Impedance Question

[noah_swanson]

Hey all, i've got an ampeg rocket reverb 2x12 amp, and recently bought a marshall 4x10 cab from a friend. Since im not finacially set to buy a head for it just yet i was wondering about the correct way to hook up this cab from my ampeg.

the ampeg has 2 12" at 16 ohms a piece wired in parallel (8 ohm output from the amp) The cab has either a mono 4ohm, mono, 16 ohm, or splits it in stero with 8 ohms on the two jacks.

This impedance business is really confusing to me, so any help with this would be much appreciated. Thanks!

[noah_swanson]

Now that i've thought about this a little more.... couldnt i wire teh 2 16 ohm speakers on the 2X12 amp in series...makeing 32 ohms, then running parallel off that to the cab at 4 ohms. that evens out to 8 ohms yes?

[Steve Conner]

No. It works out slightly less than 4 ohms. There is no way to combine the speakers you've got to make the 8 ohm impedance the amp wants. However an 8 ohm amp usually works OK with a 4 ohm load, so I would just use the cabinet in 4 ohm mono mode and unplug the combo's internal speakers.

[bob p]

steve's recommendation is how i would do it. i'd just unplug the combo's internal speakers and use the half-stack.

when calculating impedance values for multiple cabs, a good rule of thumb is that whenever you add a speaker load in parallel, the resulting impedance will always be lower than what it was when you started -- no matter what kind of load you're adding in parallel. the cabinet with the lowest Z will have the biggest impact on the final impedance. so if your cabinet with the lowest rating has 4 ohm load, no matter what you add to the system in parallel, the result is always going to be a system impedance that is lower than that.

another important thing to remember is that when you add cabs together in parallel, the power dissipation in the cabinets may not be equal. one of the cabinets may end up doing the lion's share of the work. that's why its always safest to couple cabinets with similar Z values and power ratings.

[Earl Norton]

You might try using just one side of the 4-10 cabinet in stereo but with nothing connected to one side and using the 2 -12;s in the combo. The resulting 4 ohm load should work well for the amp. If the 4 -10 cabinet is sealed you could find some added low end from the two 10's NOT connected as in a passive radiator. In any case it should sound somewhat tighter from increased machanical damping. Easy to try.