Preamps for dummies that would be me

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.

Gs..., I wouldn't realy think of any other than CAT if I had a budget for it:-)

There sure is some silly stuff being flung around here. I had just about completed writing a detailed rebuttal to it all, when my 'puter froze up and I lost everything. Right now I don't have the heart to start over again, so I'll just say for the time being: Randalle, keep a large grain of salt handy.

Theoretically, a wire should pass only signal, and a pre should pass only signal, and the original mike should pass only the original signal (of voice), and the mixing board should pass only signal, blah, blah. But the fact is, the empiric fact backed by experience - as in a concensus of the relevant peer group, which is what determines what evidence will be considered valid in a scientific discussion - is that active preamps make a large difference in sound quality in terms of musical involvement, and increasingly so as one's system increases in the ability to so catalyze that listening experience.

Do I know why? No.

Do I think its healthy to ask why? Certainly.

But the fact is that the consensus among the best system builders is that an active pre is essential.

Will it always be that way? Maybe not, but it is now.

That's why someone can talk about how passives should sound better theoretically but in practice they have a CAT.

Every piece of matter between the voice and your ears "launders" the sound. Right now, the active pre translates the musical meaning to a greater degree than the passive in the best systems.

Stereophile via Steve Stone tried to push passives several years ago when CD got going, but it failed and no reviewer I know of with a great system goes passive now. There's a reason for that and it has nothing to do with the rigors of scientific theoretics or the functional requirements of being a reviewer and swapping equipment.

If you don't have the scratch for a good tubed pre and run only a digital source, then many times a passive is the way to go. But that doesn't mean its THE way to go...

Without wishing to get too technical (because I'd probably demonstrate my ignorance) I believe that the biggest problem with a passive preamp is that of impedance matching. Passive preamps can offer a lower input impedance than some sources are able to drive. If this occurs then the sound is robbed of dynamics.

This problem is system dependant, because it depends on the output impedance of the source and the input impedance of the power amp. In a system with a very low output impedance on the source and a very high input impedance on the power amps a passive preamp might work well. There's no denying that a passive pre is transparent and cost effective in this situation.

If your source has a relatively high output impedance and your power amp has a lower input impedance then using a passive pre might not work since the output stage of the source will not be able to drive enough current into the low input impedance of the preamp. An active preamp will be able to offer a better match of the impedances in this situation, and if it's a decent preamp then it will also be pretty transparent, and will not be coloring or conditioning the sound at all. It's a hell of a lot cheaper to build a quality amp at preamp current and voltage levels than at power amp levels, so there's really no reason an active pre should be any less transparent than your power amp.

So in summary there is no blanket rule that active is better than passive or vice-versa. I believe that impedance mismatches are the core issue in system matching, and the reason that you can't apply simple rules and then buy components without first demoing them.