22/10/2021-13:11:34
RE: Mesures - Amplificateur triode PX4 VS p2500s
Bonjour,
Voici les conclusions tirées par Menno Van der Veen à l'issue d'une expérimentation détaillée sur un SE 300B.
Peut-être seront-elles des réponses aux nombreuses questions ouvertes de la discussion ou peut-être ouvriront-elles de nouvelles pistes.
Citation de l'article de Menno:
CONCLUSIONS
Based on my long experience with push-pull valve amps, plus the findings of this SE research, I would like to draw some conclusions.
1) If your goal is to hear as much details as possible, then the THD of the amp should be lowest as possible. You can apply massive negative feedback (Bruno Putzeys route), or local feedback (as in Trans-amp-4) or pre-distortion compensation (as in amp 5 with S1 = closed). My personal experience with push pull amps is: do not apply moderate negative feedback from the secondary winding of the OPT. The character of the magnetic distortions in the OPT-core, combined with NFB, creates a fog and a dead sounding amp.
2) If you wish the music to embrace you, then THD may be present, especially the second harmonic, but details get lost. Also see conclusion 7).
3) Coupling capacitor brand and construction are of the utmost importance to hear the embracing and details in a non-feedback design. Exactly the same can be said about the quality of resistors and OPT-core material. In this research I did not proof it by own measurements. In the literature there is abundant highest quality proof available (see for instance 16) and 17)) to show low level defects, caused by components. The ear can detect them.
4) Combining 1) and 2) and 3): designing a fine amp (“fine” is a subjective property) is an art.
5) The general understanding is that (in SE-amplifiers) local feedback or pre-distortion weakens the second harmonic and hardly the third. This research has proven that the third can be attenuated as well, although not as much as the second. Baxandall’s research (15) shows the harmonics above 2 to rise in level with the amount of feedback. However, that was measured on a jfet. In tube circuitry apparently things are a little different. It is not the first time that I notice the third and higher harmonics to decline directly with the amount of negative feedback.
6) There is a general understanding that the harmonics in a fine amp should decline regularly with their number. They all should stay under the masking curve of a continues tone. My research does not support that rule. Lowering the second harmonic (as in amps 4 and 5) below the level of the third improves the hearing of details without effecting the embracing character of the sound. I surely agree that higher harmonics (above the third) should decline regularly. I consider the second harmonic as a “curtain” creator.
7) More and more I give my amplifiers a damping factor below 10. Higher values by means of current feedback or overall negative feedback, tend to give the amp a “dead” character. Then the sound stays close to the speakers and does not vibrate everywhere in the listening room.
8) Testing your amp at 1 Watt level and below is a sensible thing to do. But please don’t forget to measure the power bandwidth up to the highest frequencies to detect presence of slew rate limiting induced distortion. Full power bandwidth up to 20 kHz stays mandatory.
Pour mémoire, voici la référence exacte de l'article complet:
https://www.mennovanderveen.nl/cms/image...-Menno.pdf
Bonne lecture, Jacques
Voici les conclusions tirées par Menno Van der Veen à l'issue d'une expérimentation détaillée sur un SE 300B.
Peut-être seront-elles des réponses aux nombreuses questions ouvertes de la discussion ou peut-être ouvriront-elles de nouvelles pistes.
Citation de l'article de Menno:
CONCLUSIONS
Based on my long experience with push-pull valve amps, plus the findings of this SE research, I would like to draw some conclusions.
1) If your goal is to hear as much details as possible, then the THD of the amp should be lowest as possible. You can apply massive negative feedback (Bruno Putzeys route), or local feedback (as in Trans-amp-4) or pre-distortion compensation (as in amp 5 with S1 = closed). My personal experience with push pull amps is: do not apply moderate negative feedback from the secondary winding of the OPT. The character of the magnetic distortions in the OPT-core, combined with NFB, creates a fog and a dead sounding amp.
2) If you wish the music to embrace you, then THD may be present, especially the second harmonic, but details get lost. Also see conclusion 7).
3) Coupling capacitor brand and construction are of the utmost importance to hear the embracing and details in a non-feedback design. Exactly the same can be said about the quality of resistors and OPT-core material. In this research I did not proof it by own measurements. In the literature there is abundant highest quality proof available (see for instance 16) and 17)) to show low level defects, caused by components. The ear can detect them.
4) Combining 1) and 2) and 3): designing a fine amp (“fine” is a subjective property) is an art.
5) The general understanding is that (in SE-amplifiers) local feedback or pre-distortion weakens the second harmonic and hardly the third. This research has proven that the third can be attenuated as well, although not as much as the second. Baxandall’s research (15) shows the harmonics above 2 to rise in level with the amount of feedback. However, that was measured on a jfet. In tube circuitry apparently things are a little different. It is not the first time that I notice the third and higher harmonics to decline directly with the amount of negative feedback.
6) There is a general understanding that the harmonics in a fine amp should decline regularly with their number. They all should stay under the masking curve of a continues tone. My research does not support that rule. Lowering the second harmonic (as in amps 4 and 5) below the level of the third improves the hearing of details without effecting the embracing character of the sound. I surely agree that higher harmonics (above the third) should decline regularly. I consider the second harmonic as a “curtain” creator.
7) More and more I give my amplifiers a damping factor below 10. Higher values by means of current feedback or overall negative feedback, tend to give the amp a “dead” character. Then the sound stays close to the speakers and does not vibrate everywhere in the listening room.
8) Testing your amp at 1 Watt level and below is a sensible thing to do. But please don’t forget to measure the power bandwidth up to the highest frequencies to detect presence of slew rate limiting induced distortion. Full power bandwidth up to 20 kHz stays mandatory.
Pour mémoire, voici la référence exacte de l'article complet:
https://www.mennovanderveen.nl/cms/image...-Menno.pdf
Bonne lecture, Jacques