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
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