» 29 Avr 2010 0:48
Ca dépend ce qu'on entend par "passif"
Celui-ci n'apporte pas de gain par rapport au signal d'origine, seulement de l'atténuation c'est dans ce sens qu'il est "passif" si j'ai bien compris.
Preamp Section
Today’s digital sources usually have enough output voltage (~2V) to directly drive most power amps and active speakers. The most important function of the preamp is to provide volume control, instead of voltage amplification. In this case, isn’t passive attenuation the best solution? After all, passive components provide the purest signal path.
However, passive volume control is not as simple as a volume pot. To ensure compatibility with other components, the preamp should maintain a high input impedance and a low output impedance. However, passive preamps based on variable resistors or stepped attenuators will have variable impedance at different knob positions, resulting in compromised compatibility, dynamics and bass. A better solution is to use many fixed-value resistors to form a network, and each volume setting should have its unique signal path involving specific resistor combinations to maintain desirable impedance values.
Central Station has a combination of 34 relays (NEC EE2-12NUX, $200 for 100 pieces) and a network of military-grade, metal-film resistors to help define the specific signal path at each volume setting. If I understand correctly, as the volume knob turns, a DSP chip (Wavefront AL3101) and a programmable microcontroller chip (Atmel ATMEGA8515L) compute and choose the suitable resistor combination by regulating the relays. It is complicated inside but truly simple outside. When the user turns the volume pot (not stepped), the volume changes smoothly, without any hint of the ultrafast computation and switching events occurring inside. Central Station has been reviewed by many pro-audio magazines, and none of the reviewers reported any trace of distortion or coloration from its passive volume control. I used Central Station in a wide variety of connection schemes, and there was never a sign of weak dynamics or bass, just utter transparency. For example, my Benchmark DAC1 can operate in both fixed and variable (preamp) modes, but I now prefer operating DAC1 in fixed mode and routing to Central Station for volume control. I can’t say if DAC1’s built-in volume control degrades the signal in any way, but it alters things a little. On the other hand, Central Station’s volume control does not add or subtract anything. People like to say that the best preamp has no sound of its own, which is easier said than done. Here is a totally transparent preamp for only $500, can it get any better than this? How about throwing in a good DAC and two fine headphone amps?
Moi j'en comprends que le rôle d'un préamp* est bien respecté tout en évitant les problème de coloration des actifs et d'adaptation d'impédance des passifs "cheap"
*Dans le sens: Un potard atténuateur agissant sur plusieurs sorties et disposant de plusieurs entrées.