Figuring out impedance on unmarked drivers

Discussion in 'Archived Threads 2001-2004' started by JohnnyG, Jul 23, 2001.

  1. JohnnyG

    JohnnyG Screenwriter

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    WARNING! Stupid speaker building newbie questions below!
    I have some drivers that I suspect are greater than the usual 8 ohms, but there are no markings on it whatsoever. How can I determine the impedance?
     
  2. Philip_G

    Philip_G Producer

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    couldn't you just use a multimeter across the voice coil to measure the resistence?
     
  3. JohnnyG

    JohnnyG Screenwriter

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    That's the first thing I thought of and I got a solid 8ohms, but other '8 ohm' drivers I have test anywhere between 4 and 6.5 ohms!
    Also, many of the drivers in the Parts Express catalog list 'Nominal Impedance' and 'DC Resistance', the latter often being around the 6-7 ohm mark on '8ohm' speakers.
    Soooo, I assume that these drivers are >8ohm. The next question I have is how can I turn them into 8 ohm drivers? What kind of resisters are used in an application like this and how do I calculate the value?
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    John Golitsis
    Next Big Thing Electronics
    [Edited last by JohnnyG on July 24, 2001 at 12:25 PM]
     
  4. Patrick Sun

    Patrick Sun Studio Mogul

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    You can't turn drivers into a specific impedance (lower than their rated impedance). Accept this fact, and design for when it's a higher impedance (which is not a bad thing!).
    The problem comes when the impedance of the drivers drop below what your amp can handle in terms of providing enough current to drive the impedance load. Usually under 3 ohms is bad for most conventional amps, unless it's very robust and can handle low impedances, down to 2 ohms.
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    PatCave; HT Pix; Gear; DIY Mains; DIY CC; Sunosub I + II + III; DVDs; LDs
     
  5. ThomasW

    ThomasW Cinematographer

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    Johnny
    Patrick is correct with his comments. If you put a resistor on the speaker, 1/2 of the amplifier power will go into the resistor (not a good thing).
    Actually the impedance of most conventional speakers vary with frequency. Mfgrs just take a sample of the plot and post that as the value. This is the reason for the use of the word "nominal" in the stated value.
    Here's an example of a impedance plot for a so called "8ohm" driver
    [​IMG]
    [Edited last by ThomasW on July 24, 2001 at 01:05 PM]
     
  6. JohnnyG

    JohnnyG Screenwriter

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    Fair 'nough! Thanks for the replies!
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