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Speaker OHMS..?

Started by PoorOtis, April 08, 2013, 08:17:21 PM

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Can you mix speaker ohms..in speaker replacement..or do you stay with the same ohm speaker as what you removed?? Does the same apply with a external speaker??

J M Fahey

Best is to respect original impedance.
If erring, "go up, not down" , but you lose some power.


It's actually not a great idea to even mix different models from the same maker in the same cab.  Apart from the impedance ("ohmage") there are a number of other factors such as sensitivity and resonant frequency that may mean one speaker is doing most of the work while the other is slacking off.  This is particularly true if you mix impedance where the speaker with the lower impedance will tend to hog the available power and the low impedance "tail" might wag the high impedance "dog".  There is a lot that people get away with, but it's not a great idea if you can avoid it.

Perhaps if you give us some more detail on what you are trying to do?
If you say theory and practice don't agree you haven't applied enough theory.


There are all the standard reasons why you shouldn't, but on the other hand, as long as you don;t exceed the amp's impedance requirement, it won't hurt anything.   One could point out that a 16 ohm and an 8 ohm in parallel results in the 8 ohms drawing twice the power the 16 does.   But that is only 3db.  If the 16 was 4db more efficient than the 8, it would wind up louder even at half the power.  Efficiency is a major factor.   SO in reality, you might never notice that one speaker is a little louder than the other.


    I am trying to understand this Impedance stuff, and it is making me crazy trying to totally understand.
I have a Vox AC15C2 combo amp that is 16 ohms. There is a extension speaker OUT jack in the back with a 4/8/16 ohm switch.   
  I bought a used Fender Frontman 212R that was rated at 4 ohms. I rewired the fender speakers in series that made them 16 ohms,
the same as the Vox. Then I put the switch on the back of the Vox to 8 ohms and using a Cliff Jack on the Fender to separate the amp
from the speakers, Im able to run all 4 speakers from the Vox amp power.  Im using a 12 gauge speaker 10 foot cord.

  My question is that the Vox speakers seem to be a lot louder then the fender speakers.   The Vox ones are 35 watts and the fenders are 50.

  Can anyone tell me why the vox speakers are louder ???   Should I have bought a lesser gauge wire for the speaker wire that goes from the vox to the fender ???

Thanks for your help with this.


First, the power rating of a speaker has little to do with how loud it is.  The difference between a 35w speaker and a 50w speaker is that the 50 watts will be fine out the 50 watt speaker, but would burn out the 35 watt speaker.

If you have a 16 ohm load in your Vox and a 16 ohm load in your Fender, then they both should share power equally from the amp.  But there is a lot more to it.  Not least of which is eficiency.  Every speaker has an efficiency rating.  They feed each speaker 1 watt of power and measure the loudness 1 meter in front of it.  The result is in decibels.  The higher the number, the louder the speaker.   SO really inefficient speakers with say 94db efficiency will be a lot less loud than high efficiency speakers with a rating like 103db.

If you double the power from an amp into a speaker, it will get 3 decibels louder - not a lot.  40 watts is 3db louder than 20 watts - all else being equal.  DOuble again for 3db more, so 80w is 6db louder than 20 watts.

But if you left the power the same, and just changed the speaker efficiency, it works the same way.  Put 20 watts into it, and a 97db efficient speaker will be as much louder than a 94db one as if you left the 94db in there and doubled the power.  Each 3db is as if you doubled power.  SO going the whole distance, if I replaced 94db speakers with 103db speakers in a 20w amp, it would be the same as if I increaseed the 20w to 160w for the original speaker.

For reference, a zillion Fender amps have the two input jacks - high and low.  I think your Vox has the same arrangement.  One jack is louder than the other.  Those pairs of jacks are 6db different.  That difference in loudness is 6db.

Your Vox speakers COULD be a lot more efficient than the Fenders.   It is also posible the Fender enclosure is more designed to spread sound around while the Vox is more intending to project sound out the front.  SOme speakers have tonal peaks that cut through, they may not really be louder, but are more piercing so to speak.  Those are a few possibilities.


To add...

Quote from: ctguy1955Should I have bought a lesser gauge wire for the speaker wire that goes from the vox to the fender

No, the wire gauge has nothing to do with it if it sufficient, and 12 gauge is.

You appear to have got the impedances right.  As Enzo said, both sets of speakers are getting the same amount of power, it just seems that the Vox speakers are more efficient at turning their equal share into sound.
If you say theory and practice don't agree you haven't applied enough theory.


  Thank you Roly and Enzo so much for explaining this to me.  I really like this website as Ive learned so much here and people seem to
be very patient with nubes like me. I appreciate the knowledge I have gained and just want to thank you all so much !!!!!!!!


Not always.  I have been known to have my LMGTFY moments.   :grr

Quote from: ctguy1955thank you all so much !!!!!!!!

That's what the little "[informative][useless]" buttons are for.
If you say theory and practice don't agree you haven't applied enough theory.


Quote from: Roly on April 15, 2013, 01:43:35 AMThat's what the little "[informative][useless]" buttons are for.
Thanks Roly, did not even realize those buttons existed (under posters name). 


I didn't either.   I have noticed those words there before, and just assumed they were someone's playfull nickname for a poster.  Didn;t notice they were on all of them.


I know how you feel Enzo,, Took me forever to find the *Any Key* on a computa. :lmao:


Funny again,,, I just noticed that I actually have more *chip points* than Enzo. (Headscratch????)

Crykey,,,that's a joke,, Enzo knows a lot more than me so don't take it all tooooo seriously folks. :lmao:


I have no idea where chip points come from or how you get them.


As far as I know, you got +1 chip point anytime someone click on the [informative] button and -1 anyone click one [useless], the later of which doesn't seam to happen very often.

Is my post informative, or is it just a blatant try to obtain some credit from a post totally unrelated to the original topic?  :duh

Back on topic, I see no more to add to what has already been said. Could be interesting to compare the full frequency response of the two makes of speakers your comparing (not only the rated efficiency, but the full curve for each speaker, and also the measurement process used to generate them, which can vary a lot from one manufacturer to another).