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Will this 'turn-on' speaker 'whump' be an issue long term?

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  • #16
    Originally posted by DeZZar View Post

    So what's the speaker protection circuit supposed to do? Protect them from what?
    I Think it's basically a timed relay setup. After power-on, the protection circuit timer starts which 'waits' for a few seconds for the amp to produce it's turn-on 'thump' before the speakers are actually connected, then it connects the speakers via a relay. Many if not most retail home amps have, or used to have something similar built-in.

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    • #17
      Originally posted by tomzarbo View Post
      This amp: the TDA7492:

      creates a bit of a turn-on 'thump,' a few seconds after turning on the power, but only when connected to the video preamp board It's connected to. With the board disconnected, there's no turn-on thump.

      Here is a GIF of what it looks like:

      I think the woofers will tolerate it, but the tweeters, I'm not sure how they'll like it long-term. Of course, they're crossed at 6,500 Hz, so that IS pretty high.
      Also, it's not really audible, it's just a bit of DC offset going on for a split-second.

      Should I even worry about this at all? This is for a table radio setup by the way.
      I do have a DC to DC transformer that will take the 24v and reduce to 12v and isolate it as well for the preamp faceplate. Is there a possibility that isolating the preamp that way will eliminate the thump?

      If the first 100 thumping events don't damage anything, I don't see why 10000 more would change that, until there is some other problem such as failed capacitors.

      The TDA7492 data sheet (Google found it available for download on ST's website) shows available functionality for standby and mute modes, and the external circuits needed for that. For description see sect 6.1 and for example application see figure 28 in sect 5. Perhaps you might be able to temporarily remove the heat sink (screw heads are on the bottom side of the board assembly) and solder some small wires to the pins 20 and 21 (maybe use insulated wires scavenged from scrap Cat5 cable), and assemble the additional components in a dead-bug style external and adjacent to the board assy.

      Click image for larger version  Name:	320-606_ALT_1.jpg Views:	0 Size:	251.3 KB ID:	1495110

      Click image for larger version  Name:	320-606_ALT_2.jpg Views:	0 Size:	176.9 KB ID:	1495111

      Click image for larger version  Name:	tda7492-ic-circuit-png.487315.png Views:	0 Size:	337.6 KB ID:	1495112

      While you are in there, consider upgrading C23 power supply bypass capacitor.
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      • #18
        I'm a hobbyist so I can't really tell you what I'm trying to protect my speakers from. I might as well say "spooks" or "magic spark critters". There is a loud "thump" "whump" "pop" whatever you want to call it when I turn the power on or off. I have a 2.1 amp I'm making for my nephew and it does the same thing. I have a Voxel attached to the sub output on the 2.1 amp and you can see the woofer slowly push out toward you when you turn it on and it jumps back and forth when you turn the amp off. I've heard that the whole power-on pop is bad so I'm trying to prevent it. And it sounds ugly. Nobody wants ugly sound. JRT, thanks for the reply but with my skills it would be easier for me (and far more likely) to grow a unicorn bear from a seed than solder tiny wires to chip. Me and Solder don't really get along. And in all seriousness, thank you all for trying to help me.


        • #19
          I would try adding a 330 uF non-polarized capacitor in series between the amp and the speaker. This will block DC and other very low frequencies from the speaker.


          • #20
            Series would mean only on the signal (positive/red) side?


            • Billet
              Billet commented
              Editing a comment
              Yes, only one side. The positive side seems most obvious, but a series cap would would function the same on either the + or - side.

          • #21
            Thank you for that.