Preamp function.

Hello everyone. I have always wondered what a preamplifier does. I am currently investigating valve amplification for the first time. one of the options i am also considering is a valve preamplifier to add to a pass labs power amp. i am also considering all valve. Transistor or valve, what does the preamp actually do, apart from switching and volume control. As an experiment i added a musical fidelity a3cr that is from my second sytem, to my pass xa30.5. the results were very positive and an improvement over cd player direct. i am aware the musical fidelity a3cr is not in the league of the pass, but it certainly improved the sound. Most noticeably was a quieter back ground and better musical presentation. From this i gather that a preamplifer does add something to the sound. i had always believed it purely carries the signal unaltered. If anyone here with technical kwowledge mind throwing some light on the subject, i would appreciate it( others might find it interesting too). i have read reviews where writers describe the sound of the preamp. i had believed they were actually describing the sound of the player and amplifier and the ability of the pre to carry the signal unaltered? Thank you.
james123 control the source signal in general

In Particular:

1. Pass won't be "friendly" to a passive preamplification therefore introducing active preamp in your system would significantly improve the sound. In many cases one listener may choose direct over any even super fancy preamp, but apperently not yours and the reason is rather technical than personal taste where impedance mismatch between the output of player and input of amp gets into the equation.

2. Not every tube preamp will mate great with Pass due to the low input impedance of 10kOhm also mentioned in previous para. Having an input/output impedance ratio < 20 will mean jeopardising dynamics and volume control capabilities. You may experience perfect listening on very few volume settings while when used with active solid state preamp with low output impedance your dynamics sky-rocket and you listen comfortable almost at any volume setting.

3. If you have only one source CD player, or plan to only have digital sources, you can purchase DAC with volume control or DAC-preamp. The modern ones such as Grace M903, Mytek 192, Antelope Audio, Lynx and Apogee offer great DACs with volume control, USB and multi format capabilities.

4. As to tubes, certainly they will sound different, but will they sound better than your Pass is up to you to judge. I would recommend doing as much A/B as possible before getting to all-tubes or probably simplier to build a secondary low-powered(around integrated tube amp) entry-level tube system while keeping your Pass in business.
Post removed 
I'm not a technical guy, so please excuse any incorrect terms...

The preamp drives the amp(s). It sort of buffers the electrical signals of sources and makes sure the amp sees an optimal electrical signal, from an impedence standpoint.

I've heard passive preamps sound great in some systems, and the same ones kill another system. It all depends on what it's driving. Passive preamps also generally sound best at one specific point on the volume knob, and I'm told that this is the optimal impedence match at that specific point.

Marakanetz did a far better job with the technical stuff. Hopefully I put it in layman's terms that made it somewhat easier.

In my experience, the preamp can easily make far more a difference than the amp (as long as the amp's not clipping). I think it's the most overlooked component. I stick to integrateds for several reasons, optimal preamp to amp matching is one of them.
A pre-amp does the following: 1) it amplifies the low level signals from devices to the point where power amplifiers can then take the signal and further amplify it for rated output. 2) it is a control amplifier whereby it has volume control and switching capability so that you may switch between different input sources and 3) it is an impedance matching device. The theory is that if you separate devices and power supplies you will get cleaner, less distorted signal. Therefore, older devices contained the control amp, pre-amplification, tuner and power amp in the same chasis, sharing the same power supply circuitry. When each device was placed into their own chasis separately, using their own power supplies, the sound was much better. The power supplies didn't have to work so hard and cross talk and interference between circuitry was minimized or eliminated. Amps are not designed to pick up and amplify the really low level signals from turn table cartridges and other such devices. Their input sensitivities are listed for each amp and if you place a much lower level signal at the input of a power amp and the output impedance of a cartridge/phono stage may not (typically won't) match well with the input characteristics of a typical power amp. Also, an amplifier's job is to amplify the input signal such that the output signal is a exact match for the input signal only amplified. So if you have an all in one control/pre-amp/power amp the level of design is much harder. Hence the integrated amp. It is basically a control/pre-amp and power amp in the same chasis sharing the same power supply. If designed and constructed right, you could have an amazing integrated amp. But the cross talk/signal interaction possibilities are really hard to mitigate. Also, in the audiophile world where would we be if we went back to the all-in-one single chassis system?

Thank you all for explaining what a preamplifier is. i now have a better understanding. i appreciate the detailed responses. i had been using the droplet direct for a long while. the a3cr, while no match for the pass, did something to the sound which i find pleasing. a review of the droplet suggested that there would be very minimum improvements by using a very high quality preamp. i therefore did not look into it further. however i found significant improvements in adding a preamp that now sell for very little ( 300 uk pounds) on the used market. it just that reviews should not be taken too seriously. their writing skills are very enviable though, and have used several pieces in english classes that i teach! i am now considering a quality pre. i would imagine a pass preamplifier would be a good match. anyone got any suggestions? a forum user suggested an arc reference 5. however that is beyond my set budget. thank you kindly.
I like to think of a preamp as 3 parts - input stage, gain stage, and output stage. The input stage has 2 functions - one to select the appropriate input and two to provide a high impedance so the source can drive it easily and a low impedance to the gain stage. The gain stage amplifies the signal. The output stage presents a high impedance to the gain stage and delivers the signal for output at a low impedance. The preamp also typically has some mechanism for blocking any DC voltage from the power supply to the outputs. This can be done with blocking capacitors or other devices. Some preamps do not have such blocking devices and are known as "direct coupled".

Some general comments on impedance and on the gain of the amp.

The rule of thumb of 20x input/output ratio that Marakanetz gives helps insure that the input can drive the output easily. The input side of a preamp or power amp has a high impedance and the output side of a source or a preamp has a low impedance. The high input impedance of a preamp means the source does not have to work as hard to drive the preamp - that is, it has to provide less current. Some sources struggle to provide enough current if the impedance they are driving is too low. So, if you want to have a CD player or DAC drive an amp directly, you need to be sure it can provide enough current into the amps input impedance.

Another issue to consider is the gain of the amplifier. The typical gain of a SS power amp is in the mid to high 20's dB range - say 28 dB. Some amps, like the new Emotivas, have a gain of over 30 dB, which might sound nice, but more is not always better. If you have high efficiency speakers with an amp that has a lot of gain, then the typical 2 volt signal from a CD or DAC may produce way too much volume. As others have already noted, if you end up having to use the very lowest part of the volume control you may not have enough control over the volume and, depending on the volume control, the sound might not be optimal. You even see this type of problem with some preamps, especially when the preamp has a large amount of gain.

So you should consider both the input/output impedance ratios and also the gain of the sources and amp when putting together the system.
Dtc, than you for the detailed response. it is very useful to me in understanding what a preamp is. even the musical fidelity a3cr, now old but certainly with merits- i purchsed this unit 10 yrs ago, did something very nice to the sound. i really wish i had done this test earlier. ive seen for sale a preowned pass x2.5. anyone got any thoughts on this? thank you.