8 ohm and 16 ohm

Discussion in 'Amps and Cabs' started by thesooze, Feb 26, 2008.

  1. thesooze

    thesooze Member

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    What's the difference?

    I'm just not getting ohms vs. watts for rating amps and speakers.

    I have a Maz 38 head... (at 38 watts)- can I put this into a 16 ohm cab with speakers rated at less than 30 watts? Also do ohms and watts have to match?

    For instance- I just bought a 16 ohm cab, with a speaker in it that reads 8 ohm. I'm just not understanding.:bkw
     
  2. edward

    edward Supporting Member

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    Hi Snooze,

    Ohms and Watts ...apples and oranges. Ohms is the measured resistance (sometimes referred to as "impedence") expressed in the value called "ohms," and this value must match (exactly or at least closely) with the stated impedence of what your amp says on the back panel. This is completely irrespective of what wattage your speakers or amp are rated at.

    So if your amp says "8 ohms" at the output jack, you should plug an 8 ohm cabinet into that amp. You "could" plug a 4 ohm or 16 ohm cab into said 8-ohm amp output, too (this is a 1-order mismatch that is usually ok; don't go any farther than this). But depending on the amp, this could stress the transformer (especially on older, vintage, or less durable amps) eventually causing failure. On amps with sturdy trannies, this poses no risk, but said impedence mismatch will alter the voice of your speaker ...for better or for worse is up to you as some choose to mismatch impedence because they prefer "that" tone.

    Now "wattage" is the value that expresses current. For your speaker, the stated wattage is the manufacturer's claim of maximum output a given speaker can handle. So you could put a spkr rated at 100 watts into a 5-watt amp and never be at risk blowing it. Or you could put a 10 watt speaker into same amp. Up to you. Just don't put a speaker that is rated below the amp's stated output. And in practice, you should be wary of running a spkr of the same stated output as the amp as amps often can peak at more than their rated output, especially true when running in the amp's "overdrive" channel, and particularly true of a good tube amp.

    All the above is full of generalizations, and there are as many exceptions as there are manufacturers of amsp and speakers, so don't take anything as gospel. But these are general rules of thumb that can help guide you in choosing the right cab/speaker for a given amp. Hope that helps you a bit :)

    Edward
     
  3. edward

    edward Supporting Member

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    Dang it, all that long-widedness, and I neglected to address your specific question on your particular cab.

    I am guessing that the cab you have has 2 spkrs, with each speaker being an 8-ohm, and each rated at 30 watts. It is wired "in series" which means that its impedence doubles for a total 16 ohm load ...that is the figure that ultimately concerns you. You could also, for example, wire said pair of 8ohm speakers "in parallel" which would cut the impedence in half, to a 4 ohm load for the cab.

    If your Maz head has an output jack of 8 or 16 ohms, either is fine ...try them both and see which sound you like better. Just don't put your 16 ohm cab into a 4 ohm amp output ...too much of an impedence mismatch that risks stressing out your tranny.

    As for wattage, you can simply add each speaker's stated rating: so your particular cab has a sum of 60 watt capability if you have two 30-watt speakers in there ...perfectly fine for your Maz' stated output of 38 watts.

    Edward
     
  4. GCDEF

    GCDEF Supporting Member

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    Good answers Edward. I'd just clarify this one point. Wattage doesn't really add. I'm sure you know this but for the OP, it's number of speakers X wattage of lowest rated speaker. For example a 30 and a 70 watt speaker would handle 60 watts total (30 x 2).
     
  5. HKGuns

    HKGuns Member

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    Edward's comments are spot on, there is another factor to consider for technical accuracy.

    Impedance is more than just resistance. Impedance has a frequency component to it as well called reactance.

    Symbol Z: A measure of the total opposition to current flow in an alternating current circuit, made up of two components, ohmic resistance and reactance, and usually represented in complex notation as Z = R + IX, where R is the ohmic resistance and X is the reactance. (For an AC circuit Ohm's law can be written V=IZ where V and I are the oscillating phasor voltage and current respectively and Z is the complex impedance for the frequency of oscillation.)
     
  6. JohnM

    JohnM Member

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    Isn't that backwards? I thought a 4ohm output could handle a 16ohm load, but not the other way around.

    Having a load that is a lower number than the output stresses the tranny by letting more juice than it should out, right? Whereas a 16ohm load on a 4 ohm output will just run cooler and demand less from the tranny?
    Have I lost my mind here?:messedup
     
  7. edward

    edward Supporting Member

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    Doh!! Thanks for clarifying that ...I always assume multiples of the same speaker within a given cab. We all know what happens when one "assumes" :)

    And more from you EE or professional types that have the real goods on this topic. I'm just a musical hack, albeit one that has been around the block once or twice and so occasionally has something relevant to share. There are some of yoose folks here who actually know what you're talking about, to whom I will defer. :D

    Tom, from what I recall, a significant mismatch in either direction can stress a tranny; either it's primary winding or its secondary (IIRC). But as I said, someone smack me down if I get that specific point wrong.
    :eek:

    Hey HK,
    Should I assume you are an owner as your screen name suggests? HKs are my absolute favorite!

    Edward
     
  8. HKGuns

    HKGuns Member

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    Yes Edward, P7M8 cult member as well as others.
     
  9. GCDEF

    GCDEF Supporting Member

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    You're making the common mistake of assuming solid state and tube amps work the same way. Too high an impedance for a tube amp is actually potentially more damaging than too low.
     
  10. chrisrob

    chrisrob Member

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    The short answer is: Get a cab that matches your amp. No danger. No strain. No "I hope my amp can take this." I've heard so many different things as far as what a safe difference is.

    The only undisputed truth is that a cab that matches the output of your amp is safe.
     
  11. pharmx

    pharmx Member

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    Is there an audible difference between using the 16 ohm output on the amp with a 16 ohm speaker versus an 8 ohm output on the amp with an 8 ohm speaker?
     

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