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Is overall impedance altered by adding hi-pass filter?

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  • Is overall impedance altered by adding hi-pass filter?

    Long-winded intro to a simple question. Looking for confirmation about the difference between impedance and resistance.

    I have a pair of MTM speakers, made from a kit. The crossover was designed for a specific tweeter: Vifa D26NC55. I bought the tweeters in 2009 and built the kit in 2010. Bass/midrange is great - but the high/treble was never quite there for me.

    I found out later that Vifa got bought by another firm (Tymphany?) a few years earlier and the manufacturing process must have changed. I've heard another build of this same kit from several years earlier and it sounded great.

    The problem is not the crossover. I tested it by removing a tweeter an attaching a small, bright speaker to the tweeter wires -- and that speaker was still bright.

    I wanted to replace my tweeters with the original Vifa but I couldn't find any. None that I could confirm as original. So instead, I decided to add another tweeter. I bought a pair of the AMT Mini-8 and the appropriate high pass filters. (See links)
    https://www.parts-express.com/dayton...8-ohm--275-095
    https://www.parts-express.com/5-khz-...sover--266-474

    I added the new tweeters in parallel to the existing crossover. Result sounds great to me.

    Here's my question: Have I inadvertently changed my 8 Ohm speakers into 4 Ohm speakers?

    The original kit speaker was designed as 8 Ohm. The tweeter circuit I added is also 8 Ohm. They are acting in parallel -- the new tweeter is connected to the back of the terminal connectors. If I were dealing in resistance, I'd have 4 Ohms now. But I don't think Impedance would change that much, given that so much more power is diverted to the main x-over and woofer. I have an Ohm-meter but nothing to test overall impedance directly.

    If I connected two full 8 Ohm speakers in parallel, the new Impedance WOULD be 4 Ohms, right? But since the second "speaker" I added was just a tweeter, Impedance wasn't changed much. Is that correct?

    Thoughts? Thanks.

  • #2
    Short answer: yes, that double tweeter combination is now 4 ohms.

    There's a lot more to it though, as you may imagine. That additional tweeter is only 8 ohms above where that crossover quits filtering. So it really only begins to be parallel at 8 ohms above about 5k. That's only accurate since you're using an AMT that just happens to stay right at 8 ohms at any frequency. It's even more complicated for most speakers that have resonance and inductance changing their impedance for any given frequency.

    This is not a big problem though, due to how power is distributed. Not much content actually happens up there, the power is quite low so the amplifier isn't really getting worked way up there anyway and you're likely to keep running just fine.
    Electronics engineer, woofer enthusiast, and musician.
    Wogg Music
    Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat

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    • #3
      Thanks. But tell me if this changes your answer: The new tweeter-crossover circuit is connected in parallel NOT just to the other tweeter. It's connected in parallel to the entire driver-crossover circuit of the original speaker. It's as if I connected the new tweeter/crossover across the input terminals of the speaker -- because that's what I did.

      Is the resulting total impedance still calculated at 4 Ohm? Could be. I just want to clarify the setup. Thanks.

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      • #4
        Originally posted by alanm1234 View Post
        Thanks. But tell me if this changes your answer: The new tweeter-crossover circuit is connected in parallel NOT just to the other tweeter. It's connected in parallel to the entire driver-crossover circuit of the original speaker. It's as if I connected the new tweeter/crossover across the input terminals of the speaker -- because that's what I did.

        Is the resulting total impedance still calculated at 4 Ohm? Could be. I just want to clarify the setup. Thanks.
        Wogg's answer still applies. The original XO for mid/bass limits low frequencies to the tweeter feed. So the amplifier only sees the tweeter circuit at high freq. and the mid/bass at low freq., never at the same instant (there is an overlap region but then some power to each, never full load to both circuits).

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        • #5
          The original XO for mid/bass limits low frequencies to the tweeter feed. So the amplifier only sees the tweeter circuit at high freq.
          Thanks. But I'm still not sure I'm being clear. Since I connected the new XO across the speaker terminals, doesn't the amplifier see my new tweeter circuit all the time? (I'm including the new tweeter AND filter when I say "tweeter circuit." Maybe we're defining "tweeter circuit" differently. If you just meant the tweeter driver, then yes, the amp only sees it at high frequency - but only because of the new filter, not the original XO.)

          I didn't know any other way to connect it that would guarantee I didn't mess up the existing setup. I did not want to cut out the existing "Vifa" tweeter or change it. It is definitely contributing to the sound. I just wanted to add the AMT leaving everything else as it was.

          I apologize for asking yet again but I didn't do a good job explaining the new connection.

          So what do you think? Is my amp now seeing a 4 Ohm speaker? I swear I'll stop asking!

          I've probably made a mess of it. Sounds damn good though!

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          • #6
            You're not quite catching the impedance bit, at frequency. The filter for the tweeter effectively raises the resistance of it as frequency goes below 5k. So at say 100, that 8 ohm tweeter is actually seen as hundreds or thousands of ohms, so in parallel is really doesn't matter.
            Electronics engineer, woofer enthusiast, and musician.
            Wogg Music
            Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat

            Comment


            • #7
              Originally posted by alanm1234 View Post
              Is my amp now seeing a 4 Ohm speaker?!
              Only above the crossover frequency.

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

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              • #8
                Thank you, wogg, Millstonemike, and billfitzmaurice for your explanations and your patience. It makes sense. I just couldn't tell if the slight difference in the way we were describing the setup mattered. Thanks again. (I could always add a 5k low-pass to the other xo and be back to 8 Ohms, but then the AMT might get a little louder and it doesn't need that.)

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