Preamps for dummies that would be me

First, I have to disagree with Marakanetz about the output/input impedance ratios - a preamp output impedance to power amp input impedance ratio of 1:20 or higher will be sufficient. (For instance, if your preamp has a rather high output impedance maximum of around 1,000 ohms, try not to pair it with a power amp having an input impedance below about 20,000 ohms. As a practical matter, most preamps have an output impedance lower - and most power amps an input impedance higher - than in this example, so impedance mis-match between preamp and power amp is not usually a problem anyway). Also Marakanetz, what are you talking about with your references to preamps having to "restore" the "drastically compressed and feedbacked (sic)" output of CDP's? This makes no sense to me at all.

Randalle, the basic answer to your question is gain: active preamps offer additional gain, which might be needed to listen at the desired loud volume, depending on the source material, the source component's output level, the gain level offered by the power amp, the speaker sensitivity, and the size of the listening room.

Also, controlling volume with a passive device can sometimes become problematic due to impedance-matching issues between the resistive element used for volume attenuation and the impedance characteristics of source outputs and amp inputs, as well as those of longer cable runs. Active input and output buffering around the attenuation device, as in an active preamp, can eliminate these potential issues, which can make system matching easier by providing volume control that is relatively insensitive to surrounding conditions, and by relieving the source component from having to drive long cable runs.

Ideally, a preamp could provide gain only when it is required, but most active preamps have a fixed permanent gain factor, which is then attenuated as needed by the volume control - often (as you note) to levels below that of the source's own output level (AKA below 'unity' gain). Generally speaking, the less gain - and the less attenuation - applied to the source's signal, the more transparent the sound, so passive attenuators can have a theoretical transparency advantage for sources with sufficient output drive, as long as none of the above-mentioned impedance-matching difficulties arise. (Click on my Threads and go to the one about preamp bypass testing for more related to your question.)

For most people who are just starting out into audio think that preamps are not terribly important components. Their purpose is to regulate volume. How hard can that be???

Well, I will tell you it is a lot harder than you can probably imagine (at least doing it really well). There are quite a few good speaker, amps, and sources in the world... HOWEVER, there are far fewer good preamps.

A wise audiophile once told me: 'Any given music systems signal has the most chance of being screwed up by preamplification.'

Well it is passed my bedtime...

KF

Z.., Forgive me my horrible arithmetics, even 1:10 is good enough.

The so-far called "conditioning" of the sound might(or might not) apply to "restoring" the sound after feedback on the source components. Certainly I should've placed it in quotes since whatever is lost on the first place can never be restored and that's why many prefer to call it sound conditioning. Active preamps do not neccesarily have a gain. They can have a unity maximum gain as well or very low gain that is not able to "condition" or "restore"...

Thus it's not neccesarily that 20dB-gain preamp will sound better than 2dB.

Volume control and pre-amplification in general is one of the most vast parts in audio and it certainly equally conserns passive preamps as well.

I prefere to invest more into source components rather than spending a fortune on the true active preamp that niether colour, condition or restore the signal. That's why I use passive and probably never will go active.

Aside from the technical discussion...I'm VERY glad I chased a used Aleph P to drive my lowish-impedence Aleph 2 monos. Sometimes it's simply best to let the designer (Nelson Pass in this case) design the gain stage needed to drive the subsequent ones of his own pen!

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.