The function of a preamp is to increase the gain of the source component in order for the power amplifier to gain it up to a level where it can drive the speakers. Preamps were once absolutely necessary when the source was LP's. It was, and still is, impractical to build a one-box amplifier that takes the output of a cartridge to drive a transducer; the interference from the magnetic fields of the massive power supply transformers would kill the delicate source signal.
Today, with digital equipment and dedicated phono preamps/transformers providing enough signal push to drive amplifiers, the preamp would seem to be an anachronism. Just use a passive attenuator, it removes an amplification stage which we all *know* causes signal degradation. But rarely in audio, or anywhere else, do one gets a free lunch.
To expand upon Marakanetz's example with digital: the output of a DAC is nowhere near a perfect wave form. There are extraneous high frequency components which, unhindered by distortion, transmit a lot of HF energy at the DAC output. These can and do cause IM distortion, phase instability, amp overload/clipping and can cause damage to tweeters. In other words, there's a lot of garbage riding along the output and the quality of the DAC's filtering topology will determine what gets out of the box. If you use a passive attenuator, all of what's unfiltered gets through to your amp and ultimately to your ears. That's why, I think, I cannot personally stand to listen to digital through a passive or a CDP.
An active preamp will launder the digital input and provide a clean signal source with a low output impedance; an important parameter because the capacitance of the interconnect cables are easily overcome and, also, the voltage signal to the amplifier is preserved, making it easier for the amp to work. A passive device has a high output resistance, which decimates the already low source signal voltage level to the amp and forces the source component to fight off the IC capacitance that can make everything sound worse when all's said and done. And that's an example of one of the many trade-offs in audio
But all that said, I don't use a passive because there are none I've heard that sound better than my CAT.
Today, with digital equipment and dedicated phono preamps/transformers providing enough signal push to drive amplifiers, the preamp would seem to be an anachronism. Just use a passive attenuator, it removes an amplification stage which we all *know* causes signal degradation. But rarely in audio, or anywhere else, do one gets a free lunch.
To expand upon Marakanetz's example with digital: the output of a DAC is nowhere near a perfect wave form. There are extraneous high frequency components which, unhindered by distortion, transmit a lot of HF energy at the DAC output. These can and do cause IM distortion, phase instability, amp overload/clipping and can cause damage to tweeters. In other words, there's a lot of garbage riding along the output and the quality of the DAC's filtering topology will determine what gets out of the box. If you use a passive attenuator, all of what's unfiltered gets through to your amp and ultimately to your ears. That's why, I think, I cannot personally stand to listen to digital through a passive or a CDP.
An active preamp will launder the digital input and provide a clean signal source with a low output impedance; an important parameter because the capacitance of the interconnect cables are easily overcome and, also, the voltage signal to the amplifier is preserved, making it easier for the amp to work. A passive device has a high output resistance, which decimates the already low source signal voltage level to the amp and forces the source component to fight off the IC capacitance that can make everything sound worse when all's said and done. And that's an example of one of the many trade-offs in audio
But all that said, I don't use a passive because there are none I've heard that sound better than my CAT.