How does power distribute over 2 different cab's?

[LaurensLeijn]

N00b-question about electronics ...

Per example, in parallel setup:

650w / 4Ω head
250w / 8Ω cab with 1 speaker
500w / 8Ω cab with 2 speakers​

Will the 650w be divided into ...

• 3 x 216 watt's (because 3 speakers total) ?
• 2 x 325 watt's (because 2 cab's) ?
• Or ... different story?

 

[Jaco who?]

It will divide power equally to the two cabs because the amp just sees a 4Ω load from the two cabs. That's why it can be dangerous to run two cabs of equal impedance if one of them is a 410, the other a 115, or one of them is a 410 and the other is a 210, or one of them is a 212 and the other is a 112, you get the idea.

 

[LaurensLeijn]

@Jaco who?
Thanks! This is indeed a really important aspect to know ... I was hoping for the first option though haha

 

[HaphAsSard]

I was hoping for the first option though haha

For the two-driver cab to be 8 ohms, it must have either 16-ohm drivers in parallel or 4-ohm drivers in series. Either way each of them gets 1/4 of the total power, and this for three equally important reasons: their number, their individual impedance and the way they're wired together and to the rest of the system. The amp doesn't "see" the wooden boxes, as you correctly suspected (the whole driver complement and wiring might be transferred into a single, larger cab and the power distribution wouldn't change), but neither does it "see" the driver count alone. It could deliver power equally to all three drivers in the circuit only if their impedance and wiring allowed it, for example if they were all three in parallel (regardless of being installed in two boxes) and had the same impedance, but that would necessary equal the two-driver cabinet not being an 8-ohm one, if the single-driver cab is.

 

[Mushroo]

Great advice above. It's also important to understand that your amp head is not constantly outputting 650 watts! That is the maximum when everything is turned up all the way and you are playing aggressively. Most of the time, your head is probably outputting less than 100 watts. Most bass cabinets can generate at least 90db of volume (and efficient designs 100db or more) with just 1 watt of power!

 

[Hundred proof]

Great advice above. It's also important to understand that your amp head is not constantly outputting 650 watts! That is the maximum when everything is turned up all the way and you are playing aggressively. Most of the time, your head is probably outputting less than 100 watts. Most bass cabinets can generate at least 90db of volume (and efficient designs 100db or more) with just 1 watt of power!

I wish more folks understood this, in a previous life I worked in the high end stereo business we could monitor a 350 watts per channel Audio Research power amp during most musical passages just idle at a few watts and only roll up on the watts during a massive orchestral passage.

So many new full range style cabs crave about as many watts as you want to throw at them, a lot of vintage designs were very efficient because they did not have a lot of watts to work with take an Acoustic 360 for example they carried the room pa support was non existent then so they had to.

 

[JimChjones]

It will divide power equally to the two cabs

Just to make it a bit more complicated, 8 ohms is a nominal impedance value, and the real impedance varies with frequency. If the cabs are different the actual balance may be different at different frequencies. It normally tends to balance out given a reasonably complex signal, but probably wise to make a greater allowance than you would with identical cabs.

 

[Raf Seibert]

In addition, the power rating (watts) of the cabs is nearly totally irrelevant to this calculation. The head is rated at 650W at 4Ohms. It is connected to two 8ohm cabs. Good, that means the head sees 4 ohms. As has been said, each cab will get an even share of the current, and thus, power. Now, the single driver could receive as much as 325W at max output, and that could be too much. However, as has been said, unless your bass and your head are both cranked up, you could probably get by with no harm. The power rating of the cabs has no effect on the current, and thus power, distribution to the cabs.