Originally Posted by Fjodor2000
For many amplifiers the power output does not double when the loudspeaker impedance is halved (at least not down to 2 Ohms or so). Ideally an amplifier that has 100 W at 8 Ohm should have 200 W at 4 Ohm. But many do not, and only have e.g. 150 W at 4 Ohm.
Now what I wonder is whether that only comes into play at loud listening volume?
I'll clarify with an example:
* Amplifier: 100 W @ 8 Ohm, 150 W @ 4 Ohm.
* Loudspeaker: 8-4 Ohms, depending on frequency.
Now if one is listening at a volume requiring 50 W at 8 Ohm, that would require 100 W at 4 Ohm to sustain the same power delivery to the speaker. According the the spec, the amplifier in the example is capable of 150 W at 4 Ohms (i.e. less that the required 100 W). So does this mean that it is sufficient, and that even if the amplifier would have had 200 W at 4 Ohms instead that would not improve the performance anyway?
Short answer; a loss in fidelity.
Signal compression, and spike in momentary or longterm THD & IMD, TIM, etc...
In essence all the short falls that the entrance into clip bring about.
Now if long term, your amplifiers protection will most probably real back the Voltage to protect the amplifier, in doing so THD, IMD, TIM etc, will not increase near as much but you will still incur signal compression.
Short term occurrences are very common for most of us that like to listen to our systems above 90dBc, say ranging between 90-100dBc.
Essentially all modern amplifiers are rated in dynamic terms, short term power bursts while connect to non-inductive loads - phase and load linear. Their actual usable, reliable power is always much less, than these dynamic testing standards depict. As a rule of thumb, I say take the rated, dynamic RMS and cut it in half, and one would have a fair idea of their amplifiers true, longterm, useable, and reliable power.
In this light, your 75-watt amp, would fall in the range of 35-40 watts, still quite usable. The caps will provide for momentary demand peaks, but such power is very short term and easily exhausted.
Your speakers efficiencies, the distance that you sit from them, and the average range of dB that you typically listen to them, is what determines the power demands / requirement from your amp. Your music selection of coarse, also has a weighted factor, in that some music is more dynamic than others and will let your amp breath, if you will, but most music today is so highly compressed that theirs virtually no dynamics left, its a b@ll's to the wall mix down.
If your speakers are say, 88dB efficient, on-axis, without obstruction or diffraction, at 1w/1m and you sit say 3 meters from them, in theory, with 1 watt of injection they would show and efficiency of 79dB. But in real measurement we discover that it tends to be a little less, due to the power envelops dispersing differently, than within the field of the first meter. So lets just round down to 78dB @ 1 watt.
Now lets say that you like to listen in the range of 85-95 dBc. We need to take the highest dB within the range 95dBc, to determine how much true power that you will need, outside of short term, dynamic power.
78dbc@3m = 1 watt
81dBc@3m = 2 watts
84dBc@3m = 4 watts
87dBc@3m = 8 watts
90dBc@3m = 16 watts
93dBc@3m = 32 watts
96dBc@3m = 64 watts
In this example, it is likely that power compression and increases in THD, IMD, etc, etc, will start to trifle with elements of the faithfulness of reproduction.
If someones speaker system is say 86dB efficient, these troubles would set in sooner. If there speakers are out past 3m, from their seated position, the same will be true. Lastly if they like to listen to their systems with peaks above 95dBc, they same will also hold true.
If you are someone in all three camps, then big power is absolutely required, to ensure the highest fidelity reproductions from your audio system, as a whole.
Now for an important caveat: For those of whom that listen to highly dynamic masterings, it is advisable to double your estimated RMS, long term power needs. At least if your desire higher potentials toward fidelity.