power is product of current and voltage. do your math.
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An amp will sound more powerful if it can double it's output into a 4ohm load or triple into a 2 ohm load.For instance the spec. sheet may read:40 watts into 8 ohms,80 watts into 4ohms,120 watts into 2 ohms.Many spec. sheets don't include that info. but a call or email to the manufacturer will provide you with that spec.
Yeah, my unscientific method of analysis is to look at the specs regarding power output at 8, 4 and 2 ohms, WITH ALL CHANNEL DRIVEN.
Just as an example, I have an Aragon 2004 2 channel amp that's as old as the hills. It's rated at 100wpc at 8 ohms and 200wpc at 4 ohms and weighs over 40lbs.
If you're looking at Class D amps, though, that's a whole different ball game.
Judging by power doubling, new Jeff Rowland Model 625 that outputs 300W@8ohms and 550W@4ohms is weak amplifier and that would be a mistake. Judging by weight, as Jmcgrogan2 suggested, is perhaps better, but there are exceptions. Amplifiers that have switching supplies don't require large transformers or big capacitors. In addition voltage is regulated. Mentioned Model 625 uses such supply but is very heavy (54lbs) only because of very heavy case. Power specifications are not that important since double power means only 22% louder. How is it measured? For how long? What frequency? Specifications can be some help but also can be very misleading. Amplifier with deep negative feedback can sound very unpleasant but will have great specifications.
you can get some hints from various aspects but in the end you will have to do some math to determine whether or not the power amp is capable of high(er) current.
As Jmcgrogan2 stated one aspect is sheer weight of the amp. This assumes that the weight is from the power transformer & the power supply caps. As Kijanki pointed out w.r.t. the Rowland 625 class-D power amp, you could be totally fooled with the weight of this amp; it's all in the chassis & the class-D power supply is compact & relatively light weight! So, obviously ensure that the power amp you are looking at is class-A, class-A/AB or class-AB.
As Jtcf pointed out - another aspect is wattage specs. does the manuf list the wattages for 4 Ohm, 2 Ohm & 1 Ohm. If yes, do the wattages double each time the speaker load halves? If it does most likely the power supply is very robust & can provide gobs of current (Gryphon Audio amps are often spec'd down to 1 Ohm, doubling wattage for each halving of speaker load impedance. BUT.....they DO cost a bomb! ;-) Comes with the territory) If the wattage does not double per each half of the speaker load, the power supply is robust but not that robust. I know this is a fuzzy statement so it has to be dealt with on a case-by-case basis. A power amp wattage might double for 4 Ohm load & then might go up 30-40% for a 2 Ohm load. This should tell you that a 2 Ohm load for this amp is going to be a burden & that it can support upto a 4 Ohm load quite well.
Yet another aspect is to look at the amount of power supply decoupling capacitance. Just a blanket statement here - the more, the better. Of course you cannot have any power transformer with any current capacity hooked to some arbitrary amount of power supply decoupling capacitance. It takes a large amount of charge to tank up those power supply caps hence the transformer's got to be up to the job otherwise it will hurt the dynamics. Considering that the correct transformer is used, if one has more power supply decoupling capacitance the power amp is better able to supply load transients & maybe even a short burst of current into the output stage.
Related to this the type of power supply capacitance - do they look like coke cans or do they look like a 35mm film roll. Most manuf today are using caps that lool like a 35mm film roll & many, many of those in parallel. They say that it's better for dynamics. They do have a point here - the ESR of the many parallel caps are all in parallel which means that the net ESR is small. Small cap ESR is a good thing for swift response from the power cap. OTOH, tho', the inductance of the cap leads & the physical distance of the furtherest cap from the amp output stage can add series inductance to the overall capacitance & this is not a good thing for swift response from the power cap. The other thing I don't like is that all the dozens of small caps used might not age the same way meaning that the load is not shared equally by all the caps in the array. Also, it's very likely that 12 or 20 parallel caps take up more real estate than just 2 coke can caps.
One method I like to ascertain whether a power amp is high current capable is hinted by Marakanetz - I try to find out the transformer VA rating & also the secondary AC voltage rating. By dividing VA by secondary AC voltage rating you get the current handling capacity of the transformer. Divide this current by 2 - since we have a left & right channel. Of course, no need to divide by 2 if the amp is a mono block. This is the max current the amp can output per channel no matter what the (gibberish) spec sheet says. Higher current is better esp. if you are trying to drive a difficult speaker load.
Hope that this helps. FWIW.
Bombaywalla, You mentioned important point - quality of power supply. Design seems straightforward with just transformer rectifier and capacitors but in reality it is pretty complex. Linear supply is unregulated and because of that requires a lot of capacitance to hold voltage steady. This capacitance is in series with the amplifier's output (circuit closes thru it) so anything that makes capacitor less than perfect plays role. ESR is important, being just series resistance and as such it gets lower when capacitors are in parallel. Same with inductance but then inductance of connections, as you pointed out, might add. Parallel film capacitor might help withe high frequency response or it can get things worse since it is placed across large inductance creating parallel resonance circuit that will ring. Rectifier can be hard or soft. Charging of capacitor happens in narrow spikes that starts at the bottom of the ripple and ends on the top of full wave rectified sinewave. At this peak point voltage across diode changes direction but diode is too slow to react (stores charge) for a moment conducting in opposite direction to snap back and stop conducting. Soft diodes react fast (small overshoot other direction) but snap back slow reducing that way amount of produced HF noise. Larger capacitance usually comes with larger inductance but also can cause startup problems, as you mentioned, and produce noise. Since capacitor is charged only from the bottom of the ripple to peak value, when capacitance is very large ripple is very small and charging time is very short. Such short spikes repeated 120Hz have very low average to rms ratio causing losses in transformer wire while high frequency content heats up the core. It requires transformer to be much bigger. In addition it is creating similar losses in power supply cables. Your amp might take average 1A but it can be in form of 40A spikes that cause voltage drop in power cable (and radiated noise). I mentioned all this to suggest that design of even simple thing like linear power supply, (not to mention amplifier) can be very complicated. Judging amplifier by power doubling might be oversimplifying. We look if amplifier doubles power to see if it can deliver current when speaker impedance drops, but tube amps don't do it (being power source vs voltage source) and we like their sound. In addition, for amp to double means to be perfect voltage source and that might require deeper feedback that can bring other problems. To me "doubling" describes only behavior in extreme situation that might never happen in real life. Your speaker has impedance dip above 100Hz while highest amplitudes are usually well below it, where impedance is higher.
You guys are right,I way oversimplified!Was refering to class A/B amps.Class D and tube amps certainly are different animals.My experience has been that if a class A/B amp can't nearly double it's power it has no dynamics to speak of.They don't need to use excessive neg. feedback to accomplish that,though some still do.I agree high feedback = lower distortion spec.= bad sound.