Mix 'n' match cabs with different power ratings

[andrewrx7]

Hi there,

Just after some basic info really, I would appreciate your thoughts.

A couple of questions:
[list=1]
[*]Basic power ratings between an amp head and cabinets – if I hooked up a 200w cab to a 300w (or higher) head, is there a risk of doing damage to either the speakers or the head? (this is assuming the input gain is set correctly and not being clipped). Would I be correct to assume this is OK, since the head is rated at 300w @ 4ohm, but will be around half that with 1 cab attached, which is rated to 8 Ohm, so the amp is able to supply around 150w to a single cab?
[*]What is the effect of putting two cabs that do not have similar power ratings into the head? For example, the 300w head @ 4ohm, will take 2x 8ohm cabs. But one cab is rated to 400w, while the 2[sup]nd[/sup] cab is 200w. So impedance is correct, but what effect does the different power ratings have?
[/list]

Cheers!
Andrew

[flyfisher]

[quote name='andrewrx7' timestamp='1321440532' post='1439051']
1. Basic power ratings between an amp head and cabinets – if I hooked up a 200w cab to a 300w (or higher) head, is there a risk of doing damage to either the speakers or the head? (this is assuming the input gain is set correctly and not being clipped). Would I be correct to assume this is OK, since the head is rated at 300w @ 4ohm, but will be around half that with 1 cab attached, which is rated to 8 Ohm, so the amp is able to supply around 150w to a single cab?
[/quote]

The general view is that a speaker that's driven with a potentially damaging level of power will sound bad before any actual damage is done. So that's a good starting point - if things sound good then they're almost certainly 'safe'.

For your specific example, a 300W @4 ohm amp will deliver a bit over half that at 8 ohms. I tend to use 2/3rds as a rough guide, which means about 200W at 8 ohms in this example. Thus a 200W 8 ohm cab should be fine, even at the full output of the amp - but again, let your ears be the final judge.


[quote name='andrewrx7' timestamp='1321440532' post='1439051']
2. What is the effect of putting two cabs that do not have similar power ratings into the head? For example, the 300w head @ 4ohm, will take 2x 8ohm cabs. But one cab is rated to 400w, while the 2[sup]nd[/sup] cab is 200w. So impedance is correct, but what effect does the different power ratings have?

[/quote]

The power rating of speakers does not affect [u]where [/u]the power is dissipated, only whether the speaker can handle the power that [u]will [/u]be dissipated. It's the impedance that will determine where the power is dissipated.

Thus, in this example, two 8 ohms speakers (in parallel) will present a 4 ohms load to the amp. The 300W at 4 ohms amp wll therefore deliver 300W and because both speakers have the same impedance the power will be divided equally between them. Thus each speaker will be driven with 150W, which is comfortably within the rating of either speaker.

[xgsjx]

Like Flyfisher says, use your ears & if it sounds bad, turn it down/back the lower frequencies off.

[Mr. Foxen]

Problem is, when mixing cabs, that one could be much louder and handle more power than the other, and drown out the noise of the other one being upset.

[xgsjx]

[quote name='Mr. Foxen' timestamp='1321442377' post='1439082']
Problem is, when mixing cabs, that one could be much louder and handle more power than the other, and drown out the noise of the other one being upset.
[/quote]
& that's why you're usually better having 2 of the same cab.

[alexclaber]

My two basic rules of thumb are that you'll always hear a cab distorting well before you blow it, so you can safely use a high power amp with a lower power cab, and that if you're stacking two cabs of dissimilar type (assuming they work well together), power rating or quality, then put the smaller/lower power/cheaper cab on top so you can hear it complaining if you do push it too hard.

More here: [url="http://barefacedbass.com/technical-information/mythbusters1.htm"]http://barefacedbass.com/technical-information/mythbusters1.htm[/url]

And poke around the rest of the tech info and BGM columns sections for further enlightenment. The articles on power matching are quite useful.

[flyfisher]

[quote name='Mr. Foxen' timestamp='1321442377' post='1439082']
Problem is, when mixing cabs, that one could be much louder and handle more power than the other, and drown out the noise of the other one being upset.
[/quote]

[quote name='xgsjx' timestamp='1321443904' post='1439117']
& that's why you're usually better having 2 of the same cab.
[/quote]

Yep, both good points, but the (potential) different loudness of different cabs is due to different sensitivity/efficiency of the cabs rather than their power handling as such.

A 400W cab won't necessarily be louder than a 200W cab.

Just clarifying the point, because the OP only mentioned ohms and power ratings. Sensitivity is an additional consideration.

Edited November 16, 2011 by flyfisher

[xgsjx]

[quote name='flyfisher' timestamp='1321445502' post='1439151']
Yep, both good points, but the (potential) different loudness of different cabs is due to different sensitivity/efficiency of the cabs rather than their power handling as such.

A 400W cab won't necessarily be louder than a 200W cab.

Just clarifying the point, because the OP only mentioned ohms and power ratings. Sensitivity is an additional consideration.
[/quote]
Very true.

[andrewrx7]

Thank you for the replies already, I do appreciate you taking the time.

I have been reading some other threads to, but it is sometimes quicker to ask a question and get a straight answer than wade through 10's or 100's of posts! I will keep reading anyway!

My main concern was using the two different power rated cabs from a single head. But from what I can gather from the answers here and on the sticky thread, it would be OK. I have already used them in this combination at my last gig, but wanted to check to see whether I was doing something that was totally not recommended!

[fonzoooroo]

[quote name='andrewrx7' timestamp='1321450556' post='1439262']
My main concern was using the two different power rated cabs from a single head. But from what I can gather from the answers here and on the sticky thread, it would be OK. I have already used them in this combination at my last gig, but wanted to check to see whether I was doing something that was totally not recommended!
[/quote]

Put simplistically, the power is shared evenly, so effectively you have 2 cabs which each have the maximum power of the LOWEST power cab.

Hence a 600W cab and a 150W cab driven together is just like having 2 cabs each of 150W, so you can "safely" supply them with up to 300W.

[xgsjx]

It depends on what power the amp is going to put out.
If it's a 600 watt amp, then it could send 300 watts to each of the cabs. 300 going to your 600w cab & 300 going to the 150w cab (which is obviously more than it can handle, but if the amp & ears are used sensibly, then it can still be used).

Think of it like having 2 balloons & a tank of air with 2 pipes that will split the air evenly. One balloon being able to take all that is in the tank & the other being 1/4 the size.
Both balloons can be used if you only turn the air on partially, but the small balloon isn't capable of taking 1/2 of what is in the tank, so would burst if the tank was opened fully for any amount of time. The big balloon would still take it's fill.

Hope I'm not just talking wind here!