Could someone please confirm watts and ohms?

Discussion in 'Amps and Cabs [BG]' started by lfmn16, Dec 21, 2013.

1. lfmn16Supporting Member

Sep 21, 2011
charles town, wv
I did use the search function, but I just want to make sure I understand things correctly since I need to upgrade my rig:

Ohms are pretty easy - 2 eight ohm cabinets = 4 ohms, two four ohm cabinets = 2 ohms, a four ohm cabinet and an eight ohm cabinet = about 2.7 ohms. If you have a head that is a minimum of 4 ohms, you can't run the eight and four ohm cabinets together.

I can't find this spelled out, but is there a difference in ohms between running both cabinets directly to the head and running one cabinet to the head and the other cabinet to the cabinet running from the head? I don't get why you would choose one over the other.

Watts - when pairing cabinets, it's best to use 2 of the same cabinets. However, if you don't, you are limited by the power of the weaker cabinet, so if I run a 700 watt 4X10 and a 350 watt 2X10, I shouldn't pair it with a head that puts our more than 350 watts.

Thanks to anyone that wants to confirm or correct.

2. B-string

1 Correct.
2 There is no difference with many amp heads (SS), tube output amps you may need separate cables to switch output taps or one cable to two cabs to match the load needs.
3 Not quite, you take the lowest rated cab (350 in your example) and multiply by the number of cabs (350 X 2 in your example, amp with no more than 700 watts).

3. Rob22315

Oct 23, 2010
Alexandria Virginia
EE 101

In series, Rtot=R1+R2+R3 . . .

In parallel, 1/Rtot=1/R1+1/R2+1/R3+ . . .

Two series 8 ohm cabs=16 ohms

Two parallel 8 ohm cabs=4 ohms (1/(1/8+1/8))

4. lfmn16Supporting Member

Sep 21, 2011
charles town, wv
Thanks!

5. Vince Klortho

I would revise the answer to #3 be speaker capacity is 700W but you can power it with practically what ever you want. It is possible to run a system with more power than the speakers can handle without problems (I have for decades.) Just be careful and listen for signs of issues.

Remember that you have a volume knob and it does not have to be set on 10.

Or 11 either.

6. MunjibungaTotal Hyper-Elite MemberGold Supporting Member

May 6, 2000
San Diego (when not at Groom Lake)
Independent Contractor to Bass San Diego
Very true. I run my MB800 or MB Fusion 800 heads into my 8-ohm Bergantino AE210 often. The AE210 is rated at 400 watts and the MB amps are rated at 500 watts into 8 ohms. I also run them into my 4-ohm CN212 which is rated at 700 watts and the amps are rated at 800 watts into 4 ohms. I've never had a problem in either instance. I also know what an overdriven cab sounds like, so I don't go there.

Also for practically all solid-state heads there is no difference between running a cab to each output and daisy chaining cabs. Do one or the other based on convenience. Some amps have only one output jack, so you have to daisy chain to do more than one cab.

7. OOZMAN

Jun 16, 2010
Australia
Doesn't daisy chaining cabs put them in series? So if you have two 4 ohm cabs and daisy chain them, you will end up with an 8 ohm load to the head. That's what my speakers did anyway. Not sure if my speakers are just oddball though.

8. Vince Klortho

Usually daisy chaining places speakers in parallel. If yours are connected in series then yes, I think they are oddballs.

9. MunjibungaTotal Hyper-Elite MemberGold Supporting Member

May 6, 2000
San Diego (when not at Groom Lake)
Independent Contractor to Bass San Diego
No. Daisy chaining them puts them in parallel, and two 4-ohm cabs will present a 2-ohm load to your amp. How do you know that your cabs were in series and presented an 8-ohm load to your amp? What kind of cabs are they? If you connected them with standard speaker cables, I can't think of any way the cabs could have ended up in series.

10. dincz

Sep 25, 2010
Czech Republic
Apologies if I'm stating the obvious, but the comments about #3 are valid only if the two cabs have the same impedance.

The standard for instrument cabs is that jacks are wired parallel.

The same for jacks on integrated amps.

If someone had altered a cab's jacks to series and sell it on the used market, you would hope they would let you know

It's easy to check though.

12. Rich McCoy

Apr 8, 2013
If your amp puts out 250 watts at 8 ohms, and 400 watts at 4 ohms you could use one cab rated at greater than 250 watts or two cabs rated at 200 watts each.

13. RickenBoogie

Jul 22, 2007
Dallas, TX
Wattage ratings on cabs are misleading. The basic rule is, use common sense, and listen for any signs the spkrs are being stressed. Otherwise, use what you like, but don't expect more than any particular cab is capable of delivering.

14. Smooth_bass88Groove itSupporting Member

Oct 31, 2006
AMERICA
Yeah but you don't refer to it as "ohms". It's called impedance. i.e. The impedance of this cabinet is 4 ohms. Or... What's the impedance of two 8 ohm cabinets?

15. B-string

Ohms is the measurement of AC impedance or DC resistance, the same "yardstick" is used for either.So I'm not quite sure what distinction you are attempting?

16. Rob22315

Oct 23, 2010
Alexandria Virginia
Yes, this is 'more correct'. Impedance has both a DC component - reisitance; and an AC component - reactance. Speaker impedance in specs is the DC component or resistance. The unit of measure is ohms for all of this but absent a representation of the AC component, it's resistance. The DC component is a number, the AC component can be very complex depending on the number of reactive components and the circuit design.

17. experimental bassist

Mar 15, 2009
I always take wattage limitations with a grain of salt.

Chances are a driver will reach it's mechanical limitations far sooner than it will reach its wattage (thermal) limitation.

In other words, a speaker will fart out before it will melt.

User your ears, and you'll be fine.

18. dincz

Sep 25, 2010
Czech Republic
It varies wildly across the frequency range but the specs show nominal impedance. If it was resistance, the specs would list resistance not impedance.

The DC resistance isn't of much interest but it's typically around 3/4 of the nominal impedance.

19. lfmn16Supporting Member

Sep 21, 2011
charles town, wv
Thanks to everyone for their input!

20. MunjibungaTotal Hyper-Elite MemberGold Supporting Member

May 6, 2000
San Diego (when not at Groom Lake)
Independent Contractor to Bass San Diego
All this time I thought it was "ohmage." LOL.