A quick guide to calculating ohms!

Discussion in 'Amps and Cabs [BG]' started by Ívar Þórólfsson, Mar 19, 2003.

1. Ívar ÞórólfssonMmmmmm...Supporting Member

Apr 9, 2001
Kopavogur, Iceland
Hi all!

I apolagize if this has done before, but I could not find this in the excellent amp faq that Joris compiled.

There has been an abundance of ohm related questions here, so here I give you the quick answer to calculating ohms.

R = ohm
R1 = ohm of cabinet 1
R2 = ohm of cabinet 2 etc.

R1 * R2 * R3 (etc.)
----------------------- = R
R1 + R2 + R3 (etc.)

For example, if you have two cabs, a 8ohm cab and a 4ohm cab and you want to know how many ohms that will make, you simply use the formula here above.

8 * 4
-------- = R
8 + 4

So, 8 * 4 = 32
and 8 + 4 = 12
Finally 32 / 12 = 2.66666....ohms.

If the total ohms number is lower than your amplifier specifications recommend, it is not safe to use that particiliuar combination of cabinets as it would most likely result in a fried amp.

On the other hand, if the number is higher than your amplifier specifications recommend it is safe, but the amp won´t be pumping out as many watts as it could have.

As a sidenote, if your amp is capable of 2ohms, and you have a 8ohm cab and a 4ohm cab, which is safe in this instance. The 4ohm cab will be much louder then the 8ohm one.

Hope this helps!

2. wushuguy

Jan 9, 2002
Flagstaff, AZ
Thanks! I never really knew the formula.

3. Ívar ÞórólfssonMmmmmm...Supporting Member

Apr 9, 2001
Kopavogur, Iceland
->Bump in case anyone else is interested<-

4. Bass Mule

Does it matter if your cabs are running in series or parallel?

5. 5stringDNA

Oct 10, 2002
Englewood, CO
Good info Icez- thanx!

6. Richard Lindsey

Mar 25, 2000
SF Bay Area
Yes. That formula is for parallel, which is the most common way for two cabs to be connected.

7. Phat Ham

Feb 13, 2000
DC
That formula only works for 2 resistances. You'll get the wrong answer if you use it for 3 or more resistances. The proper equation to use is:

1/R = (1/R1) + (1/R2) + ... + (1/Rn)

8. NJL

Apr 12, 2002
San Antonio
you da man!! very cool!

bump

9. tufnuts

Agreed. It's better to use the correct formula, then derive the formula for your application.

1/R = 1/R1 + 1/R2 + 1/R3 + ... + 1/Rn

In our (two cab setup), R3 thru Rn do not apply. They equal a resistance of near infinity.

So, if R1 = 8 and R2=4

1/R = 1/8 + 1/4 + 1/oo

Note, 1/oo approaches 0...and will get there eventually So we just let it go to 0.

1/R = 1/8 + 1/4 + 0

1/R = 1/8 + 2/8

1/R = 3/8

Algebra Time!

1*8 = 3*R

R = 8/3

R = 2.667

Icez is right, but this is for you 2 2x10 / 1x15 (etc) users. Or for anyone that really loves equations.

10. miccheck1516Guest

Feb 15, 2003
Ireland
bump, more people need this, or a physics text book.....