Why do amplifiers sound different?

Oh one more thing, Just look at what a typical solid state amplifier says in their power rating. 

150 watts into 8 0hms and 250 watts into 4 ohms. 

They are telling you that thier output will vary depending on the impedance. 

That's why amplifiers sound different!

Actually it isn't. Most speakers are designed around the idea that they are 'voltage driven' which is shorthand for saying that the amp driving them will put out the same voltage regardless of the impedance of the load. In this way, the speaker and amp are 'plug and play'. This concept was originally pushed by MacIntosh and ElectroVoice in the late 1950s. It took about 15-20 years for that idea to become widespread in the industry. For more on this see:http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html


Another issue is distortion, and nearly all amplifiers make audible distortion. The ear converts all forms of distortion into some sort of tonality. For example, most solid state amps made run feedback, and so suppress the lower ordered harmonics. But the higher orders are there because if feedback is insufficient, it will generated higher ordered harmonics on its own.

The ear perceives the higher ordered harmonics as brightness and harshness. That's why solid state has had a reputation of harshness and brightness for the last 50 years, despite being flat when measured on the bench.


So most of the differences we hear between amplifiers, their 'sonic signature' is actually their distortion signature- the relationship of the various generated harmonics to one another (and of course the IMD they make). Harmonics of a lower order can mask the presence of higher orders, if that lower order has enough amplitude. The lower orders, the 2nd, 3rd and to a lessor degree the 4th, are innocuous to the human ear and mostly contribute to 'warmth' and 'bloom' (audiophile terms).

All tube amplifiers have output transformers so they aren't affected by impedance fluctuations.

This statement is false. Transformers transform impedance; it goes both ways (else Kirchoff's Law would be violated). So if the load on the secondary varies, the load on the power tubes varies by the same proportion. People like the smoothness of tubes because the lower ordered harmonics they make are able to mask the higher orders. You can see this if the harmonic spectrum is measured; tube amps actually make more higher ordered harmonics than solid state amps do!

If you can run enough feedback, then the feedback will not cause the higher ordered harmonics that it does otherwise. To do this takes a lot of work, and most solid state designs have not been up to the task (and no tube amps are either). What's needed is 35dB of feedback at 20KHz. The old Futterman OTLs had 60 dB of feedback at 100Hz, but owing to insufficient Gain Bandwidth Product (a combination of bandwidth and gain) the feedback at 10KHz was **considerably** less. This happens with a lot of amps.


So you can see now that the distortion signature is actually what is influencing the differences we hear in amps. It sound for all the world as if its a tonality, and it is because of how the ear perceives distortion.

Your statement that tube amps have flatter realized frequency response than solid-state amps is demonstrably false. If you look at the Stereophile’s measurements into a a simulated loudspeaker load (NOT into 8 ohms of pure resistance), you will find that tube amps tend to be much less flat than solid-state amps. This is in large part due to the higher output impedance of a typical tube amp.

The flattest amps into reactive loads are those with lowest output impedance; for example, the PuriFi 1ET400A module, with its 65 micro-ohm output impedance.

Its important to keep in mind that no speaker is actually flat.


And for that matter, they have fallen well behind amplifiers with the idea that they are 'voltage driven'. So the simple fact remains that to get the most out of any amplifier or any speaker, its mate needs to be vetted properly.

Coming from a electrical engineering background,

I’m sorry. Still, many audiophiles have much worse handicaps. (Not really, but trying to stay positive!) I recommend a complete memory wipe, followed by a month studying The Compete Guide to High End Audio by Robert Harley. With a lot more listening and a lot less engineering there may be hope for you yet.

"...Coming from a electrical engineering background, amplifiers theoretically should all sound alike as when measured into an 8 ohm load, their frequency response is extremely flat..."

What ever measurements we make, they really don't tell us how amplifiers sound. I can't tell you what measurements would tell us how amps sound but so far nobody knows what those measurements might be. 

Based on his posting history,  not worth my time to engage.  Have fun with it, team!

I'll say my usual: The greatest impediment to advancing an audio system is the audiophile.   :)

I disagree with the OP in total, but I guess it doesn't matter. He asked and answered his own question.

OP has a depth of understanding comparable to that of Julian Hirsch in 1970.  Since then a lot has arisen to pique the curiosity of both listeners and amp designers. The effect of global feedback was illustrated by Matti Ottala. Richard Marsh published his findings on capacitors. Bob Carver exposed how protection circuits can intrude on music reproduction. All applied to supposedly transparent SS amps with ruler flat response “from DC to light”. Today, we have D Class surging in the market with Hypex and Purifi. Long live the curious!

There are at least three factors involved in how an amplifier sounds:

The engineering of the equipment (and its measurements).
The perceptual and psychological mechanisms involved (ear, brain).
The interpretative habits of the listener (habits, preferences -- taste).

What that says to me, at least, is that I can take with (more than) a grain of salt any claim beginning with "Coming from a electrical engineering background, amplifiers theoretically should all sound..."

The author of such a sentence simultaneously claims and disclaims their own expertise. This is the intellectual equivalent of stepping on a rake.

Hello vanson1. Question your assumptions. Not all tube amps have output transformers. Numbers given us by machines cannot tell a complete story to human beings. There are lots of women with 36-24-26 figures, same weight, same height; but are they all the same? Different capacitors with the same electrical specifications sound different. It's easy to demonstrate. Manufacturers of different amps use different parts. All 12AX7 tubes do not sound the same; ask any "tube roller." So . . . any assemblage of parts will have it's own "sound" due to the interaction of the signal it's handling with all the various parts. An iron wire has different characteristics than a copper wire or an aluminium one. If you are a beginner, a budding audiophile, you are entering a wonderful hobby which is mucher cheaper and safer that discovering the differences between all the ladies with simlar "specs."  A little experience will lead to a great deal of education, and a lot more questions. Enjoy the music1