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I have heard a lot (!) of misconceptions and anecdotal explanations for why 100w of tube amp is way louder than 100 of solid state (SS) amp. Now, knowing that 100w is always 100w, why is a 100w tube amp indeed perceived as being a lot louder than a 100w SS amp? Using the same signal source and speakers, obviously.

Is it because of how they clip differently? Is it the different frequency response perhaps? Are they rated by the manufacturers under different conditions?

A scientific/technical explanation would welcome.

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  • \$\begingroup\$ I don't have any sources on this, but I once read that Tube Amps had more harmonics that are similar to the ones caused by sound being reflected by spatial features, walls, etc.. That might unconsciously make us think that the sound is "filling the environment" "better". Typically louder sounds have more characteristics caused by spatial features, so it might be correlated. \$\endgroup\$ – Wesley Lee Oct 23 '16 at 1:34
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    \$\begingroup\$ Because the tube amp goes to 11. \$\endgroup\$ – Olin Lathrop Oct 23 '16 at 12:05
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One specific datapoint : some years ago I made some measurements on a 12 watt valve amplifier (Leak TL/12+) it met its rated distortion specification (0.1%) at 12W rms (at 1kHz).

However, by the time distortion reached 1% it was generating 20W rms, while most transistor amplifiers would have started clipping immediately beyond its rated power.

I stopped there and didn't find out where it reached hard clipping or even 10% thd.

(At high levels, very little of that distortion was 2nd harmonic, thanks to the push-pull output stage)

As for why : with transistors, it is easy to add gain and thus employ more negative feedback, and the emitter followers typically used in the output stage are relatively linear; so it is easy to maintain linearity right up to the point where the output hits the supply rails.

With a valve amplifier, additional gain stages are expensive, so you have to meet your linearity target with relatively little negative feedback. That means designing the hardest bit - the output stage - to be reasonably linear across as much of its range as possible, and restricting the rated power to that range. There are tricks to increase that linear range somewhat, such as connecting the screen grid to an intermediate tap on the output transformer, somewhere between V+ (pentode connection) and anode (triode connection) - tap at 43% for the so-called "ultra-linear" connection.

Beyond that range it will still amplify, and produce additional power, but no longer meet its distortion specification, up to some limit where real clipping will occur. Triode-connected gives the smallest linear range (therefore the lowest rated power) but, by the same token, offers the largest reserve of power in overload.

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Perceived by who? There are a lot of misconceptions with regards to audio equipment. There are people who believe that the following affects the audio perception: type of materials in the AC power connector, grade of the copper in the speaker connecting cables, number and type of connectors, etc. There are also people who believe that vinyl records sound better than CDs. There is no engineering basis for these opinions but that doesn't stop the claims that you see on the Internet. There are 10 watt tube amplifiers on sale for more than $10,000. Are they better than a 10 watt solid state amplifier? Not by any engineering standards. However, it is difficult to argue perception. A 100 watt audio amplifier, either tubes or solid state, will produce exactly the same output when driven by a 1 kHz sinewave into the same speaker. Under these conditions, the loudness will be the same. However, it has been argued that tube amplifiers tend to clip less sharply then solid state amplifiers. Thus, an overdriven tube amplifier might sound better, not necessarily louder, than a solid state amplifier due to lower amplitude harmonics. But that is not normal operation. The frequency response of either amplifier can vary based on the design but generally solid state amplifiers have flatter responses. If the tube amplifier has an output transformer, that will affect the range and flatness of the frequency response which will alter the sound somewhat depending on the range of the input signal. Again, though, the difference in loudness of the amplifiers should not be significant. You can't give a scientific explanation for any of these effects because they are dependent on the individual listener. I don't believe there are any engineering tests that would show any loudness difference between 2 amplifiers rated for the same power that are not overdriven and are connected to identical loudspeakers.

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    \$\begingroup\$ Spın̈al Tap: When you've put your amplifier to ten, you ask yourself: what comes beyond? \$\endgroup\$ – Janka Oct 23 '16 at 0:46
  • \$\begingroup\$ @Barry I agree with what you have writen. The reason for why I posted this question is because as an engineer I cannot understand or find a good enough justification for what my ears hear. I have tried several amps, and there seems to be a trend :\ Tube amps, to everyone that I have put to the test, have a perception of increased loudness when using a tube amp. I cant explain it... \$\endgroup\$ – AmiguelS Oct 23 '16 at 0:58
  • \$\begingroup\$ @AmiguelS Perhaps the amplification factor is simply higher on the tubes? You say "100W", but turning the volume to "8" on different amplifiers might be a different point on the 0.0-1.0 scale of amplification. \$\endgroup\$ – chrylis Oct 23 '16 at 8:21
  • \$\begingroup\$ I have performed this test with 15W amps, at full volume. \$\endgroup\$ – AmiguelS Oct 23 '16 at 11:16
  • \$\begingroup\$ Tube amps have a soft limit defined at some distortion level, beyond which there is more power. Semiconductor amps have harder limits beyond which there is very little power. Often it's a distortion limited rating due to the magnetics. \$\endgroup\$ – Sunnyskyguy EE75 Oct 1 '17 at 20:51
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I believe the perception comes mostly from players of musical instruments, and less from audiophiles. That's why there is a wide range of opinions. Engineers apparently are too condescending to suggest an objective measure for "musical instrument loudness". Sine waves and part-per-million distortions are not the right metrics for this application. The engineering answer, however, is plain and obvious.

Tube amplifiers provide smoother signal clipping, which produces less level of higher harmonics of musical tones. These harmonics are the ones that hurt musician ears. Therefore, with tube amplifier, a player can hit strings or claviers with higher amplitude without feeling the sharp discomfort from higher overtones, and the main tone amplitude gets quite higher producing noticeably louder RMS sound.

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Amplifiers are usually specified for distortion at around maximum power. Low order non-linear distortion increases with increasing output power, so max power is the 'worst case' place to measure it. Both tube amps and solid state (SS) amps suffer from this, though tube amps have rather more. A modest amount of low order harmonic distortion can sound relatively pleasant and 'warm', often adding something 'extra' to the music, so the high distortion from tube amps is not generally perceived as a vice.

When they go into overload, SS amplifiers 'hit the rails', whereas tube amps 'go a bit further round the corner'. Needless to say, the former sounds nasty, the latter can often be forgiven. This means that for equivalently rated tube and SS amps, the tube amp is more tolerant of mis-set levels.

Push-pull class B output stages on SS amplifiers have another distortion mechanism, cross-over distortion. This type of distortion tends to be independent of output power, so in ratio terms is worst when the output signal is small, just where you'd expect the amplifier to be performing best. This type of distortion tends to be high order, so is un-musical , harsh, 'wearing' and generally not liked.

Unfortunately, many of the first SS amplifiers to be made commercially available were very bad for crossover. A quite passable distortion level at full power of 0.1% on a 100W amplifier could become 3% when outputting 100mW. Although rock tends not to have these dynamic ranges, classical music does, and musicians hated the violence that was done to delicate passages on a few instruments.

Designers rapidly improved the design of output stages, improved the stability of biassing with respect to temperature, and got crossover distortion under control at all output levels, but by then, SS amplifiers had already got a very bad press. This was made worse by a) there were already a large number of amplifiers out there and b) bad ones were still being built and sold as the design knowledge did not pass quickly from the good to the bad manufacturers.

As music is such a subjective personal experience, it's difficult to correlate enjoyment with audio fidelity. With large sums of money to be made catering for the luxury end of the audio market, the less scrupulous manufacturers have little incentive to disabuse the buyers. It's well known and demonstrated in tests that music sounds better when there's a shiny tube amp glowing on the counter, than when there's a non-descript black box there, whatever is producing the sounds.

A personal anecdote on the musical effect of low order harmonics, which I'll try to keep brief. When I first started my music collection as a student in the mid '70s, I had little money, so first bought a decent deck and cartridge, so as not to damage my record collection, and listened with emitter followers and headphones. Next year, I built the Practical Electronics 'Gemini' 50W stereo amplifier, but initially could only afford a couple of tiny 3W car speakers from a jumble sale. Nevertheless, Annie Austere from Fruupp's Prince of Heaven's Eyes sounded glorious, huge swathes of big sound, a chorus of ... you get the idea. Listening again, the effect is rather like much of Quadrophenia!

Two terms later, I finally built my KEFkit IIIs, in 120 litre cabs. Now I was going to listen to that track the way it should be heard. It sounded OK, so I turned it up. It still sounded OK. I turned it up some more. Eventually, the windows were rattling, my ears were hurting, the neighbours were complaining, and it still sounded OK. The technicolor swathes of glorious sound had been the distortion from my way under-powered speakers. Sigh!

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It depends on the kind of watt. Peak ? RMS ? in what load ? I remember seing an amplifier advertised for 200W because it was stereo 2×100W in 4 ohm and the seller did not even know if these were watt RMS (which I doubted). So this 200W amplifier was capable to output a reality of 25 watts RMS per channel in 8 ohms.

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The best amplifier is a solid state amplifier that does not distort. But tube amps fail with soft limiting and 2nd harmonic distortion from the gain asymmetry and soft saturation so can produce more pleasing distorted sounds when over driven.

This does not make them better but makes them push more power at 10% distortion that is less noticeable with 2nd harmonic bass rather than higher odd harmonics from SS clipping. 2nd harmonics are due only to asymmetric tube gain for class AB output state.

The best professional gear is the one you can afford that does not distort.

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Transmission of sound is non-linear. It is assumed to be linear over a short range, but really it isn't. The dependancy on pressure of the speed of sound makes sine waves tend toward saw tooth waves (peaks travel faster and troughs travel slower) which is odd order harmonics. Valve amps generally give odd order harmonics and so can reasonably be perceived as a louder sound but further away psycoacoustically.

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