Ohms question.

[BAD RONBO, KiLLeR DWaRfS]

would that analogy apply to an argument with the wife ? (on her end, i mean !)

Sounds like you have some experience in this!

( X 3 my friend !)

[FretlessOnly]

What he meant to say is

320W into one 8Ω cabinet; or
240W into each of two 8Ω cabinets (4Ω total)

That's exactly what I meant to say. A simple 4 for 8 was all I was missing.

My GK pushes 480W into one 4Ω cab, 320 into one 8Ω cab, and 240W each into two 8Ω cabs in parallel.

[cjj]

Can someone define headroom definitively? Is it the difference between amp output and speaker watt rating? What is the "proper" differential?

Headroom is the short term (much less than a second) ability of an amp to output more power than it is rated for. For example, an amp with a power supply that has a lot of current delivering capability and is rated 100W but with a 3dB headroom rating means over a very short time period it can deliver 200W. That could happen on the initial attack of a note when the bass string is plucked.

Headroom the difference between the nominal signal value and the maximum undistorted value.

A more general way to think about this is the ability to put out more power than required on average. This can mean, as John said, the ability to run over the rated power, or just having more power available than you actually need.

So, say you have a 200W amp, but with your particular speaker setup and where you happen to be playing, you are running the amp at something like 150W continuously. You then have 50 watts of headroom before you start hitting the limits of the amplifier's capability.

As John said, this can happen on initial attack as well as other times for brief periods. If you are always running up against the limit, these higher power moments risk driving the amplifier into clipping, which can sound bad, or possibly destroy things like speakers or even the amp itself, especially with solid state power amps...

[beatlefreak]

Yes, the other advantage with keeping some headroom is that your speakers don't have to work as hard.

[FretlessOnly]

Sorry to carry this thread further afield, but I think that John B. had his question answered.

So, how does one figure out headroom with a rig? Say you have a head rated at 400W and a speaker rated at 600W.

1. Is headroom a function of both the head and cabinet?
2. Obviously headroom is not something stamped on the head as a wattage is. How is it realized?
3. Do user's manuals usually go into headroom?

Bottom line, how do you "get it" and what are the typical ranges (both values and units) that are desireable? I realize that the simple answer is "that depends," but let's take it past that if we could.

Thanks.

[jps]

I have been learning some interesting things regarding headroom (Max, and otherwise ) and speaker power rating capabilities over at Talkbass. The "standard" theory for headroom is to have more power than your speaker can handle, say, 1000 watt head for a 500 watt speaker so that you never run out of clean, undistorted power; the idea here is that you will hear any distress in the speaker before actually causing any damage to it. With this scheme you only have 3dB of headroom, not much, and you will run out of this little bit of headroom very quickly. So, now we think we should get that 4000 watt power amp , this way we will have a whole lot of headroom; the problem here is that a speaker is rated in thermal watts, something the manufacturers do not mention. There is also a mechanical limit to a speaker driver, measured as the XMax and XLim; these mechanical ratings describe the total voice coil displacement in relation to the magnet gap/structure, and is much lower than the thermal power rating, perhaps by 1/2 to 2/3 the power rating we see in spec sheets. It has been suggested by some speaker gurus over at TB to basically just match up the amp's power rating to the speaker's, at least for musical instrument applications, the former theory is still applicable to PA system use for various reasons.

[johnallg]

Well said.

Bass-ically, LOL, you want more power in the amp then you are going to use, so as not to drive the amp into distortion at the output because this puts high current flow in the speaker voice coils which causes heating they cannot dissipate and the resulting fried speaker. Save the distortion for the preamp or effects.

[gibsonlp]

Afaik tube amps won't get any power increase when putting more speakers and taking down the impedance.
For example - my SVT will deliver 300w in 4ohm and 300w in 2ohm, the speakers will work easier, but the amp will deliver the same amount of power.

[jps]

That is because the output transformer has two independent secondary windings, one each for 4Ω and 2Ω loads set with the impedance switch on the back of the amp. This provides the same output power to the different loads.

[beatlefreak]

+1

[gibsonlp]

I don't have any switch at the back of my svt

[jps]

Does yours have separate speaker jacks for 8Ω, 4Ω and/or 2Ω? If so, you shouldn't need that switch, just use the appropriate jack for the same load the speaker provides.

[BAD RONBO, KiLLeR DWaRfS]

the svt automatically switches from 4 to 2 ohms when you plug a second cab into it. the main cab plugs next to the pre amp in/out jacks, and the secondary cab plugs in at the outer right side on the back panel. not more power when you plug in a second cabinet, but an illusion of more power as you've just spread out the wall of sound due to more speakers.

[jps]

an illusion of more power

Hmm, good name for a band:

Illusion Of Power