If tubes amplifiers measure poorly, why are they perceived as sounding better?

[Randolf]

The conflation of "high source impedance" and "tube amplifier" is inaccurate.

Maybe inacurate but typical (and to my understanding inherent to the design of tube amps). Lets look at some Stereophile tube amp reviews:

• Air Tight ATM-1 2024 Edition: 0.93 ohms at 20Hz, 0.69 ohms at 1kHz, and 0.71 ohms at 20kHz / 1.85 ohms at 20Hz, 1.28 ohms at 1kHz, and 1.3 ohms at 20kHz
• Octave Audio MRE 220 SE: 1.87 ohms in the bass and midrange, rising to 2.7 ohms at 20kHz / The output impedance in DF High mode was half that in DF Low mode
• Zesto Bia 200: 11.5 ohms at 20Hz, 13.6 ohms at 1kHz, and 11.3 ohms at 20kHz / 6 ohms at 20Hz and 1kHz, increasing slightly to 6.85 ohms at 20kHz / 3.2 ohms at 20Hz and 3.7 ohms at 1kHz and 20kHz
• Octave Jubilee Mono SE: 2 ohms (including the series resistance of 6' of speaker cable) at low and middle frequencies, rising to 4.5 ohms at the top of the audioband
• Doshi Audio Evolution: The output impedance, specified as 0.5 ohms, was 0.54 ohms at 20Hz and 1kHz, rising to 0.67 ohms at 20kHz.

The Stereophile measurements also includes the effect on the frequency response for a simulated dummy load.

 

[SIY]

Maybe inacurate but typical (and to my understanding inherent to the design of tube amps). Lets look at some Stereophile tube amp reviews:

• Air Tight ATM-1 2024 Edition: 0.93 ohms at 20Hz, 0.69 ohms at 1kHz, and 0.71 ohms at 20kHz / 1.85 ohms at 20Hz, 1.28 ohms at 1kHz, and 1.3 ohms at 20kHz
• Octave Audio MRE 220 SE: 1.87 ohms in the bass and midrange, rising to 2.7 ohms at 20kHz / The output impedance in DF High mode was half that in DF Low mode
• Zesto Bia 200: 11.5 ohms at 20Hz, 13.6 ohms at 1kHz, and 11.3 ohms at 20kHz / 6 ohms at 20Hz and 1kHz, increasing slightly to 6.85 ohms at 20kHz / 3.2 ohms at 20Hz and 3.7 ohms at 1kHz and 20kHz
• Octave Jubilee Mono SE: 2 ohms (including the series resistance of 6' of speaker cable) at low and middle frequencies, rising to 4.5 ohms at the top of the audioband
• Doshi Audio Evolution: The output impedance, specified as 0.5 ohms, was 0.54 ohms at 20Hz and 1kHz, rising to 0.67 ohms at 20kHz.

The Stereophile measurements also includes the effect on the frequency response for a simulated dummy load.

The high output Z is not because of the active device chosen, it's because of poor engineering endemic to contemporary fashion audio. A competent engineer can get the source impedance of a tube amp to 0.1 ohm without great difficulty. A really good engineer can do even better (see: Krohn-Hite, for example).

edit: And of course there's lots of solid state amps in the fashion audio segment with similarly high output impedances.

 

[SIY]

That is a surprisingly popular thing to do in some circles.

Of course, so is stuffing $300 worth of boutique parts into a $30 "valve preamp" using repurposed Soviet or Chinese HF pentodes at truly weird operating points -- and raving over the warm and velvety sonic character imparted to that harsh, sterile digital stream fed through said "preamp" into a harsh, sterile solid state amplifier.

My old friend Morgan Jones refers to these as "heavily designed, lightly engineered."

 

[Randolf]

The high output Z is not because of the active device chosen, it's because of poor engineering endemic to contemporary fashion audio. A competent engineer can get the source impedance of a tube amp to 0.1 ohm without great difficulty. A really good engineer can do even better (see: Krohn-Hite, for example).

edit: And of course there's lots of solid state amps in the fashion audio segment with similarly high output impedances.

Hmm, couldn't find any Krohn-Hite audio power amplifier. Anyway I checked ChatGPT's opinion on that with 2 questions just for curiosity:

1) Why do tube power amplifiers have high output impedance?
...
Conclusion:
In short, tube power amplifiers have high output impedance because of the intrinsic characteristics of tubes, the use of output transformers, a design that often avoids significant negative feedback, and the preference for a certain tonal signature that comes with higher impedance. These factors contribute to the unique sound that tube amplifiers are known for, which is valued by many for its warmth and character.

2) Are there any tube power amplifiers with low output impedance?
...
Examples of Tube Amplifiers with Low Output Impedance:
Audio Research Reference Series
These amplifiers often use a combination of push-pull tube designs and sophisticated negative feedback circuits to reduce output impedance. They are renowned for balancing tube warmth with improved control and precision.

Conrad-Johnson Premier Series
Conrad-Johnson is known for combining tube warmth with improved damping factors, offering tube amplifiers with relatively lower output impedance compared to traditional designs.

VTL Siegfried II
VTL (Vacuum Tube Logic) produces high-end tube amplifiers like the Siegfried II, which uses advanced circuitry, including feedback mechanisms, to lower output impedance and provide better speaker control.

Rogue Audio M-180
The Rogue Audio M-180 is a push-pull design that offers relatively low output impedance for a tube amplifier, offering a good balance between tube characteristics and solid-state control.

Conclusion:
While traditional tube amplifiers are known for their higher output impedance, some modern tube designs incorporate feedback, hybrid technology, and carefully engineered push-pull designs to achieve lower output impedance. These amplifiers aim to combine the best of both worlds: the characteristic warmth and harmonics of tube amplification with the damping control and stability offered by low output impedance. However, it's important to note that the unique qualities of tube amplifiers are often tied to their higher output impedance, and achieving low output impedance might result in a different sonic character.

Sounds reasonable to me. So I tried to verify the low output impedance claims:

Rogue Audio M-180: just 0.33 ohm at 1kHz from the 4 ohm tap in both ultralinear and triode modes, though this did rise to 0.57 ohm at 20Hz and 0.9 ohm at 20kHz. From the 8 ohm tap, the output impedance ranged from 0.57 ohm at 1kHz to 1 ohm at 20Hz and 1.2 ohms at 20kHz.

VTL Siegfried Series II Reference: The lowest impedance was in triode/MDF mode, at 0.88 ohm in the midband and 0.9 ohm at the extremes of the audioband.
...the output impedance rose a little with each reduction of feedback, reaching 1.36 ohms at 1kHz and 1.4 ohms at the frequency extremes
The output impedance in tetrode mode was only slightly higher than in triode mode, varying from 0.9 ohms at 1kHz/MDF to 1.53 ohms 1kHz/LDF

I still believe that equalization due to higher output impedance is one reason why tube amplifier may sound different. Others might be lower electrical damping due to higher output impedance / lower damping factor (probably less significant due to speaker cables and crossover between amplifier and driver setting limits on the benefit of high damping factor) and added harmonic distortion (perhaps the most important factor?).

 

[mhardy6647]

My old friend Morgan Jones refers to these as "heavily designed, lightly engineered."

The whole cheap distortion box phenomenon (which seems to have ebbed somewhat from a peak ca. 5 or 6 years a decade or so ago) befuddled me from day one.
The really remarkable befuddlement corollary (again, from my perspective) has been the race to the bottom in terms of the tube types employed in these gadgets -- and the ensuing aftermarket race to find boutique tube types (at least relatively speaking) to tube roll into them! oy vey.

Lookin' at you, Nobsound E6, in particular!

What do you guys think of the newly released Nobsound E6?

Its a super small directly heated 2J27S tube preamp. It uses a 5v dc plug but according to nobsound the anode voltage is stepped up to 105V. If its true then this thing could be an actual tube pre.

www.audiosciencereview.com

Note that a can opener (or equivalent) is necessary to remove the metal shields on these odd little 1.5 V filament, direct-heated (for battery use), deeply microphonic Russian Loktal pentodes. EDIT: In fairness, the E6 apparently ships with Chinese-made versions of the Russian originals.

PS I am chagrined to discover that I already used the phrase race to the bottom in the context of the tube choice for this particular little doorstop almost exactly two years ago in the above-referenced thread. Does that count as short term memory deficiency?

 

[SIY]

I still believe that equalization due to higher output impedance is one reason why tube amplifier may sound different. Others might be lower electrical damping due to higher output impedance / lower damping factor

They're the same thing. Damping factor is directly proportional to the reciprocal of source impedance.

Yes, a high impedance amplifier can show audible frequency response changes irrespective of whether its active devices are transistors or tubes. As you surmised, this is an effect achievable (if desired) with just EQ, or if you want to be more profligate, a series resistance. But again, just because contemporary fashion audio tube amps are usually NOT engineered to give a reasonably low source impedance (or, for that matter, engineered, period) does NOT mean that a tube amp necessarily has high source impedance, nor that this is a function of the active devices chosen.

 

[jxdking]

What's the best tube amp so far on the ranking chart?
Old Dynaco ST-70?

 

[GXAlan]

What's the best tube amp on the ranking chart?
Old Dynaco ST-70?

Yes, but my now-sold Sonic Frontiers SFS-80 is even better (but only tested with an E1DA Cosmos ADC).

Sonic Frontiers SFS-80 Review (Vintage Tube Amplifier)

Sonic Frontiers SFS-80 Tube Amplifier: A "new" benchmark to beat from 1992 Up next on testing gear that would be impossible to ship safely... the Sonic Frontiers SFS-80 tube amplifier. 70 lbs, all concentrated in the transformers. Believe it or not, Sonic Frontiers is a company we all know...

www.audiosciencereview.com

 

[jxdking]

I own a Dynaco st-70. It sounds just like my other solid state amplifiers.
The output impedance is less than 1 Ohm as I measured. Basically, the output amplitude won't budge much before and after you hook up 8 Ohm load.

My philosophy is, all good amps should sound similar, no matter tube or solid state. Bad amps sounds all different.

 

[SIY]

Yes, but my now-sold Sonic Frontiers SFS-80 is even better (but only tested with an E1DA Cosmos ADC).

Source impedance of those was about 0.2 ohm. IIRC, it was a Joe Curcio design.

 

[Randolf]

The effect is better to see when you match the curves in the middle of the spectrum (500 Hz?)

Ok, here we go: