Why do amps sound different?

Dazzdax,

That is still just one group of people's opinions. Transistor amps have always "measured better" than tube amps, as defined by standard bench tests devised by engineers. That is not sufficient to prove they are better, it's a belief that it proves it does.

Now this swiss amp has heavy competition from amps that measure much worse than perfect, for the title of best amp. It might well be in the top 2%, but one of the amps given title of best amp include Lamm's medium power zero NFB high distortion 6C33C low damping factor triode SET. How could that happen? Because it's still subjective.

I happen to own a battery powered Class T (form of Class D) amp from Red Wine Audio. I used to be devoted to tube amps, but this is better in many ways for a moderate cost amp driving medium sensitive speakers. It's as modern a design as there is for amps, and measures well and sounds excellent. It has less personality than tube amps, yet more transparency than most transistor class A or AB amps and most tube amps as well.

Since there is no perfect source and speakers, why should it be "correct" to own a perfect measuring SS amplifier? What if it processes the signal a little that offers the illusion of richer instrument harmonics that got bleached out in the recording process? This could be considered better, not defective.

Until source and speakers become perfect, I will never be pursuing the perfect "straight wire with gain" electronics to go with them. I will search the best complementary electronics to go with them, within my budget. A lot of that comes from my custom built tube preamp where the parts and circuitry were all selected for overall sound quality, not bench measurement perfection.

So where is the value in bench measurements for amps? Simple: factory quality control. An automated way to see if the completed product is within expected tolerances to ensure uniformity of them going out the door without the more expensive and impractical method of listening tests for all.

Kurt

Shadorne,

The levels of distortion that can ruin an amp can be so small that measurement is impractical, especially for a guy without very expensive modern test equipment. And like I was trying to say, isolating the distortions in special cases are nearly impossible. In wine tasting, it's trivial to add 5 ppb of the diluted chemical that makes it taste corked. There are many chemicals to worry about in wine, and the levels that are needed to ruin the wine are all different. The chemicals can be isolated and added individually to find out this exact level needed to stay under to be safe. In audio, we don't have those numbers. We can't isolate the contaminants without changing something else with it. All distortion claims are scientifically invalid as a result.

For example, back in the early 1960's it was declared that THD levels under 2% were inaudible. Then in the 1990's it was stated that it better be under 0.1%. The difference was the different content of the average amp's harmonic spectra, between the 1960's tube amps and the 1990's transistor amps. And that was just the tip of the iceberg.

I can hear the difference in resistors in an amp. What are the distortion levels caused by resistors? Almost nil, unmeasureable except to the best equipment available, like down -120 dB. That's about 0.0001% distortion. But its a different kind of distortion. A lot of it is some HF ringing from spirally wound (inductive or possibly capacitive) laser trimmed resistors, some also from magnetic nickel or steel construction in it with hysteresis distortion.

Capacitors have more impact, especially coupling caps. The different dielectrics produce different levels and different types of distortion: dielectric saturation that bends the linearity of the charge/discharge cycle, different dielectric absorption distortions under dynamic time domain conditions, dielectric hysteresis distortion, and frequency dependent ESR and ESL shifts.

The greatest distortion generator in tube amps are the magnetics of interstage and output transformers. The main one being the large saturation and hysteresis distortion. Then there's the imperfections down in the microscopic level at the magnetic domains. Some domains don't respond well to small signals and low level detail might be obscured at low volume levels. Nickel is better than silicon steel for the low signals and should be used for anything before the output transformer.

Those kinds of distortions often don't really show up well in repetitive waveform measurements. And if they do, they just ride on top of a bigger and more recognized distortion, or it looks like it's all from one known source.

Again, isolating the audible small distortions that ruin the sound is a near impossibility. But some distortions are small and very annoying in limiting performance, one of the worst offenders being the distortions of different capacitor dielectric material. Yet for high level distortions that are seemingly more benign are the magnetic transformer distortions.

In transistor amps, the worst offenders are the transistors themselves IMO. Lots of high order distortion that need plenty of NFB to try to get rid of. And the typical vertical MOSFET has huge modulating input capacitance loading, which has shown to be a big negative to the sound. Luckily we now have lateral MOSFETs that go a long way to solving that problem, somewhat more expensive and hard to find, but are featured heavily in Ayre amps.

And now to the most controversial topic: wire and connectors. Does it have a distortion? If so, can I prove it? The answer is yes. The cell phone companies found the problem for the first time and measured it for the first time with the most expensive test setups. It turns out that transmitter/receiver stations for cell phones have to have remarkable low distortion in the RF cabling and connectors in order to work. One channel might be transmitting 100 watts out while the adjacent channel is receiving only 10 microwatts on the same cable. That's a problem if there's IMD on the cable, just about any IMD.

So they set out to measure the distortion of the cables since it appeared it was not good enough. They were right. The distortion measurement is called Third Order Intercept, where the third order harmonic would reach the fundamental at a theoretical output. It was discovered they needed +130 dBm TOI to get the job done and they saw it wasn't reaching it. To fix it, they re-designed the connectors by silver plating them. Then the distortion of the cabling systems went down to an acceptable level. You can buy silver plated RF connectors now for this problem.

Anyway, it seems some people can hear connector and cabling distortions well. And there's some evidence to back up their claims.

Kurt

Although alluded to before, it seems that the amount of negative feedback is a big issue.

2 amplifiers can easily have the same bandwidth, but one running with feedback and one without. The one without will likely sound more relaxed, since it lacks the global feedback which enhances odd-ordered harmonics which in turn behave as loudness cues. We are not talking a lot, like Kurt says- hundredths of a percent is all it takes to make the difference.

A further complication is the idea of 'constant voltage' output, which is the same as doubling the output power as the load impedance is cut in half. Some speakers are designed to expect this (B&W 802). Other amplifiers are designed with the idea of "constant power" in mind- that is that power does not change regardless of the load (tube amps are good examples of this). Such amplifiers, sometimes referred to as 'current source' amplifiers, have a higher output impedance and an entirely different class of speakers exist to accommodate them (Sound Labs and horns for example).

In fact two paradigms of design and measurement exist in audio today:

http://www.atma-sphere.com/papers/paradigm_paper2.html

This is about more than just simply matching components, but that is what you have to do. Normally one paradigm will take over in a field of endeavor but that did not happen in audio because the 'prior art' (tubes) did not go away like they were supposed to- too many people like them.

Another interesting thread with great contributions by everyone.

Is someone able to explain the difference between "current paradigm" and "voltage paradigm" amplifiers?

Amfibius, take a look:

http://www.atma-sphere.com/papers/paradigm_paper2.html

'Current source' is a Voltage paradigm term for amplifiers that have a high enough output impedance such that they exhibit (or at least try to exhibit), constant power with respect to load. Usually this refers to a tube amplifier, but not universally.

"Voltage source' is again a Voltage paradigm term for an amplifier that can express a constant voltage regardless of load. Another way to put this is that power is doubled as the load impedance is halved (or halved as the load impedance doubles). Usually this refers to a transistor amplifier, but again, not universally- there are always exceptions because this is not about tube/solid state.