# 4ohm +8ohm cab =?

Discussion in 'Amps and Cabs [BG]' started by Randy W Lopez, Feb 23, 2004.

1. ### Randy W LopezSupporting Member

Oct 19, 2003
Dearborn Heights, MI
Endorsing Artist-MESA BOOGIE
Not exactly sure what type of ohm load I'd be putting on my amp using a 4ohm cab and a 8 ohm cab. I'm using a SWR 900 and from what I understand, 4 ohms is the minimal it will take. What do you all think? Thanks

2. ### Circus

Jul 8, 2003
Spruce Grove Alberta
I would assume that running an 8 ohm and a 4 ohm would not be good for your head since you would be running at more than 4 ohms.

3. ### ccbass71Supporting Member

Using 4 ohm and 8 ohm cabinets together would load down the amp at 3.2 ohms. If your amp's minimum rating is 4 ohm you could potentially damage the amp. I have seen people use amps in this situation and there was no problem. BUT I recently saw a guy run two 4 ohm cabs with the amp bridged at 4 ohm and after the show the plastic rack that the amp was in was REALLY hot. If you want to use two cabs, I would suggest going with two 8 ohms or get another amp that can handle 2 ohms.

ccbass71

4. ### Electricmayhem

Dec 18, 2003
NH
I wish I understood it...

Just figured out the basic points of resistance...this is too much!

5. ### seanmI'd kill for a Nobel Peace Prize!Supporting Member

Feb 19, 2004
Impedance really isn't that hard.

If the cabs are in series, you just add the impedances. Eg 8 + 8 =
16. That's why most amps/cabs don't have the speakers in series,
adding a second cab makes the setup quieter!

So, most cabs are in parallel. The general case for parallel cabs of
impedance A and B: (A x B) / (A + B)

Eg (8 * 4) / (8 + 4) = 32 / 12 = 2.7

However, for the normal case of two parallel cabs of equal impedance,
just divide by two.

A further note: No matter what you do, two cabs in parallel will
always come out lower than the lowest cab. So if your amp can do a
minimum of 4 ohms, and you have a 4 ohm cab, you cannot safely add any
cabs in parallel.

6. ### ChenNuts44

Nov 18, 2001
Davenport, IA
Series: R1 + R2 + ... + Rn = Rtotal

Parallel: 1 / ( 1/R1 + 1/R2 + ... + 1/Rn)

7. ### embellisherHoly Ghost filled Bass PlayerSupporting Member

Actually, 2.67 ohms, not 3.2

8. ### Mattski

Jan 6, 2003
Cleveland, OH
Yep...2.67 Ohms (but I just round to 2.7)

9. ### Whappo Grande

Feb 9, 2002
Santa Clara, CA.
Manager: AccuGroove Speakers
Your SWR 900 will do 400 watts @ 4 Ohms per side or 240 watts per side @ 8 ohms.

This means that in stereo, you could run your 8 Ohm cab @ 240 watts & your other channel could run @ 4 Ohm with 400.

Mark
AccuGroove.com

10. ### Jim Wheeler

Dec 15, 2003
Winston-Salem NC
If I'm right:

Running in mono, you'll find the 4 ohm cabinet receiving twice the power delivered to the 8 ohm cabinet. This can be used to advantage, making one cabinet louder than another, affecting the final balance.

11. ### ccbass71Supporting Member

Oops! You are right it would be 2.67ohms not 3.2. My bad. Either way it still wouldn't work.

12. ### Randy W LopezSupporting Member

Oct 19, 2003
Dearborn Heights, MI
Endorsing Artist-MESA BOOGIE
Math . Not my strong subject.
I thank everyone for their assistance.
Let's add to this, if I'm running biamp mode, I assume I'm using 2 different side's of the power amp which each excepting what ohm load I put on it. 1 side 4, the other side 8.
Problem is the 15 is 8ohm and the 10's are at 4. I assumption is you generally want more power to your lower end, which in actuality will be the opposite.
But then again, the 10's do have a wider dynamic range then the single 15.
Opinion's?

Aug 9, 2003