Old 19th February 2013
  #1
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Ohms and wattage

I have a peavey classic 30 combo and I want to use an external cab. Currently the cab has a 8ohm speaker. I want to match Impendance by putting a 16ohm speaker in the cab. The new speaker is 75 watts. Is 75 watts too much for the combo to handle? Does wattage have to also match? I've read Impendance does. Thank you
Old 19th February 2013
  #2
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Originally Posted by NoPro View Post
Does wattage have to also match?
No. Speaker wattage is just the maximum (or close) that the speaker can handle. If you were using a speaker rated at less than the power amp you might have a problem.
Old 19th February 2013
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Quote:
Currently the cab has a 8ohm speaker. I want to match Impendance by putting a 16ohm speaker in the cab.
How would you "match" an 8 Ohm speaker by putting in a 16 Ohm?
Old 20th February 2013
  #4
JBP
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Yeah, I'm curious about the 8/16 ohm thing. I do think that higher ohm speakers are more efficient and sound a bit better, so I can see why you may want to do it, but I'm confused by why you think it matches.

Doubling the ohms will mean that you draw half as much wattage. If the original speaker is 100 watts, you can use a replacement that is rated at 50 watts.
Old 20th February 2013
  #5
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The Classic 30 came stock with a 16 ohm speaker, and the minimum impedance it can safely handle is eight ohms. So, plugging a 16 ohm speaker into the extension jack will put you below the minimum impedance that the amp will handle. If you had a 16 ohm speaker in it still, an extension cabinet with a sixteen ohm speaker in it would bring you to eight ohms (the internal speaker is wired parallel to the extension speaker jack), which would be safe.
Old 20th February 2013
  #6
The formula for working out the resistance of two combined speakers is R1 x R2 divided by R1 + R2. A 16 ohm and a 16 ohm yields 16 x 16 (=256) divided by 16 + 16 (=32) = 8 ohms. If you mix a 16 and an 8 you get 16 x 8 (=128) divided by 16 + 8 (=24) which gives you 5.3 ohms total.

The wattage is irrelevant. You have to match the impedance of the speaker to the amp.
Old 20th February 2013
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If the internal speaker is 16 Ohms and the speaker in the extension cabinet is 16 Ohms, the with the paralleled speaker output on the Classic feeding them both you will get 8 Ohms. Yer good to go.

Let's review: If you wire two speakers in parallel you will divide the impedance, which results in an impedance of 8 Ohms if using two 16 Ohm speakers. If wired in series the impedance will double so two 16s would equal 32 Ohms.

Quote:
Yeah, I'm curious about the 8/16 ohm thing. I do think that higher ohm speakers are more efficient and sound a bit better, so I can see why you may want to do it, but I'm confused by why you think it matches.
Poppycock. The rated efficiency of a speaker (listed in dBs) is different than the rated impedance (expressed in Ohms).
I also challenge you to hear the difference between a Celelstion Greenback 8 Ohm model and the 16 Ohm version.

When presented with a different impedance load an amp may perform more or less efficiently. Not the driver.
Old 20th February 2013
  #8
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Quote:
Originally Posted by ears2thesky View Post
How would you "match" an 8 Ohm speaker by putting in a 16 Ohm?
The combo is 16ohms. The cab needs a 16ohm not 8ohms(the current ohms in the cab)
Old 20th February 2013
  #9
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Originally Posted by chucks View Post
No. Speaker wattage is just the maximum (or close) that the speaker can handle. If you were using a speaker rated at less than the power amp you might have a problem.
Thanks for this info!!
Old 20th February 2013
  #10
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If you double the impedance 8 ohms to 16 ohms you halve the power of your amp
Wattage = voltage square / impedance as your voltage will be constant doubling your impedance halves the amount of power your amp has to supply.
What this means as long as your impedance is greater than the original speaker you will not hurt your amp however it will not be as loud.
beg borrow or steal a resistance meter and make sure you have the same or more resistance than your original speaker setup and you will not have a problem.
If you are replacing one 8ohm speaker with two 16ohm speakers in parallel they can be half the wattage as the original (provided it dident fail).

Hope that all makes cents
Old 20th February 2013
  #11
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Quote:
Originally Posted by Format C: yes View Post
If you double the impedance 8 ohms to 16 ohms you halve the power of your amp
Wattage = voltage square / impedance as your voltage will be constant doubling your impedance halves the amount of power your amp has to supply.
What this means as long as your impedance is greater than the original speaker you will not hurt your amp however it will not be as loud.
beg borrow or steal a resistance meter and make sure you have the same or more resistance than your original speaker setup and you will not have a problem.
If you are replacing one 8ohm speaker with two 16ohm speakers in parallel they can be half the wattage as the original (provided it dident fail).

Hope that all makes cents
This applies to solid state amps, but tube amps are a whole different game. Some amps cannot tolerate much of an impedance mismatch, and are at risk of cooking the output transformer if you try it.
Old 20th February 2013
  #12
JBP
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Originally Posted by ears2thesky View Post
Poppycock. The rated efficiency of a speaker (listed in dBs) is different than the rated impedance (expressed in Ohms).
I also challenge you to hear the difference between a Celelstion Greenback 8 Ohm model and the 16 Ohm version.

When presented with a different impedance load an amp may perform more or less efficiently. Not the driver.
You make a good point about the amp. I also agree that a lot depends upon the design. A particular 8 ohm speaker may be more efficient than a particular 16 ohm speak — the cone is heaver, etc.

But it's not poppycock. If all things are equal, the 16 ohm speaker should be more efficient. The speaker does it's work by converting an electrical current to a magnetic field that the moves the speaker. The magnetic field is what creates resistance, which is measured in ohms. In other words, more ohms equals a stronger magnetic field. In addition, more ohms also means less current. (see ohms law.) So more ohms means a stronger magnetic field with less current drain.

More work with less current. That's the definition of more efficient.
Old 20th February 2013
  #13
JBP
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Quote:
Originally Posted by Format C: yes View Post
If you double the impedance 8 ohms to 16 ohms you halve the power of your amp ...

Hope that all makes cents
Yeah, that's a good point. (It's the other side of the coin.) But I've found that the greater efficiency makes up for some of the lost wattage unless you're having to operate at the margin of what the speaker will do. But that's a situation I try to avoid because equipment typically sound better with headroom.
Old 20th February 2013
  #14
JBP
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Originally Posted by Frodebro View Post
The Classic 30 came stock with a 16 ohm speaker, and the minimum impedance it can safely handle is eight ohms. So, plugging a 16 ohm speaker into the extension jack will put you below the minimum impedance that the amp will handle. If you had a 16 ohm speaker in it still, an extension cabinet with a sixteen ohm speaker in it would bring you to eight ohms (the internal speaker is wired parallel to the extension speaker jack), which would be safe.
Ah, that makes more sense.
Old 20th February 2013
  #15
JBP
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Originally Posted by The Famous Yard View Post
The wattage is irrelevant. You have to match the impedance of the speaker to the amp.
I get what your saying on the amp side, and maybe I misunderstood the question, but put 100 watts in a 50 watt speaker and you'll quickly learn it's not irrelevant.

Ug, I feel like half these posts are more about miscommunication than gear. Not that I think it's trivial. We gotta get this crap right if we are actually going to help somebody.
Old 21st February 2013
  #16
Quote:
Originally Posted by JBP View Post
I get what your saying on the amp side, and maybe I misunderstood the question, but put 100 watts in a 50 watt speaker and you'll quickly learn it's not irrelevant.

Ug, I feel like half these posts are more about miscommunication than gear. Not that I think it's trivial. We gotta get this crap right if we are actually going to help somebody.
You're right that the wattage is relevant; maybe I wasn't being very clear. I was replying to the OP asking if the 75w speaker would be too much for the 30w amp. I was just emphasising that the impedance match is the main thing, and maybe assuming that it was common sense that you shouldn't put 100w through a 5w speaker. But yes, we should be absolutely clear because you never quite know who knows what.
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