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.
In fact, it's all about the system's integrity.

Yes, basically the active preamp is designed not to only bring the signal to the line level but to restore drastically compressed and feedbacked output of the cheap CD-player with bi-polar output stage for example and bring it to the amp that also has quite large amount of feedback.
A good active preamp is usually built with small amount of feedback and can be a great interface between the components. The output impedance of preamp and input impedance of the poweramp have to match approximately 1:1000
Most-likely all active pre-s have a low output impedance that can match well with low input impedance of the SS amps. The low input impedance of the amplifiers is caused by deep negative feedback that highly limits the input sencitivity.

No, no active preamp shall be designed to 'condition' the sound but in fact they do despite being designed not to.

Yes, if you have invested into the high quality source component CD-player or analogue; if you have low neg. feedback on your poweramp, high input sencitivity all you need is just input selector and volume control i.e. passive preamplifier.
Just of the tech experience I can add the following:

Any preamp that is using variable electronic or op-amp feedback with bi-polar high-gain output stage will certainly deplete and compress the sound drastically. The sound change is very similar to turning-on Dolby NR system to the cassette or any tape as you plug in such pre- between your components v.s. no preamp.
The same amp with regular R/C feedback tend to colour the sound and make it extra-bright due to high instability of the bi-polar elements.

So choose either tubes MOSFET or JFET preamp that most-likely have low neg. feedback, linear and does only minimal 'conditionings' to the sound.
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...

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.
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.
OK, let's try again...

About what Gs5556 said:
CDP's and DAC's have steep HF filters kicking in at around 20KHz to reconstruct the analog signal and keep HF digital artifacts at bay. In contrast, any competent active preamp will have its upper -3dB point (and often even the -1dB point) well out above the audioband. Given that truth, what do you propose to be the magic mechanism in active preamps that is supposed to "launder" your allegedly dirty signal coming from the CD source and make it "clean" for the power amp? As I and many others know from experience, the direct, unattenuated and unpreamped output of a CD source cannot not harm your amp, speakers, or ears on any basis other than possibly that of sheer volume.

You then posit a contradiction, by suggesting (correctly, if not comprehensively) that a passive attenuator can interact with the source's output impedance, the amp's input impedance, and the cables' capacitance, to cause mis-matching and insertion losses which can combine to roll off response prematurely at the LF and/or HF ends of the audioband, sapping signal fidelity and drive vs. an active preamp - but then put forth in the same breath your contention that a passive attenuator will pristinely pass along all of your alleged digital HF garbage to the amp unhindered vs. an active preamp, which you suggest will somehow filter this alleged spuriae that you claim is part of the CD output signal.

Well, which is it? - it can't be both. In any given situation, a passive attenuator will either preserve the fidelity of the input signal or reduce it vs. an active preamp, and the same thing applies the other way 'round; you can't simultaneously but selectively argue that passive attenuators will *both* preserve and destroy fidelity (as well as making the same argument in reverse for active pre's), depending on what's conducive to bolstering your position in the debate. As for your claim that you "personally cannot stand to listen" to a CD source not run through an active preamp, I have to assume this rhetoric is based as much on what you (erroneously) think about the output of CDP's and the action of active pre's, as it is on what you actually hear. (Which isn't to say that in any particular instance, one might not prefer the sound of their CD source run through an active preamp [especially as compared to its sound run through some other sort of attenuator, or volume-controlled in the digital domain], but that it won't be for the 'reason' Gs5556 alleges.)
If a passive preamp did the job better than my Ayre for the same price or less (and had phono as magical as the Ayre), I'd buy it in a heartbeat. My main goal iis end product sound quality and not fancy smancy equipment (unfortunately, I have found that if you really want to get world class sound.... well, you need to dish out at least national class dolars...) Eeeking that last %10 of heavenly sonic quality from your speakers tends to be pricey at best.

I am the first to tell folks that if I could get the same sound out of coat hangers used as speaker cables as I could my Nordost SMP speaker cables, I'd be the first to raid all the closets in the house. (I remember an audio designer telling me that monoblocks and coat hanger could sound very good. I asked him to demonstrate, but he declined.) LOL.