# wattage calculation

Discussion in 'Amps and Cabs [BG]' started by scorpionldr, Jun 17, 2004.

1. ### scorpionldr

Dec 10, 2002
Dirty Jersey, USA
ok, now as everyone hear knows, I'm getting an XM200. now, i hear that with or without an 8 ohm extension speaker that i'm fine, but here's the kicker. I've heard that i can hook up 2 extention speakers if i wanted to somewhere, so say that I buy 2 8 ohm extention speakers, and add that along with the built in 4 ohm speaker? now what i know is that the amp is 200w@2 ohms, and i know that the cabinet is 4 ohms.I heard if I take one 8-ohm extention, that I would get 200@2.6 ohms. so I took a calculation that i found on the forum at one point

R=a cabinet with impedance

R1*R2*R3
---------
R1+R2+R3

4*8*8
------=256/20=12.8
4+8+8

so now what happens??? am i going to have 200w@12.8 ohms?? it sounds highly unlikely, but i'm really interested, because since i'm getting cash soon, i might just be interested.
oh and please don't try and recommend any other amp, i'm just interested in getting an accurate calculation of the watts, nothing more

2. ### Petebass

Dec 22, 2002
QLD Australia
You've actually got 2 ohms.

Try it this way:-

First calculate the 4 Ohm speaker with one extension cab:-

4*8
----
4+8 = 2.67

Now add the next 8 ohm cab:-

2.67+8
-------
2.67+8 = 2 Ohms.

3. ### scorpionldr

Dec 10, 2002
Dirty Jersey, USA
So in general, it's not going to do anything. I just get one extention cab cuz I'll get the extra .6 ohms and the full value of the 200 watts, whereas completely solo i just get a loud 150

4. ### Petebass

Dec 22, 2002
QLD Australia
Not really. It will definitely be louder with 2 extension speakers than 1. Don't put too much emphasis on the wattage because you need a hell of a lot of extra watts to make an audible difference. Decibels are logarithmic so the difference between 150w and 200w is barely audible. To double your volume, you need to multiply your watts by 10. Doubling your watts to 400w will only get you an extra 3dB.

The other way to get an extra 3dB is to add an extension speaker. Not only is cheaper than buying a new amp, it brings the speakers higher off the ground and closer to your ears. Plus the speakers themselves will couple to add more percieved loudness again, and more bottom end. If you're like me you'll EQ that extra bottom end out, reducing the overall speaker movement, meaning you can push the whole rig harder again. Just don't clip the amp!

PS:- when you're calculating impedance for more than 2 resistors, use this formula instead of the one in your original post.

Rs = 1/(1/R1+1/R2+1/R3+.+1/Rn) (n resistors in parallel).

so
R3 = 1/(1/4+1/8+1/8)
= 2 Ohms.

Jun 14, 2004

6. ### Frank MartinBitten by the luthiery bug...

Oct 8, 2001
Budapest, Hungary, EU
Yep, the proper way of calculating resistance is:

Series.
R = R1 + R2 + R3 + ...

Parallel:
1/R = 1/R1 + 1/R2 + 1/R3 + ...

BUT REMEMBER: at lower impedances the amp will run harder and hotter