The smoothest amp you've heard?

from a sonic perspective "smooth" doesn't present itself as something I am attempting to ascertain, or looking for in a amp? I have listened to many amps, I never recall saying "that amp is so smooth". I'm not trying to sound disrespectful. I think its a useless adjective when evaluating sonics of a component. Do you think a sound engineer says "we have to do it again, not smooth enough" IMO, Its a pointless term.

@jakesnak  Its not. In fact, if the amp isn't sounding nice and smooth, to me its an indication of higher ordered harmonics and I will be looking for a reason to work on the circuit 'till I have it right. It sounds to me like you've not experienced an amp that is actually smooth. My recommendation is to listen to the difference between a solid state amp and a tube amp (one in good condition of good design; not something hauled from a 1960s console...). If you've only heard solid state, you might have heard hundreds of them and not heard anything to justify the term. There are smooth sounding solid state amps but they IMO/IME are quite rare.


One qualifier: When/if you are interested enough to do this as an audition, make sure both amps are happy with the loudspeaker used. If that is the case, you won't have any problem hearing this difference- its literally why tube amps are still around after tubes were declared 'obsolete' back in the 1960s. Normally when the prior art is succeeded, it goes away. But tubes are still in production; they have been 'obsolete' now for longer than they were the only game in town! That's happening for a reason and that reason is literally 'smoothness'.

I don't understand what smooth even means?

Perhaps others who have posted above can help me with my (lack of) understanding?

@jakesnack  @david_ten  Read my first post in this thread. The explanation is in the first sentence.

Exactly how or why they would introduce phase shift down to 1/10th the cutoff frequency isn't revealed or explained.

In any case, why just those frequencies? What about 20kHz cutoff frequency causing a 2kHz phase shift?

A 20Hz cutoff will induce phase shift to 200Hz, robbing the amp of impact (since that is how the ear perceives phase shift in the bass).


Its filter theory. When the amp rolls off, it does so on a 6dB/octave slope. A 6dB slope causes phase shift to 10x or 1/10th the cutoff frequency (-3dB point) depending on if the cutoff is the low frequency point or the high frequency point, respectively. So you need 2Hz bandwidth to properly reproduce 20Hz and 100KHz bandwidth to do 10KHz.


There is a way around this, by introducing so much feedback that the amp is able to correct phase shift. That amount needs to be at least 35dB, but the caveat is that the 35dB value has to be the same at all frequencies from 20Hz to 20KHz. Most amplifiers made in the last 70 years are not capable of this feat since it requires a lot of gain bandwidth product.

many tube amps are closed in on top, not airy. so they may be butter smooth, but it’s a circuit artifact overlayed over all the music, and not ’the’ music.

@mikelavigne The quality of smoothness is a lack of higher ordered harmonics; also a lack of IMD. 'Airy'-ness is arguably a different quality which probably needs more clarity to make sure we're talking about the same thing.


In my book, 'airy' has to do with the speed of the amp; many tube amps are not all that fast (15Volts/usec is common). The other thing that this might refer to is phase shift. If there is a roll off in the amp that isn't all that far from the audio band, for example 50KHz, this will introduce phase shift down to 1/10th the cutoff frequency. The ear interprets this sort of thing as a coloration (since it covers a spectrum rather than a single frequency); if the phase shift is due to roll off, it will interpret is as 'dark' or 'slow'.

Not all tube amps behave as you describe of course (hence your use of the word 'many' rather than all). If the amp has bandwidth to 100KHz then its going to have minimal phase shift and won't sound dark. How it generates the higher ordered harmonics is a different matter, but there are tube amps that are both smooth (lacking audible higher ordered harmonics) and 'airy' at the same time.