Just back from Amersfoort and only around 300 messages to catch up on, ya'll disappoint me

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That's a great point. I'd never thought of it like that, but it makes a hell of a lot of sense. Pretty much tosses outt the notion that only higher-power-rated speakers demand more power than lower-rated ones.


Just to play both sides : a higher rated speaker might be mechanically stiffer and/or less efficient, therefore requiring a bit more power to get as loud as a more efficient, 'easier to drive', speaker. Looking at the efficiency numbers can be interesting. 3dB less sensitive and you need 2x the power for the same volume... But the higher rated speaker still wouldn't "suck" more out of the amp. It just does as well it can with the power delivered.

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If the amp is intentionally delivering more voltage than 12V, which is what the car is supplying, doesn't that mean that you always run that risk? Or do you mean more voltage than the power supply is able to generate?


Sorry, should have been clerarer - More than power /higher voltage than the power supply can generate.
The internal supply goes to two power rails. Imagine them as roof and floor. The output transistors are connected to these and can make the output any voltage inbetween, but the roof and floor [minus losses in the transistors] are the limit for the voltage on the speaker outputs. Ask the peak voltage of the output, due to input signal level and gain setting, to swing into the sattic and basement and it just won't happen; for the 'tallest' parts of the signal it will just peg to roof or floor.

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And assuming that the speakers are 4Ohms, that means that the power rating of an amp directly tells you how much voltage it can produce by taking the square root of four times the power rating. So a 75W amp can produce up to 17.3V and a 300W amp can produce up to 34.6V. Of course, nothing's rated in voltages, but that at least helps me get my mind around it.


Internally it is likely to be a touch higher in order to give a bit of margin and for some internal losses, such as the saturation voltage (voltage that will still be across the transistor even while we tell it to be fully "on") of the output transistors.
But yes, playing a 0dB (and beware of any filters) 50/60 Hz sinewave and measuring the voltage on the speaker terminals is a way of checking how much power it really makes.

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But virtually all car speakers are 4Ohm, right? So that's a non-issue unless you get a non-standard speaker or the speaker becomes damaged some other way first.


As others have said, most normal car speakers today are 4 Ohm. There are exceptions - for instance some 'premium' OEM packages run lower ohm speakers (with amps designed for it).
The Ohm rating is nominal.
If you measure the DC resistance with a meter, you'd probably be around 3.5 Ohmfor a "4" Ohm speaker.
As you feed it an AC signal, a mainly reactive component (due to the coil) gets added and the total impedance is around 4 Ohm, but may vary some with frequency.

Subwoofers, specially in the upper power ranges, are less standard, I know there are [nominal] 1, 2, 3, 4, 6 and 8 Ohm [per coil] subs - look hard enough and you can probably find other values as well. Mainly as a way to get the number of subs you want and a final load suiting the amp. For 3 subs and a 1 ohm stable monoblock amp take 3 dual coil 6 Ohm subs and wire all six coils in parallell...
But one shouldn't wire stuff up just any wayimaginable - asymmetries in wiring scheme and or coil impedances will give diffent subs differing amounts of power.


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So that means that, since a high-rated speaker doesn't put any more strain on an amp than a low-rated one, and since a more-than-double-rated speaker should be able to handle the worst case from the amp, then the safest thing to do is get speakers that are rated more than two times the amps rating, yes? Ignoring playing sane volumes and economics.


Pretty much...There is a faction that claims the speaker (mainly subs) are cooled less well on a square wave than the same frequency sinewave because of the membrane 'standing still' for parts of the cycle, whiich woulld point to more than 2x. But as it [the mebane] still moves back and forth x times per second I'm sceptical, but have no proof either way.

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I'd say go with roughly the same ratings and be sensible with the gain setting, trust your ears.

So even with all this stuff that tells you that speakers can get damaged when they can't handle the power that the amp sends to them, you still run your fronts and subs with under-rated speakers. That tells me that setup and sanity are probably bigger factors than most of this anyway.



Not forgetting that the "marketing factor" in both speaker and amp ratings can upset any calculated setup anyway...
As long as the gain setting is lower than the required setting for the rated power [given the input signal level from the HU], the max power delivered from the amp will be less than the rated power. If you set things up with 0dB on the empeg and adjust the gains to the lowest of a) no higher than it still all sounds clean or b) your ears cant take more.
I'd use an 'energetic' piece of music for this. Do one speaker set at a time. Then listen to them all together: a set to loud compared to the rest? Turn than gain knob down, don't turn the others up - just like with the equalizer, cut, don't boost... And if you after that never go over 0dB on the empeg your speakers should be quite safe.
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/Michael