Originally Posted by commsysman
The key thing about amplifiers is to understand that amplifier performance is synergistic with the speakers.
The above would seem to imply that the amplfier's performance is always changed in audible ways by the attached speakers. If that is what was meant, then it is not representative of the modern world.
In general, lower powered amplifiers (and tube amplifiers) have a higher output impedance, which means that they interact with the varying impedance (and reactance) of the speakers to a greater degree than those amplifiers with a very low output impedance.
Conflating tubed power amps and low power SS power amps is a bad idea. There's no reason why a low powered SS amp would have a high output impedance, and the test bench and listening room confirm that in the vast majority of cases.
This can result in non-linear frequency response, and in the case of highly reactive speakers it can also cause the amplifier to distort.
The good science: An amplifier with a high source impedance will have frequency response variations that are related to the frequencies where the speaker load presents a low impedance load to the power amplifier.
The bad science: A high source impedance can actually reduce the tendency of a power amp to distort with a reactive speaker because it won't apply high current to the speaker in order to get the reactance under control.
This is not very surprising if you look at the frequency versus impedance curve of a speaker, where it is common for the impedance of a given speaker to vary from 3 ohms to 20 ohms at various frequencies, and the phase shift of speaker voltage and current to be as much as 45 degrees. Some amplifiers behave very badly when the speaker shifts the current phase that much.
The confused science part 1: Reactive loads can cause problems two different ways. Reactive loads can cause a poorly-designed amplifier to become unstable and oscillate. The oscillations can build up and damage the amplifier and/or the speaker. This is rare in modern amplifiers. Reactive loads can cause problems even with amplifiers that are stable. Loads that are both reactive and present a low load impedance can cause overloading in output transistors of a special kind called Secondary Breakdown. Modern amplifiers generally use output devices that are resistant to this problem. Back in the early days of SS amplifiers, output devices of this kind pretty much didn't exist, and damaged amplifiers were more common.
The confused science part 2: It's not load reactance that causes durability problems, its the combination of a low load impedance and lots of reactance in the load that causes durability problems. This is important because there are speakers that are very reactive (inductance or capacitance) at some frequencies but also have high impedances at those frequencies.
For example, many speakers have an impedance "bump" in the upper midrange and low treble area where the impedance often goes as high as 20 ohms. This will tend to make the speaker/amplifier combination have more output at these frequencies, which is often characterized as a "forward" or "bright" sound. This is of course more of an issue with a smaller amplifier that has a higher output impedance, which reacts more to the change in load.
Same problem as I pointed out above. The idea that a low powered SS amplifier necessarily or commonly has a high source impedance is simply false.
This characteristic may be attributed to the amplifier OR the speaker sounding "bright", when in fact it must be looked at as the characteristic of that speaker in combination with that amplifier; the blame is due to the way they interact ( the quality of the speaker drivers etc. is of course an issue also).
In modern times the previous several paragraphs are a lot of discussion of something that is now pretty rare with SS amplifiers. Fact is you can assume that in general a power amp won't have its operation audibly affected by the loading caused by almost all speakers, and you will be right.
Tubed amplfiers come in 2 kinds - those that are trying to act in the way that good SS amps do more-or-less naturally, and those that are trying to go the opposite way as good SS amps. The latter kind of power amp is frequently called a SET or Single-Ended Triode. Seriously, these amplifiers pretty much look just about everything that is known about designing good power amps in the eye and run in the opposite direction. Interestingly enough, you can simulate much of what they do by simply hooking a power resistor between 3 and 20 ohms in series with your speaker, and hook it to a good SS power amp.
The reason higher-powered amplifiers tend to sound better often has very little to do with the higher power per se (which is often unused), and more to do with the very large power supply and and resulting low output impedance of the amplifier. This makes the amplifier capable of maintaining a relatively constant output even when the speaker impedance and reactance vary a lot over the audio frequency range.
The above paragraph is pretty much 100% audiophile myth. The low output impedance of SS power amps is readily achievable with fairly modest power supplies. The actual source of the low output impedance is the output stage design. The circuit features that lead to low output impedance are local and global inverse feedback in and around the output stage itself.
There are some higher-priced amplifers that do not have terribly high power ratings, but still sound very good with a wide range of speakers. This is often because they have larger power supply transformers and capacitors than other amplifiers of similar power, which lowers their output impedance.
The audiophile myth faucet seems to now be fully turned on. ;-)
I've done some bench and listening tests where I intentionally took a relatively low powered power amp (car stereo - about 30 wpc) and experimented with various power supplies. One of the power supplies I tried was designed to power a laptop computer and could only put out a couple of amps before it would trip out its short circuit protection. I could clip the amp out at its usual power level for clipping with a stout power supply, without tripping out the laptop power supply because music has a high peak to average ratio and the average current drain was low. The whimpy power supply had no effect on the power amps output impedance or ability to develop high peak currents into reactive loads.
Also, it has been found that coupling capacitors in the signal path cause distortion, and more expensive amplifiers use quite expensive capacitors to eliminate the audible loss of quality cheaper ones cause.
Actually, it is common to build power amps with no
interstage coupling caps at all, and perhaps just one input cap to block DC that might mistakenly come from the signal source. As long as the capacitor has a large enough capacitance to not significantly decrease the low frequency response of the amp, the distortion being speculated on above just doesn't happen.
As a rule perfectly adequate capacitors are widely sold for reasonable prices. The idea that special "audio grade" caps do any audible good is yet another audiophile myth. I've never seen an audiophile capacitor upgrade article that showed any objective improvement in performance over the same piece of equipment with ordinary capacitors in good condition. It is possible for capacitors to fail in use, and broken capacitors are like just about any kind of part - broken ones can make equipment fail to reach its potential.