I've been reading up on amplifiers and I'm a little confused on how things actually work. I currently have some decent low end speakers and am looking to upgrade to a more robust setup. My receiver right now is an Onkyo TX-SR605 and all of my speakers have an impedance of 8 ohms. Some of the speaker lines I am interested in have an impedance of 4 ohms. The TX-SR605 does not support 4 ohm speakers and it does not have pre-outs for anything but the subwoofer.
So if I were to purchase 4 ohm speakers I would need at least a new receiver if not an external amplifier. My question is at what level of speakers do you actually _need_ an external amplifier. From reading these and other forums it seems like certain brands of speakers are always paired with stand-alone amplifiers. If I were to purchase a receiver that is in the $1500 range such as an Onkyo TX-SR876 (140wx7 @ 8ohm) or a Denon AVR-2809CI (115wx7 @ 8ohm) can those actually power good mid-range speakers? This would be for a small room now (11x13x10) but the system would eventually be placed in a slightly larger room.
Do Paradigm Studio 100's or Rocket floorstanders need standalone amplification? Those are 8 ohm but the Studio 100's list their max power as 210 watts. Even for a large room or if you wanted an extremely high volume can a good $1500 receiver do the job well? For 4 ohm speakers like Triad Gold LCR's or Thiel speakers or PSB Synchrony One's can you effectively drive those with a $1500 receiver in a 5.1 setup?
I think part of my confusion is that you can get a $1500 receiver that can push 140wx7 that also handles all kind of processing and connectivity but a stand-alone amplifier that does 140wx8 and nothing else is $2400. Are stand-alone amplifiers like $50/foot speaker cable or are they actually so much better than the all in one AVR's?
You have asked a complicated question which vexes everybody involved in this complex problem of the loudspeaker/amplifier interface.
There has been about as much ink spilt on these forums on this technical question than any other.
Let us start with the loudspeaker. First of all manufacturer's specifications are pretty much meaningless, and tell you just about nothing concerning the difficulty of load or otherwise to an amp.
First please understand that a speakers impedance is not constant. If the enclosure is ported there will be two sharp peaks around the tuning frequency. A sealed enclosure and a properly damped TL has one bass impedance peak. Most speakers these days have narrow fronted cabinets, which result in a first order bass roll off starting between 600 Hz and 400 Hz depending on driver size and cabinet width. Good speakers have compensation for this. This compensation always involves dropping the impedance progressively below the roll off point. Now the power divide is 400 Hz for most program material. So half the power is required below 400 Hz. However if the impedance is dropping, that doubles the power requirement below 400 Hz, so that two thirds or so of the amp power is required below 400 Hz, where the amp is seeing a progressively dropping impedance. There may be drops in impedance around crossover points also. On the whole better speakers will be effectively four ohm speakers no matter what the manufacturer says the "nominal" impedance is.
Now the problem gets more complicated than that. Loudspeakers do not present resistive loads, to amplifiers, they present inductive and reactive loads. This results in the voltage and current being out of phase to varying degrees. This results in the concept of apparent and true power. Usually the apparent power is significantly higher than the true power. The problem is that the amp has to provide the current to meet the apparent power requirement. The upshot of this is that the amplifier actually sees an impedance lower than measured.
To make matters worse amplifiers are tested and specified into resistive loads, and not the sort of loads presented by loudspeakers. In order to discern the complexity of load presented by a speaker to an amplifier, you need to see a plot of impedance with frequency. Also you need a plot of the phase angle between voltage and current with frequency for the speaker. Very, very few manufacturers release this data. They would very much prefer that no one knows.
Now let us consider the amplifier. The first thing to realize is that the voltage developed at the speaker terminals into a given load determines the current through the load and therefore the power. Now lets say we have an amp that can generate a maximum voltage swing of 40 volts. Into an eight ohm load that amp will deliver 200 watts. Voltage squared (1600) divided by resistance (8). Now if that amplifier is a good one, it will maintain 40 volts across four ohms and will deliver 400 watts. If it is a very, very good and usually expensive amplifier it will maintain 40 volts across two ohms, and deliver 800 watts. The current required is 4, 10 and 20 amps respectively.
Now lets take the usual case of a budget receiver. Usually, the maximum current available is pretty close to what is required to meet spec into 8 ohms.
So if 4 amps is the maximum current, then the power into four ohms will be 100 watts and the power into two ohms will be 50 watts. If it is a 100 watt receiver then it will deliver 50 watts into four ohms and 25 watts into two ohms.
The situation may be worse than that, as the attempt to deliver high current close or beyond its maximum power, into clipping range, may trigger the amp protection and shut it down. Frequent activations of protection circuits will greatly shorten the life of the amp or receiver.
So what do you get from this. Well amps that have power supplies and output devices big enough to deliver high currents in the 20 amp range are worth their money, and not in any way comparable to "funny" wire.
That if you limit your listening levels then you reduce current demands, and a humble amp may give you your desired results. If you want clean high power and long life from your amp, then you have to pay the piper.
One other point, a 3db increase in the sensitivity of a speaker is equivalent to doubling your amp power. However make sure the rating of the sensitivities is the same. If it is one watt one meter spec, then it is equivalent. However often sensitivity is quoted as 2,83 volts 1 meter. Now 2.83 volts is the voltage that will put one watt though an 8 ohm load. So if a speaker is four ohms it is going to take more power from the amp to produce that one watt. As I told you impedance on the whole drops in the bass frequencies, thus increasing power demand in those frequencies. So you really need to have an idea of a speakers impedance curve to get a handle on the sensitivity when the 2.83 volt 1 meter spec is used. This is just one more of many ways speaker manufacturers confuse you, and lay a trap.
The real answer to this problem, I think will be the placing of high efficiency class D amps in the speakers, one for each pass band, and using active electronic crossovers in place of passive ones. That way power can be optimally matched to load. This will prevent us from leaving an awful lot to chance like we do now
