Amp and speaker question

Treewig1

Hi.  Can anyone tell me the difference between a 10 watt amp with an 8 ohm 75 watt speaker and the same 10 watt amp with an 8 ohm 25 watt speaker? Thanks 

RiftAmps

One speaker can handle 50w more than the other, that's it. Doesn't necessarily mean that one is louder than the other, nor that they sound any different (but they could).

Treewig1

Thanks @RiftAmps. So, in practical terms, so long as the speaker wattage exceeds the amp wattage, it doesn't matter (won't blow up!)

RiftAmps

@treewig1 Standard practice is to ensure that the speaker is rated for twice the power of the amplifier, this ensures that it can handle the peak power of the amplifier, which can often be 1.8-2x. Some people push this, others don't.

Treewig1

Thanks for taking the time to help @RiftAmps.  I'm very grateful.  Kind regards.

ICBM

With valve amps in particular (which are inherently less hard on speakers than solid-state ones) it's common practice to cut it finer than that, so the speaker goes into thermal compression - which typically starts to occur at over about half the power rating - but it's complicated by some speaker manufacturers being much more optimistic about power ratings than others, so it can be hard to tell just how much margin you really have. In general having the speaker rated for double the power of the amp is safe with any amp and any speaker brand - with some (particularly most Celestion models) having the speaker only rated for more than the power of the amp is fine.

With a 10W amp and a 25W speaker you're below the point at which that's an issue though, so in general there will be very little difference between that and a 75W speaker, other than that different speakers sound different and have different sensitivities (efficiency) - which are not related to the power rating. Contrary to popular myth there is no reason you need to 'drive a speaker fully', so the 75W one will work just as well. There are a few speakers which can sound a bit stiff and harsh at *very* low power inputs, but 10W should be enough for any of them.

Treewig1

Thanks @ICBM.  That's very clear.  Much appreciated.

DJH83004

As always IC and Rift spot on the money, the only general point I would add is that (in my experience) higher power rated speakers 100watt+ don't drive as well with lower wattage amps, I guess because of the mass of the voice coil and stronger suspension, need to be overcome by the amp and so lose so subtlety at lower amplitudes (and lower peaks to overcome the natural intertia). I'm only saying this because I have installed higher rated speakers (150watt) in my 2 x 12" test rig, as I am fed up with blowing them! and they are definitely less sensitive with lower power amps.      

ICBM

DJH83004 said:

As always IC and Rift spot on the money, the only general point I would add is that (in my experience) higher power rated speakers 100watt+ don't drive as well with lower wattage amps, I guess because of the mass of the voice coil and stronger suspension, need to be overcome by the amp and so lose so subtlety at lower amplitudes (and lower peaks to overcome the natural intertia). I'm only saying this because I have installed higher rated speakers (150watt) in my 2 x 12" test rig, as I am fed up with blowing them! and they are definitely less sensitive with lower power amps.      

Yes, that's true with ones that aren't really designed as guitar speakers - which is rare for ones rated at over 100W, they're usually bass or PA speakers which *can* be used for guitar, if that makes sense.

The last time I blew a speaker testing something was when I connected an amp with an intermittent DC output fault directly to a V30 cabinet… a few seconds of normal sound followed by a loud pop and smoke, which was annoying! I usually try to be more careful than that with solid-state amps - I'm not even sure most 150W speakers would withstand it.

DJH83004

Good point IC, they are PA speakers, just so I can test a variety of amps on the same rig, and the biggest problem is big MOSFET amps going short and putting the DC supply rails across the speakers, but they do tend to tolerate it just enough for the fuses to blow. I do of course have lower rated cabs / speakers for guitar amps. 

ESBlonde

All the essential points about power handling have been coverred above. Different makers use different methods to measure but Celestion typically add pink noise (full range energy) to the driver and leave it for an hour to see if the coil glue will melt. So as ICBM says that is fairly over spec'd.
In terms of what you hear, the sensitivity of the driver can make the amp appear much louder or quieter. Again the human ear can play tricks because  certain frequencies might be more or less sensative in a given brand/type and the ear picks up on the whole speaker being louder because these frequencies cut through.

If you like the sound of a driver in the given box and it matches the amps specs, you're good to go. If you don't like the sound, change it.