Speaker watt rating

[DrNomis_44]

We could also examine the differences between a typical solid state guitar power amp, and a valve guitar power amp, first thing we would notice is that the valve power amp uses less components than the solid-state power amp (assuming they are both made using discrete components), second thing we would notice is that the valve power amp uses less negative feedback than the solid-state power amp, this is mostly due to limitations in the valve power amp's output transformer which limits the amount of negative feedback that can be applied.

Applying negative feedback to a valve power amp does three beneficial things I can think of, firstly it reduces harmonic distortion, secondly it flattens the frequency response, and thirdly it lowers the output impedance of the valve power amp, of course, solid state power amps use negative feedback too but they tend to use a lot more than valve power amps, if you try to increase the amount of negative feedback in a valve power amp, at some stage the valve power amp is going to become unstable in it's behavior at some frequencies in it's frequency response, why does this happen?, it's mostly due to phase-shifts caused by the windings in the output transformer, now, because most solid-state power amps found in guitar amps don't use an output transformer, like valve amps do to isolate the speaker from the HT supply going to the power valves, they don't suffer from the same phase-shifts and so can afford to use higher levels of negative feedback, and it is because solid-state power amps use higher levels of negative feedback that they inherently have much lower output impedances, this means that the solid-state power amp can supply more current to the speaker.


There is one downside to solid state power amps, when the solid-state power amp is run below the point where it starts clipping, everything is great, soon as you start running the solid-state power amp harder into clipping you run the risk of burning-out the voice coil in the speaker because you're effectively putting pure DC across it.

This is where valve power amps are at an advantage, since if the valve power amp is driven into clipping, the output transformer is going to limit the current being delivered to the speaker.


Here's an interesting fact, the impedance of a speaker is actually not constant for all the frequencies that the speaker can reproduce, it actually varies, at some point in the speaker's frequency response the impedance will rise to a peak, this is where the speaker is operating at maximum efficiency due to resonance, therefore the impedance-rating of a speaker is actually a nominal rating.

[JimC]

because you're effectively putting pure DC across it.

Myth. You're not putting DC across it at all. You're just burning it out with too much power. Its simply that there's more energy - and specifically more high frequency energy in a distorted signal than a clean one.