What does a pre amp do

I have been trying to get an answer to this question, but so far have not been enlightened

In a modern system, the role of a pre amp is really questionable.

Historically, a pre amp would provide quite a number of real functions such as source selection, impedance matching, maintain volume consistency from different sources, volume adjustment, mute, balance, even tone control, high/low filters

Today, most people have a single CD source with XLR interconnection, which standardized impedance and signal level. Some CD and DAC (such as wadia) has digital volume control. In fact wadia does recommend people to connect directly to their amps

At least in thoery, the best preamp is no preamp. Even the preamp manufacturers agree as evident by the extent they go to separating power supply, minimize signal path. Then there is the passive preamp

So here is the question, is preamp a historical appendix that audiophiles are afraid to get rid of due to the fear of missing out on some unknown elements of music. Or is it really a percived need created by good marketing programs to exploit audiophiles

The obvious answer may be to try it out with and without preamp in the same system. The results I have heard so far are controversial, some claims much better transparency and clarity while others claim there is a lack of dynamics and less musical

I do not believe answers from "experts" who happens to be in a sales position. They always give a very affirmative answer and yet refuse to let you do an A/B comparision.

Most modern preamps are an empty box, and do we need to pay high prices for a piece of equipment with no known function
I believe that preamps have a very important function. They provide control functions, source and output switching, and pre-amplifying of any source items which may be lower than necessary for the needs of the amp input.

As for analog volume controls in preamps, most are not that great, and could be improved, but there are some really good ones. On the subject of digital volume controls, the general consensus is that they are no good, due to "throwing bits on the floor". There are many articles regarding the subject of digital volume controls, and I haven't seen many favorable comments about them.

I also wonder about just what is the definition of a "modern system"? Does that mean CD only? What about analog turntables, and maybe FM tuners? Are these not part of a "modern system"? And that doesn't even consider any HT functions.

There have been a number of threads posted here about the direct connection vs thru a preamp. Many answers depended upon the type of gear being used, and the number of sources that the user wants.

Some people prefer to use a preamp and some prefer to go direct. The results may be better either way, depending upon gear used and user preference. However, I think that it is quite erroneous to consider preamps "a piece of gear with no known function".
Everyone needs some sort of volume control. Just considering a CDP as the only source you have several options, depending on your system. I'm sure, though, that there are better outboard volume controls and preamps than what are included in probably all CDPs. If they are worth the extra is for whoever to decide. Using my softest CD, I skipped my preamp, and the sound was quite the mixed bag, so at least with my equipment, the (active) preamp has a beneficial function.

Check out the Ultimate Attenuators: http://www.tweakaudio.com/products.html
Maybe you will find those as interesting as I did.
Contrary to your first stated assumption, there are a great many of us who use a tuner, a record playing system, a tape deck, and a CD playing system that does not have balanced connections or a volume control, as well as other sources. For those of us misguided souls, a preamp is kinda handy for providing gain, volume control, RIAA equalization and source selection and to balance the stereo signal between channels, in case our hearing is better in one ear than the other or a room problem causes the imaging of our speakers to be off-center. So preamps are not merely empty boxes, do serve a function, and are not a perceived need created by marketing programs designed to deceive. If your only source is a CD player with balanced outputs and integral volume controls, and you are happy with the sound from it, then you don't need a preamp, and no one is putting a gun to your head to use one.

By the way, why don't you try out with and without a preamp in your own system and listen for yourself instead of relying on what others have said? If you tell me you can't because your CD player has no volume control, then I'd say you could definitely use a line stage, either active or passive, unless you are willing to put passive attenuators on your amp and adjust them with each disk (and possibly track) you play.
You have got to be kidding! Buy the best pre-amp you can afford or you are missing the boat. One of the most important pieces in the chain.
I have two passive preamps and one tube preamp. The passives seem to pass along very balanced music and well defined bass, while my tube unit bloated those lower frequencies. However, I greatly preferred the midrange of the tube preamp which was rounder, more dimensional and absolutely involving in a way the passives couldn't match. And for that reason even with the muddier bass, I listen with the tube preamp in place because I love what it adds.
Apologies for any misunderstanding, surely the traditional preamp functions are still used by some, with mulitple sources or need volume control

The question should have been phrased as, is preamp necessary in a system with single source and built in volume control

Unfortunately my cd player does not have a volume control and I do not have an outboard DAC either, so I cannot try this out myself

There are numerous equipment on the market that may eliminate the need for volume control in preamp, such as Wadia, Musical Fidelity pre CD, outboard DACs with volume

So the question is in such scenario, what is the supposed value of a preamp
Just about every CD player, even expensive ones with a volume control use a cheap pot or digital attenuator and possibly a set of opamps. It's better to use the fixed output and control the volume with any number of good quality passive preamps or standalone attenuators.
I have the Resolution Audio RC50 and RC 55, and the Audio Aero Capitole. Each has internal volume control that matches the quality of many high end preamps and supposedly also have very good output stages. I do not know why, but they do not sound right unless passed through a preamp. They all sound a little flat, lacking tonal colour and hard in the upper mid range on up - whether feeding my Plinius SA102 or my custom made valve monos. The improvement is not huge with a preamp, and you pay the price in transparency if it is not a darned expensive one too. In my case I use a high quality buffer stage between my CDP and amp. That way I get the result with no major downside (no pots or switches) for low cost. As I say, I don't know why, but maybe it is difficult to locate a good line stage within these smallish box CDPs. I have heard that the Wadia 850 and 860 (much bigger boxes) do not suffer this same problem.
I have an AA Capitole running directly into my amps. I don't feel I am missing anything. I'm getting a preamp to run my LP with so I will try the test of my AA Capitole with and without a preamp. My plan is to just hook up the preamp when I want to listen to LP but if there is a REAL improvement to running my CD through a preamp I will find out. This could be the case but it just doesn't seem like it would be. My ears will tell me.
Redkiwi, thanks for sharing your experiences. I have heard comments similar to yours regarding flat, lacking tonal color and a bit hard. There appears to be no reasonable explanation for this

I do not agree with Rgodin that even expensive CD players use a cheap pot or digital attenuator. Please read write ups on Wadia and Musical Fidelity site regarding their CD players with volume control

There appears to be some good observations on this subject with no shortage of opinionated or emotional people, but the real answer still seems to be elusive

For those who think I am against using a preamp, I DO have a Mark Levinson preamp in my system. It would be nice to find out what value this expensive box is adding

Please educate me with facts and data not rhetorics
Wadia's site clearly states that there a loss of resolution unless the digital volume control is run at maximum (no attenuation). The more the attenuation, the higher the loss of resolution. If your amp has average gain and your speakers are more than average in efficiency, you are going to have the volume much closer to the bottom of the settings than the top.

I wasn't able to find a discussion of Musical Fidelity's volume control on their site. Is there a link ?

If transparency is what you want, then test driving a Placette passive might be worth the effort. It's not overly expensive, but supremely transparent and your money back if you don't like it. www.placetteaudio.com
Rgodin, many thanks for the info on Placette. It is worth looking into

I read the info on Wadia website and the understanding I have is that there are switches at the back of the unit that can be set to match the input signal level of the amp. It also states the volume is manipulated in the digital domian which does not introduce any losses or distortions. In fact Wadia does so much DSP to shape the noise based on empirical data from listening tests, the volume adjustment is just another step within this DSP which is taking place already.

As for Musical Fidelity, my understanding is that they actually have a preamp section (from their preamp product) integrated into the pre CD. MF is known to do that and spares no expensive to achieve good results. The Trivista SACD is actually a CD and a SACD inside the same box. Unlike other SACDs which is usually weak in CD playback due to sharing of same circuits which are optimized for SACD. This is one of the main reasons I bought the MF Trivista

Back to the original question of pre amp, notwithstanding that this is a known controversial subject, we should continue to get different views from people who have experimented. This does not mean all of us have to come to the same conclusion, but all of us will be making more informed decisions in pursual of our hobby

Happy listening
The stage of gain that a preamp induces is critical to complete the musical signal. Its like reversing the DNA of the signal and playing it back. (riaa curve)

Passive preamps just dont make it. People think they sound better but in the end, and on whole, they don't.

Forget the new wave of thinking, get a killer tube pre-amp and get happy.
I should add, the problems I reported on above were more with the Res Audio than the Audio Aero and I still run my Audio Aero into my tube monos. But the Res Audio runs through an SCE HRS before my Plinius SA102 - and there is no doubt the SCE HRS is worth its cost, the cost of a PC and an extra interconnect.
I share your views on passive preamps. But the Res Audio players do not have a passive preamp in them. They have an active line stage that that follows the volume control and that, on paper, looks as good as any decent silly state preamp, with oodles of drive and low output impedence. Maybe it is not as good as claimed - don't know. But I find that the problem manifests itself, even with a preamp in place, if I run the Res Audios at lower than full volume. Again, I doubt you can point your finger at the quality of the volume control, but it appears more likely to be the problem than the output stage. I think I recall hearing that the volume control is the same as that used by Rowland. My theory is that when implemented in a CD player that it attracts more RF problems the more resistance is introduced into the signal path. I have that theory for no other reasons than that; one-box CD players are tricky beasts to keep digital nasties out of the analogue (which is why DVD players sound bad IMO - much more digital rubbish sprayed around inside the box); the effect on the sound increases as volume is attenuated; and, the effect sounds awfully like RF problems I have experienced at other times and in other systems.
I use my Audio Aero CD player in place of a preamp. I am of the opinion that, unless you are playing vinyl, the standard preamp is of minimal value. Really well-mixed CDs require almost no treble/bass adjustments, especially since the Audio Aero seems to already add a bit to the bass, anyway.

But here's an interesting aside. I recently had an early 1950s-era Fisher AM/FM tubed tuner restored and it has a preamp tht has multiple equalizaton settings for different LPs, depending on the label. Weird, but I guess a really senstitive preamp can have value...for vinyl.