question for the experts about power amps

[David Gencay]

is less than 0.05 total harmonic distortion and less than 0.05% from0.015 watts to 150 watts intermodulation distortion ok specs this day in time thanks

[pauln]

Not an expert, but modern distortion specs have little relationship to how the amps sound. There are some amps that have very low figures, but the methods the manufacturers used to get them that low can make the amps sound like crap. There are other amps with rather scary figures that sound wonderful. The quality of the circuit design and parts used is a better predictor. Also depends on the rest of the system, especially speakers.

There are about a dozen known types of distortion and probably some more types that aren't known yet. It's all pretty complicated so chosing components is very difficult. Here are two ideas that might help. The first is about how to weigh the distortion contributions of the components when they play together. The principle applies to the relative contribution of good qualities as well. The second is an old methodology for ordering the selection of components for best chance of compatibility for good sound.

The total distortion contributed by the various components in the signal path is not arithmetic (you can't just add them up); the total distortion is found using quadrature... like this:

Let's say these are your component stages and their distortions

preamp .5%

power amp 1%

Speakers 3% (really good ones, most hit 30%)

If you add them up you get 4.5%, and you might assume that the contributions of each component were:

3/4.5=66% speakers

1/4.5=22% power amp

.5/4.5=11% preamp

...but that is not how it's done.

What you do is take the square root of the sum of the squares

Square each one... .5^2=.25 1^2=1 3^2=9

Add up the squares... .25+1+9=10.25

Take the square root of that total... SQRT(10.25)=3.2

So, the total distortion is 3.2%, not 4.5%

To see the individual components' contributions to the total as a percent, use their individual squares divided by the sum of their squares

9/10.25=88% from the speaker

1/10.25=10% from the amp

.25/10.25= 2% from the preamp

So, notice now how little the preamp actually contributes, but how large is that of the speakers... so where might you focus your effort to improve?... (speakers!)

This kind of goes along with an old rule in audio that says, "Speakers first"; meaning that the first thing to originally select or later upgrade should be the last thing in the signal chain (speakers). The reasoning is that this provides the basis for properly evaluating subsequent upgrades for the upstream components. If your speakers are lousy, how can you know if one amp sounds better than another? But if your speakers are good, comparisons are much easier. Likewise, if your amp is lousy, how do you evaluate phono or digital sources?

The idea is to work backwards through the path; now days people start with the room itself, then speakers, amps, then sources... this provides the highest probability of getting a string of equipment that sounds good. It's an old idea but I think it still makes a lot of sense.

So what speakers do you have? What amp are you looking at?