Preamps for dummies that would be me


This is one of those “why is the sky blue” questions that I feel like an idiot for asking, but here goes.....

Other than switching inputs and controlling volume, what exactly does an active preamp do? If that were all to it, we'd all be using passive preamps. I've tooled around the web looking for articles, but I'm not really hitting anything. I've seen some veiled references about 'conditioning the sound' for the amp. Whatever that means.

So what, other than a fixed and usually too high output level, is coming out of the CD player(or tuner or whatever) that the input of the amp does not want to see. Thanks in advance for not slamming my ignorance.
randalle

Showing 1 response by seandtaylor99

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.