4 ohm / 8 ohm mix ???

[Lippy1]

I've been living with hauling around a combo amp for a while now but I'd like to switch to a couple cabs and a lightweight amp so I can leave one cab at home, one cab at church, then just transport the amp. This way I would also have the option to use both cabs if the needed. So I have a line on a 4 ohm 12, and I was just wondering what issues this might cause? I realize that I will need a 2 ohm capable amp, but would the second cab also need to be 4 ohm to balance the signal from the amp?

Thanks.

 

[JackANSI]

Or you can just get one amp and cabinet and leave the combo one place and the amp/cab at the other.

You can also make an adapter box to make your two 4 ohms into one 8 ohm.

 

[UncleBalsamic]

edit: Kind of pointless reply.

 

[anonymous02282011]

4ohm + 8ohm = 2.67 ohms

the 4ohm will be louder

anything else?

 

[Lippy1]

Thanks for the quick replies.

If I have one 12 that is 4 ohm and one 12 that is 8 ohm, will they be different volume levels because of the different loads on the amp?

 

[anonymous02282011]

4ohm + 8ohm = 2.67 ohms

the 4ohm will be louder

anything else?

 

[Lippy1]

Sorry, I guess I already received an answer to the volume question.

So the major drawbacks would be that the amp would need to be 2 ohm capable and the 4ohm cab would be louder? Doesn't seem to be a big deal...

 

[scottbass]

Thanks for the quick replies.

If I have one 12 that is 4 ohm and one 12 that is 8 ohm, will they be different volume levels because of the different loads on the amp?

Yes. The 4 ohm cab will draw twice as much current as the 8 ohm cabinet. Power = current squared times resistance. For example, if your amplifier puts out a total of 12 amps of current:

8 amps squared X 4 ohms = 256 watts through the 4-ohm cab.

4 amps squared X 8 ohms = 128 watts through the 8-ohm cab.

The only way the two cabs will be at the same volume level is if the 8-ohm cab is VERY, VERY, VERY more efficient than the 4-ohm cab, which is not gonna be the case.

EDIT: Yes, I know these are really impedances, not DC resistances, and the true values are different. But they are proportional none the less. Different absolute values, same effect.

 

[Fretlessboy]

as long as the 4 ohm cabinet has twicw the number of speakers as the 8 ohm you will be fine

 

[Rick Auricchio]

+1 to scottbass' answer.

(Dennis, each cab appears to be a 112 from what I gather.)

 

[seamonkey]

another +1 to the scottbass and frettlessbass answers
All else being equal, the cab getting twice the power will only be 3db louder, unless there's multiple cones difference involved here. 3db is only a small difference, and it depends at what frequency the levels are different at.

 

[lomo]

Thanks for the quick replies.

If I have one 12 that is 4 ohm and one 12 that is 8 ohm, will they be different volume levels because of the different loads on the amp?

Yes. The power through the cab is inversely related to its' impedance. The 8 Ohm cab will get 1/3 and the 4 Ohm 2/3 of your power (watts). That's why people with cabs of differing impedances will usually use, for example, a 4-Ohm 410 and a 2 Ohm 210, to get the same wattage through each driver. Other combos can also work well, however. I've used 8 Ohm 210 and 8 Ohm 112, or the same 12 with an 8 Ohm 115. However, these combinations are not balanced as far as the volume from each cab/driver.

 

[RickenBoogie]

If you're starting from scratch, why not just buy stuff that matches up well?

 

[JustDavid]

+1 to RickenBoogie. Best to do it right.

But mixing & matching great deals on used gear is something I must plead guilty to.

The 4 ohm will get twice the power. But what level difference will that create? All things being equal, I get 3 dB also. But all things aren't equal: it could happen that your 8 ohm cab is louder.

 

[FuzzBoxVoodoo]

I run a 8 ohm 2x10 and a 4 ohm 2x12 and it seems to be a good balance as the 12s can take a little more juice.

 

[Rick Auricchio]

The 4 ohm will get twice the power. But what level difference will that create? All things being equal, I get 3 dB also. But all things aren't equal: it could happen that your 8 ohm cab is louder.

Correct, because we don't know the overall sensitivity of each cabinet to use as a starting point. Even then, sensitivity figures have no standard measurement.

Worse, without full response curves, there's no way of knowing in advance what the characteristic voicing of a cab might be. The quieter cab might have a peak that you just hate, even though overall it isn't as loud as the louder cab.

We're stuck with the trial-and-error method of listening and choosing.

 

[Lippy1]

So I could construct some kind of pad to make the 4 ohm speaker look like 8 ohm?

Or you can just get one amp and cabinet and leave the combo one place and the amp/cab at the other.

You can also make an adapter box to make your two 4 ohms into one 8 ohm.

 

[Rick Auricchio]

So I could construct some kind of pad to make the 4 ohm speaker look like 8 ohm?

That would be terribly inefficient. Effectively you'd be installing a 4-ohm resistor, which would convert power to heat. Its frequency response would be different from a speaker, which includes inductance in the equation; the amp may sound different.

The adapter mentioned is a series-connection cable. This would connect both cabs in series, as opposed to the usual parallel connection. This solution is somewhat viable for identical cabinets.

With different cabs, one set of speakers may impose a big change on the response of the others, due to different inductance. Not generally what you want.

Also, there is the danger that a helpful assistant may simply wire the cabs in parallel like everyone else does. If you don't spot the error, you fry your amp.