What makes different amps produce different sound?

Eldartford: Where did you get that information? To my (admittedly limited) knowledge, everything you've said is wrong. Most instruments produce ALL harmonics, but some wind instruments, if I'm not mistaken, produce only ODD harmonics. (Remember that the fundamental tones are themselves odd harmonics!) Also, while it is true that tube amps tend to have more even harmonic distortion than odd (or is it the other way around?), tube tend to have more of BOTH even and odd-order harmonic distortion than typical solid state designs.

Why anyone thinks that added distortion of any kind should make something sound more musical is beyond me.

I think it was Art Dudley who suggested that the color of the face plate had an effect on the sound of amps and preamps.

Some other variables that affect the sound of an amplifier:

Humans are relatively insensitive to even-ordered harmonics and astonishingly large amounts are usually not objectionable. Even ordered harmonics tend to add 'bloom' or 'fatness' (in the bass) and will obscure detail if in large amounts.

Humans find odd-ordered harmonics much more objectionable, in fact higher orders (about the 7th and beyond) are used by the human ear/brain system as loudness cues. Transistor amps with high feedback tend to have very low distortion except for higher/odd ordered harmonics, which is why many transistor amps will have a harder sound. Tube amps, particularly those with low or zero feedback, will have very little high/odd ordered harmonics and will have a softer sound.

Bandwidth plays another role. Generally, due to phase shift/group delay effects, the cutoff frequency of many amps can be audible. For example a 10Hz cutoff will introduce audible artifacts to about 100 Hz, resulting in a bass shyness. Amps that cut off higher, for example 20Hz, will have cutoff artifacts to 200Hz. High frequency cutoff issues generally follow similar rules- to 1/10th the cutoff frequency. A 50KHz cutoff will introduce artifacts down to 5KHz. These are all frequencies that humans can hear very easily! -and is why so many manufacturers strive for wide bandwidth.

There are a number of other influences, but it is getting late and these are good for starters. .

Unsound, I read an article in Stereophile about ten years ago about an experiment with speakers. The listeners were told that different colored grill cloth was used so they would be able to tell the difference between the multiple speakers being tested.

They ran the listening tests for the speakers and the listeners determined which had certain sonic characteristics. There seemed to be somewhat of a consensus as I recall. The ones with the blue grill cloth had the better bass... and so on.

The funny thing about the test was, all the speakers were the same except for the color of the grill cloth. I doubt that these people would qualify as audiophiles. I don't remember who did the test, I think it was a university in the US, but unsound's comments reminded me of the test.

Bomarc...I really don't know where I learned what I posted: it was long long ago, and I know I have seen it many times. See also the comment by ralph@ above.

Sometimes there is confusion about numbering of harmonics. If you do a Fourier analysis of an audio signal, what we call the fundamental is the first harmonic. DC is the zeroith harmonic. When speaking of an AC signal, harmonics are usually numbered as multiples of the fundamental.

Perhaps brass instruments do produce the "unmusical" harmonics. This explains why they sound so strident compaired with strings.