Bass Rig Ohmage/Wattage?

[ChromeTsunami]

This stuff always confuses me, so I'm hoping someone here with the correct knowledge can help me out? It's a wattage/Ohm question...

I'm currently using TC-Electronic rig comprising a BH550 amp (550w, 4 Ohm) into 2 x RS115 (400w, 8 Ohm) cabs and it produces a sweet bass sound ideal for my style. But I have just bought a BH800 amp and a K-410 (600w 8 Ohm) cab because of an unmissable deal that Andertons here in the UK have on at the moment, and want to incorporate that into my rig. The idea being one rig at one side of the stage and another at the other side (kind of stereo). perhaps with different effects from each rig to give an awesome mind-blowing wall of bass sound. I'm using a transmitter from my bass and have two receivers, one for each rig.

So here's the question; Will the BH800 amp drive a stack of the K-410 and one of the RS115 cabs safely and what level will I be safe cranking it up to? If it's all ok I think I may also buy another K-410 so the rigs are similar each side.

Thanks in anticipation for any pointers or comments.

 

[Redbrangus]

What's the problem with running a pair of 115s on one side and the 410 on the other? Seems like that's a more even match.

Edit: There's no technical reason you can't run one of the 8-ohm 115s with the 8-ohm 410; that equals a 4-ohm load, so it's safe enough. What might be an issue is that each cab gets half the amp's output, so the 115 will dissipate half the output in watts, while the 410 will get the other half, with each 10" driver receiving a quarter of the power the 115 is getting. [Further edit:] So presuming your amp is 800 watts at 4 ohms, the 115 will be getting half the power -- 400 watts, it's full rated capacity. The 410 will get the other 400 watts, so each of it's 4 drivers will be dissipating 100 watts.

 

[ChromeTsunami]

What's the problem with running a pair of 115s on one side and the 410 on the other? Seems like that's a more even match.

Actually that may be a better solution, as I could run the BH800 on the 2 x 115s and other side BH550 into the 410s.

Thanks

 

[rickdog]

Unless you're on a really wide stage, separating speakers like that is fairly likely to cause weird dead spots and boomy spots in the room.

 

[Wasnex]

Tube amps and solid state amps have some significant differences you need to be aware of.

Tube amps use an output transformer to match the high impedance of the tubes to the low impedance of the speakers. Ideally you want a perfect impedance match. In other words if the speakers provide a 4 ohm load, you want to hook it up to a 4 ohm output on the amp.

As long as the amp sees the expected impedance, it will make the same amount of power. In other words if you hook an 8 ohm cab to an 8 ohm output, or a 4 ohm cab to a 4 ohm output, the amp sees the expected load and will be able to make it's max power. However if you hook an 8 ohm cab to a 4 ohm output, or vice versa, you create an impedance mismatch. This will cause the amp to work harder and make less power.


Solid state amps do not have output transformers. Instead the output devices are pretty much coupled directly to the speakers. The output section tries to produce a constant voltage regardless of impedance. If voltage is held constant and impedance is reduced, power goes up. If the amp can hold the voltage perfectly constant, the power will be doubled as you change the speaker load from 8 ohms to 4 ohms.

The reality is the power supply in most amps will sag a bit as more current is pulled from the amp. So the power usually does not double (100%) when you cut the impedance in half. A power increase of 66% is probably somewhere close to the average.

An important factor you need to be aware of with solid state is the minimum rated impedance the amp can drive. As I mentioned, current increases as you reduce the impedance. More current equals more heat. So eventually as the impedance continue to go down, the current will reach a level that produces so much heat that it destroys the output devices. A 4 ohm minimum impedance rating is very common. A small quantity of amps are capable of pushing a 2 ohm load.

For the specific power an amp can make at 2, 4, or 8 ohms, consult the amps specifications. As an example, the Mesa Subway amps can drive 2 ohms minimum. Rated power is 400W at 8 ohms and 800W at 4 or 2 ohms. There is an impedance switch on the back of Subway amps. I believe this switch reconfigures the power supply and some other circuits, so the power stays at a safe level (800W). If you forget to put the amp in 2 ohm mode, you could potentially destroy the amp. At the very least you should expect it to go into protect mode. The switch should be set for 2 ohms any time the load is under 4 ohms. So if your speakers provide a load of 2.67 ohms, set the switch to 2 ohms.


How to figure impedance when you have multiple cabs.

Series: Simply add the impedance of each cab. For example if you have an 8 ohm cab and a 4 ohm cab in series, the impedance is 8+4=12 ohms.

Parallel: Here it depends on whether each cab has the same impedance or not. If the impedance of each cab is the same, simply divide the impedance by the number of cabs. For example if you have two 8 ohm cabs: 8/2=4 ohms. If you have three 8 ohm cabs, 8/3=2.67 ohms.

If the cab are not the same impedance, you have to use a different formula. This is the formula I suggest because it can be expanded beyond two cabs:

For example, if you have three cabs just continue the sequence:

If I had three cabs with the same impedance I would simply divide the impedance by the number of cabs. For example 8/3 = 2.67 But this works as well:

Or if the impedances are dissimilar

 

[ChromeTsunami]

Tube amps and solid state amps have some significant differences you need to be aware of.

Thanks for your comprehensive answer - a lot to take in but much appreciated.