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Thread: Ohm Boy

  1. #1
    Senior Member captdg's Avatar
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    Ohm Boy

    Which is better , an amp with 60 watts at 8 ohms or 100 watts a 4 ohms? Which speaker would be less likely to be damaged by volume?

    Thanks
    DG

  2. #2
    Still a Junior Member Albrecht Smuten's Avatar
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    I've been told that the input impedance of the speaker must be equal or higher than the output impedance of the amp.
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  3. #3
    SuperD Boogie's Avatar
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    Having the power and using the power are two different things. Personally, I like having a lower rated speaker being pushed hard but I know there are limits. I also know that I don't play my 100w amps anywhere close to their limits, so a 60w speaker is more than plenty. It also depends on how many speakers you're driving. That's why a 4x12 with 25w (16 ohm) greenbacks sounded so great when pushed by a 50w.

    A 100w amp will drive a 4 ohm speak much harder than an 8 ohm speaker, so comparing a 60w at 8 ohms is tough. If it's just about surviving maximum load, then just pick up an EV12 series (about 300w rating) equivalent and don't look back. But if you have a tone preference, post that up. Too many options.

    An impedance mismatch is OK with a tube amp if it's reasonable (nothing below 4 ohms typically) and can yield good results.

  4. #4
    The speaker's resistance in ohms is generally unrelated to its power handling capacity. Speakers are offered in different ohm ratings mainly so that they can be combined in various ways to offer the resistance the amplifier wants to see.

    And of course as time went on in guitar speaker design, speakers were given the ability to handle more and more power.

    Unlike transistor hi-fi equipment, a tube amp usually doesn't put out more power into speakers with lower ohm loads. In the hi fi world you'll see a transistor amplifier's rated power at 2, 4, or 8 ohms differ. But even in the hi fi world, it's common to use an amp with a much higher power rating than the nominal power handling ability of the speakers to give the amp more headroom in short-burst peak passages. Then the speakers would have fuses in line to prevent damage to the speaker if a power rating is exceeded for a given time.

    A speaker's ability to handle higher power is all about its magnet structure, voice coil components, and the ability of the speaker to handle large excursions without being destroyed.
    Last edited by LSchefman; 03-07-2013 at 08:22 AM.
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    Senior Member captdg's Avatar
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    My Reason for asking was due to the fact that I own a PRS guitar, I did get a Vox Valvetronix..Not that PRS amps are not great, I am a neophyte and wanted the modeling before I bought a bunch of pedals. There were four choices..VT 20 (30w RMS @4 ohms) VT 40 (60w @ 8 ohms) VT 80 (120w @ 4ohms) and VT 120 (4ohms x2 speakers) . But like Mr.Schefman posted, I was thinkin back in the day of Hi-Fi and Klipsch horns where ohms were used to convey what a speaker could handle without "Ones brother in law comes over intoxicated and says "Crank IT Up!".. So , maybe fallaciously I went with the VT 40 because I thought 8 ohms were better than 4 to minimize speaker damage.
    Its weird..I feel like a traitor for not owning a PRS amp.

    DG

  6. #6
    When I saw the thread title I figured you were talking about Carlos.

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  7. #7
    Senior Member jfb's Avatar
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    Quote Originally Posted by ]-[ @ n $ 0 |v| a T ! View Post
    When I saw the thread title I figured you were talking about Carlos.

    I thought he was talking about Chris Poland. Crazy pinky shred! Poland should rock a PRS.
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  8. #8
    Quote Originally Posted by ]-[ @ n $ 0 |v| a T ! View Post
    When I saw the thread title I figured you were talking about Carlos.

    You know, those would make awesome truss rod covers on a PRS...
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  9. #9
    SuperD Boogie's Avatar
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    Quote Originally Posted by captdg View Post
    ... I went with the VT 40...Its weird..I feel like a traitor for not owning a PRS amp.
    My son has a VT40 and it's a fantastic amp...and LOUD. The impedance is only used for matching the output of the amp. If it's the stock speaker, it will handle what that amp has in store easily. Good stuff.

    There's plenty of time in this life for a PRS amp. No need to rush it. Enjoy your Vox.

  10. #10
    Senior Member captdg's Avatar
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    I like hangin with my "Ohmies"

  11. #11
    Senior Member captdg's Avatar
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    That's what I thought they were.

  12. #12
    deus ex machina
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    Impedance is not a synonym for resistance. Resistance is the opposition to the flow of direct current (DC). Impedance is the opposition to the flow of alternating current (AC). Impedance is a complex value that contains a real component (resistance) and an imaginary component (reactance).

    Impedance (Z) = SQUARE-ROOT(R^2 + X^2), where R = resistance, X = reactance, and the symbol "^" denotes raised to the power of

    X = Xl - Xc, where Xl = inductive reactance, Xc = capacitive reactance

    Xl = 2 x Pi x F x L, where Pi = 22/7ths, F = frequency in Hertz, and L = inductance in Henries

    Xc = 1 / (2 x Pi x F x C), where Pi = 22/7ths, F = frequency in Hertz, and C = capacitance in Farads

    The take away here is that resistance (R) in a DC circuit is constant whereas impedance (Z) in an AC circuit changes with respect to frequency because Xl and Xc are frequency dependent. The impedance of a speaker changes with respect to frequency, which is why speakers carry a "nominal" impedance rating.

  13. #13
    Senior Member captdg's Avatar
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    Is it similar as when an opera star can break a glass by using the right frequecy?

  14. #14
    deus ex machina
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    Quote Originally Posted by captdg View Post
    Is it similar as when an opera star can break a glass by using the right frequecy?
    No, that phenomenon is caused by hitting the resonant frequency of a glass. At resonance, the sound energy applied to a glass causes it to oscillate wildly until it literally tears itself apart.

  15. #15
    Senior Member captdg's Avatar
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    I learned all my Physics from Neil Degrasse Tyson.. I guess I need to open a book sometime!

    I guess the resonance phenomenon would also apply to the Tacoma Narrows bridge.

  16. #16
    Senior Member vchizzle's Avatar
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    Quote Originally Posted by captdg View Post
    I like hangin with my "Ohmies"
    Ohm no

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