The sound of the vacuum tube amplifier is determined by its damping factor!!

[whazzup]

Probably the experts are already well aware of this, but still, seeing the frequency response comparisons as well as the test to make the solid state amp sound like the tube amp makes everything so clear now. And why speaker amplifier synergy was such a thing in older days.

Video starts from the amp comparisons (Japanese with English subtitles)

 

[Speedskater]

Actually it is the amp's output impedance with respect to frequency and the multi-driver loudspeaker's impedance with respect to frequency that determines the sound.

 

[whazzup]

Actually it is the amp's output impedance with respect to frequency and the multi-driver loudspeaker's impedance with respect to frequency that determines the sound.

I see.... so higher output impedance of tubes leads to lower damping factor which in turn leads to relatively poorer control of the speakers and that acts like a loudness filter on the frequency response.

 

[Gregss]

And that freq response with the speaker gives large variations. W=Good part of why some amps don't go well with some speakers.

 

[egellings]

Actually it is the amp's output impedance with respect to frequency and the multi-driver loudspeaker's impedance with respect to frequency that determines the sound.

It's the interaction of those two impedances.

 

[MAB]

Here is an amp with high output impedance (~5 Ohms from the 8 Ohm tap, ~2.6 Ohms from the 4 Ohm tap).
8 Ohm tap into various simulated loudspeaker loads:

4-Ohm tap into various loads:

Wavac SH-833 monoblock power amplifier Measurements | Stereophile.com

Sidebar 3: Measurements "Perhaps I should just measure one of the amplifiers," I said, eyeing the eight chassis that comprise a pair of the Wavac SH-833 monoblocks, all of them sitting at the business end of Mikey's room. Even so, as neighbor Wes Phillips and I manhandled the 550 lbs of...

www.stereophile.com

Those are dramatic variations, very audible.

Most tube amps don't have enough distortion to be clearly audible, despite the common statement. Likely the Wavac SH-833 has enough that it is audible.

If I wanted the above EQ of the Wavac, I would use DSP even if the SH-833 was reasonably priced.

 

[Speedskater]

I see.... so higher output impedance of tubes leads to lower damping factor which in turn leads to relatively poorer control of the speakers and that acts like a loudness filter on the frequency response.

Well no, it's not about damping factor or control.
It's about the series circuit of amp output impedance and loudspeaker impedance. Each of these varies with frequency at different rates.
This is a voltage divider circuit so the signal voltage at each driver voice coil varies by this in addition to the crossover control.
* * * * * * * * * *
The Stereophile measurements of vacuum tube amplifiers have a chart of the amps output impedance.

 

[Speedskater]

Damping factor only affects the bass frequencies and most vacuum tube amplifiers have a high enough damping factor that it is of no concern.

 

[NTTY]

Amplifier Output Impedance (Damping Factor) and Speakers

 

[Keith_W]

Erin on why amplifiers sound different. It's the output impedance.

 

[sq225917]

And its everything before the output interaction, noise, FR, imd, thd.

 

[solderdude]

I see.... so higher output impedance of tubes leads to lower damping factor which in turn leads to relatively poorer control of the speakers and that acts like a loudness filter on the frequency response.

It actually leads to a modification of the frequency response dictated by the (varying) impedance of the connected speaker due to a different voltage division per frequency.
The actual 'damping current' is not changed much as most speakers do not rely on electrical damping.
In some cases an audible roll-off at the lower and upper end of the audible range can also have an effect.

 

[MaxwellsEq]

I see.... so higher output impedance of tubes leads to lower damping factor which in turn leads to relatively poorer control of the speakers and that acts like a loudness filter on the frequency response.

Not exactly...

Tube amplifiers generally operate at high voltages and relatively low currents (compared to a solid state amplifier). These tube voltages and currents don't suit typical domestic loudspeakers, so the tubes feed the speaker via a transformer (long pieces of wire wound round a core). These transformers drop the voltage and increase the current.

The transformers generally have higher output impedance than an equivalent solid state amplifier. This can mean the impedance curve of the speaker is more imposed on the music than with an equivalent solid state amplifier. The transformers also can suffer from saturation especially at low frequencies.

 

[fpitas]

The better tube amps back in the day used various schemes to get a reasonably low output impedance, like lots of negative feedback. These days though, "audiophiles" have convinced themselves that negative feedback is bad. It certainly seems that the majority of tube amps these days are simply special-effects boxes.

 

[Redacted]

You probably won't hear much (bad) effect with a damping factor >>10, but you may well with very low DF, in your bass drivers.

My modest amp has 2mΩ (Max) <1kHz and 4mΩ < 20kHz, so with 8 ohm speakers that's a DF of 4000, or "sufficient". I think this is typical these days.

The other thing is that like speakers, the output impedance of an amp is complex, as in LCR, not just R. This matters more if transformers are in the output stage - like in my guitar amp.

I have never heard anyone (at least nobody who can design amps that sell in quantity) rail against (no pun intended) -ve feedback. The facts and figures are measurable...

 

[fpitas]

You probably won't hear much effect with a damping factor >>10, but you may well with very low DF in your bass drivers.

My modest amp has 2mΩ (Max) <1kHz and 4mΩ < 20kHz, so with 8 ohm speakers that's a DF of 4000, or "sufficient". I think this is typical these days.

The other thing is that like speakers, the output impedance of an amp is complex, as in LCR, not just R. This matters more if transformers are in the output stage - like in my guitar amp.

I have never heard anyone (at least nobody who can design amps that sell in quantity) rail against (no pun intended) -ve feedback. The facts and figures are measurable...

Just spend a few minutes on a subjective audio forum if you want to learn the horrors of negative feedback. Obviously, competent designers employ it.

 

[Mart68]

I was reading the other day the theory that at least some early transistor amplifiers did not use enough negative feedback, so only low order harmonic distortion was reduced allowing high order harmonics to become audible, and this was why they did/do not sound so good i.e a bit rough and grainy.

I have no idea if that's true or even credible?

 

[Redacted]

Just spend a few minutes on a subjective audio forum if you want to learn the horrors of negative feedback. Obviously, competent designers employ it.

I too, am an engineer, so I don't have time to go and read the hysterical stuff!
Everything in engineering is a compromise where you make stuff that doesn't nicely obey you, become good enough for the job at hand with trickery.

 

[Redacted]

I was reading the other day the theory that at least some early transistor amplifiers did not use enough negative feedback, so only low order harmonic distortion was reduced allowing high order harmonics to become audible, and this was why they did/do not sound so good i.e a bit rough and grainy.

I have no idea if that's true or even credible?

Negative feedback reduces gain, but increases bandwidth. I'm guessing that if you widen BW beyond human hearing, more distortion harmonics will be allowed at the output. Unless you are referring to some masking effect or IMD, I can't see the assertion being true, BUT early transistor amplifiers likely needed as much gain as possible to keep component costs down.

 

[HalSF]

I’m feeling unenlightened so far in this thread, apart from learning about the ideological stance at AsR that approaches the appeal of tube amplifiers with a kind of huffy, impatient, generally pejorative reductiveness that refuses to contemplate any sympathetic hypothesis around legitimate technical explanations for enjoying tube sound, whatever that actually is.

Is it something more interesting and intelligible than just bad measurements and impedance matches and euphonic distortion and nostalgia, all the stuff that make listeners who seek out and enjoy that sound bad deluded people in this corner of the internet? Damned if I know.

Gonna go watch Erin’s video and see if that helps.