Unfortunately, grounding can be a real headache, as I'm sure you've come to realize...
                              Last edited by neildavis; 07-09-2017, 04:24 PM.
                              Free Passive Speaker Designer Lite (PSD-Lite) -- http://www.audiodevelopers.com/Softw...Lite/setup.exe

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