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How should I calculate max gain on a DSP amplifier?

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  • How should I calculate max gain on a DSP amplifier?

    I have a question regarding setting up of a DSP amplifier for a speaker with a lower power rating than the amp. I realize there is the issue of peak powers to deal with, but assuming the objective is to prevent someone blowing the speaker by turning the volume up to maximum. Here's my train of thought:

    Amplifier has a 50W rating:
    https://www.parts-express.com/dayton...h-dsp--325-126

    The speaker has a 25W continuous rating

    If I make sure the output at the end of the DSP chain is -3dB down from the input, the amplifier should never produce output enough power to blow the speaker.

    Do you think this holds up? It feels a bit too simple to be true.

  • #2
    The 2 x 50 W rating is simply the KAB chip's thermal dissipation limits. All else being equal, the power out is dependent on two things: PS voltage and speaker impedance. The ROT for a chip amp in BTL mode (ex. the KAB) is: P rms = (PSv-1)2 / (2 x SPimp). With a 24 V PS, you get ~ 33 W rms at < 0.1% THD into 8 ohm speakers. Near double that for for 4 ohm drivers. If it's a 8 ohm driver, I wouldn't worry about the 25 W rating. It's good to have some head room for peaks that won't stress the driver. No if your using a PS at a lower voltage you can calculate the power. At 12 V, power into 8 ohms is ~7.5 W rms, and into 4 ohms is ~15 W rms.

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    • zx82net
      zx82net commented
      Editing a comment
      Thanks for the insight

  • #3
    People (like me) routinely use amps capable of quickly destroying the drivers if run at full power continuously. That's not how music works, though. You need power for the peaks; the average power will sometimes be 20dB (or 100x) less than the peaks. So your 50W amp is good for a whole 0.5W average if you want to preserve the peaks.
    Francis

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    • #4
      Originally posted by zx82net View Post
      Do you think this holds up? It feels a bit too simple to be true.
      It is too simple to be true. The only way to be sure an amp can't blow a speaker is with a limiter, which restricts the voltage input to the amp so that the voltage output of the amp can't exceed either the thermal or mechanical limits of the speaker. If the DSP you use has that capability all well and good, but most do not.

      www.billfitzmaurice.com
      www.billfitzmaurice.info/forum

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      • #5
        Is not cranking it up to 11 out of the question?
        Francis

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        • #6
          Just leave a note. I'm sure it will be fine.

          Comment


          • #7
            Originally posted by fpitas View Post
            Is not cranking it up to 11 out of the question?
            Indeed, that would normally be my approach.

            The problem is avoiding kids or the uninitiated turning it up to 13!

            Unfortunately, these days "turning it up to maximum" seems standard practice for a lot of people.

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            • #8
              Sigh. Again. First of all, the power rating of the amp is into a specified resistive load for when the distortion skyrockets to some insane value. You can look at the TI data sheet to understand what the basic chip does. Then look at the power supply you are applying and the amount of heat sink on the actual board. By their graphs, you can see how much power it could sustain without blowing up at what frequencies. Looks to me like a nice little 10W into 8 Ohm amp. Really nice actually. You wil probably use less than 1 or 2.

              Speaker rating is for CONTINIOUS sine wave input. It is when the voice coil will get too hot and fail. It has almost nothing to do with amplifier rated power. What does, is the amount of power it can handle at any given frequency in relation to the mechanical excursion of the cone. In other words, when a really big amp can bottom out the voice coil.

              Investment tip: Hearing aid manufactures. Between ear buds buds, clubs, and car stereos, we have at least two generations getting older with sever hearing damage ( not even including out vets) and more to come.

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              • #9
                Originally posted by tvrgeek View Post
                Speaker rating is for CONTINIOUS sine wave input.
                Speaker Pe ratings are determined using noise and sweeps. http://xlrtechs.com/dbkeele.com/PDF/...0EIA-426-B.PDF
                www.billfitzmaurice.com
                www.billfitzmaurice.info/forum

                Comment


                • tvrgeek
                  tvrgeek commented
                  Editing a comment
                  Not all. Most are continuous tone as it is a function of heat dissipation. It is average current so how you do it does not matter

              • #10
                Originally posted by tvrgeek View Post
                Sigh. Again. First of all, the power rating of the amp is into a specified resistive load for when the distortion skyrockets to some insane value. You can look at the TI data sheet to understand what the basic chip does. Then look at the power supply you are applying and the amount of heat sink on the actual board. By their graphs, you can see how much power it could sustain without blowing up at what frequencies. Looks to me like a nice little 10W into 8 Ohm amp. Really nice actually. You wil probably use less than 1 or 2.

                Speaker rating is for CONTINIOUS sine wave input. It is when the voice coil will get too hot and fail. It has almost nothing to do with amplifier rated power. What does, is the amount of power it can handle at any given frequency in relation to the mechanical excursion of the cone. In other words, when a really big amp can bottom out the voice coil.

                Investment tip: Hearing aid manufactures. Between ear buds buds, clubs, and car stereos, we have at least two generations getting older with sever hearing damage ( not even including out vets) and more to come.
                Getting back on topic here: the aim is to prevent someone who doesn't know what they are doing from blowing up the speaker. Any practical ideas? I'm happy to make the assumption that people are not going to deliberately try to destroy it with pure sine tones. Let's assume music, just for the sake of argument.

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                • #11
                  I already told you. Use one of these: https://dbxpro.com/en/products/166xs
                  But I don't think it's cost effective for your application.

                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

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                  • #12
                    Hmm, I certainly won't fit a rack mount compressor in the box, but thinking along those lines, perhaps this is a practical alternative:
                    https://ez.analog.com/dsp/sigmadsp/f.../100522#100522
                    I'll give it a shot.

                    Comment


                    • #13
                      Originally posted by zx82net View Post

                      Getting back on topic here: the aim is to prevent someone who doesn't know what they are doing from blowing up the speaker. Any practical ideas? I'm happy to make the assumption that people are not going to deliberately try to destroy it with pure sine tones. Let's assume music, just for the sake of argument.
                      Fuse

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                      • #14
                        TVRGeek and myself disagree, a bit, on the clean power these amps will put out. They're pretty clean up to clipping - a function of the PS voltage. The KAB module has a pretty big heatsink. I don't see a problem getting 65 W rms sustained into a 4 ohm driver with 24 V supply (as I have out of it's TDA3116 chip).

                        What driver are you using? The thermal limit of the driver is most likely a second consideration to Xmax. Most all drivers run out of excursion before hitting their thermal limits.

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                        • #15
                          I'm actually working on a couple of designs, the figure of 25W was for the sake of discussion. The driver I'm most concerned about is actually this one:
                          https://www.parts-express.com/tecton...4-ohm--297-216
                          (10W continuous, 20W peak)
                          I'm using a sharp 165Hz high pass in the DSP. So I think over excursion shouldn't be an issue, I may be proven wrong.

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