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Why do amplifiers sound different?

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. 

Usually + or - less than 0.2 dB. Your ears can't detect that. 

What makes them sound different is the fact that speaker impedance various with frequency. All solid state amplifiers that do not have output transformers vary their output slightly depending on the impedance they see at each frequency. 

That's why matching amplifiers to speaker matters. 

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

That's one reason they sound better to most people. 

Odd vs even order harmonics is another but that's another discussion. 
vanson1

Showing 1 response by mike_in_nc

mike_in_nc
1,125 posts
 
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.
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