What is a preamp?
The word "preamp" is used in many different ways by different manufacturers, marketers, and users. It is one of the most widely-interpreted terms I see when discussing audio gear; if you ask for a "preamp", you might as well be asking for a "furniture" -nobody will know exactly what you want. A preamp may be built into an instrument, a pedal, a rackmountable unit, a mixer, a sound card, or a variety of other forms; and a preamp is also the input stage of every amp "head".
Sometimes people confuse preamps with DI boxes; this is an easy mistake because so many devices include the functions of both. A DI, which stands for "direct injection", is a device that converts a signal from unbalanced to balanced, and sets the output level and impedance of the signal so that it will be ideal for connecting specifically to a microphone preamp input on a mixer. There are many preamps which also include a DI output; and there are many units marketed as DI's which also contain functions associated with a preamp, such as tone shaping or gain boost. But just because many products have overlapping features does not mean you can assume that any preamp will always work as a DI, or vice-versa.
A preamp may be designed to do any of these things:
-Increasing the gain
-Changing the tone
-Lowering the output impedance
-Converting from unbalanced to balanced
...Or any combination of those, to any degree.
Gain just means the amount your signal level is increased. The amount a specific preamp increases your gain is a critical thing to understand, because every device your signal will be sent to (power amp, mixer, instrument amp head, etc.) has an ideal range of signal level it expects in order to operate best. Not every preamp can effectively drive a power amp, for example. Some preamps have no gain at all. Others may be intended to increase the signal level just enough to overdrive the input of a tube amp. Some preamps have a gain control, while others (like onboard bass preamps) have a fixed amount of gain. Either way, they usually have a "volume" knob which just passively turns down the overall signal level at the very end of the preamp circuit.
Tone can include anything from EQ controls, to "warmth" or other subtle qualities, to outright distortion. Some people want lots of tonal changing and EQ controls, other people want absolute transparency, and of course everything in between.
Impedance can be described as the efficiency of the signal transferring from one piece of gear to another. An illustration I like to use is pedaling a bicycle uphill. If you are in the right gear, pedaling uphill requires some energy; but if you are in the wrong gear, it requires a lot more energy, and after a while you may not have the strength to go any further. An ideal impedance relationship is a very low output impedance number connecting to a very high input impedance. That is being "in the right gear". If your instrument or device has an output impedance that is too close to the input impedance of the device you're connecting it to, you will be in the wrong gear, and your signal may be too weak. That weakness may result in a lower signal level or a dull tone.
Preamps are usually "active", meaning they require a power supply. This is because it requires energy to boost a signal, and it is easiest and cheapest to design the other functions of a preamp using small powered components. There are a few products out there marketed as "passive preamps", but with passive components the energy of your signal can only be rearranged or dissipated, it cannot be increased. Passive boost is possible, using inductors and transformers to "rob" energy from one frequency range in order to boost another range. In the world of Hi-Fi there's a lot of voodoo, questionable science, and exaggerated claims around passive preamps, so be wary. EQ's may be active or passive. Active EQ usually has the ability to boost signals, and the ability to act on very specific frequency ranges; while passive EQ is mostly about cutting rather than boosting, and acts on broader and less specific frequency ranges.
A common question is "can this preamp drive a power amp?" The answer is that you need to consider the maximum output level of your specific preamp, and the input sensitivity rating of a specific power amp--because they are not all the same! Power amp input sensitivity is rated in volts; the most common ratings are .7V, 1V, and 1.25V, but you may find others. Many pedal preamps, onboard bass pre's, and older rackmount pre's do not put out a signal strong enough to drive a power amp with a 1.25V or higher rating. If you try a certain preamp and it sounds weak, that's probably the reason. Similarly, if you get great sound from your preamp driving one power amp, and weak sound with the same pre driving a different power amp, the problem is not that the second power amp has weak tone! This is a very common misunderstanding.
The real problem is usually that the second power amp had an input sensitivity that was too high to be driven well by your preamp. Unfortunately most preamps don't specify their average output voltage. So the best thing you can look for is a statement from the manufacturer that the pre has a "line level" output. Line level can be anywhere between .7V and 1.4V on average, so the phrase "line level" is still no guarantee, but it's a start. The next thing to look for is a statement that the output is "-10 dBu", "0 dBu", "0 dBv", or "+4 dBu" line level. -10 dBu is associated with older gear and entry-level gear, and it won't properly drive a power amp with a 1.25V input sensitivity, but it may work OK with one that has a .7V sensitivity. 0 dBu output is an ideal match for the .7V input, but it may not be quite up to driving a 1.0V or 1.25V sensitivity. 0 dBv (note the v instead of a u at the end) is ideal for 1.0V sensitivity. An output rating of +4 dBu is exactly what you want for driving a power amp with a 1.25V sensitivity. If your power amp is rated for 1.4V, you need a preamp with UNUSUALLY high output.
Another spec you can look for in a preamp is dB gain (amount the signal is boosted). A "clean boost" pedal may commonly offer 20 or 30 dB of gain, but it may take 50 or 60 dB gain to bring the output of a bass or guitar up to the level needed to drive a typical power amp.
Another factor is whether the output of the preamp is balanced or unbalanced. An unbalanced signal uses a regular instrument cable containing two wires, typically with a 1/4" plug on the end. A balanced line uses three wires, and may have an XLR (microphone type) plug or a TRS (stereo) 1/4" plug. You need to verify which of those types of connection is most ideal for the next item you're going to plug the preamp into. There are some devices which can receive both balanced and unbalanced connections, but you cannot assume that about any one piece of gear. The instrument input of an amp head is unbalanced; most pedals are unbalanced; a DI output is balanced; many rackmount processors are balanced. There are some preamp-to-power amp combinations where an unbalanced connection will be significantly lower (-6 dB) in level than a balanced connection. This is another cause of weak sound, so if the levels seem lower than you expected, double-check the manuals for both devices to confirm whether you needed a balanced connection for best operation. In a power amp's specs this may be shown as two different input sensitivity levels for unbalanced or balanced input.
-back to the top-
So in sum, when you are looking at preamps, whether they are in pedal form or rackmount or built into an active instrument, you need to consider whether its output is at the correct level and impedance to properly drive whatever device you plan to connect it to; and then make sure it has the right type of output connector plug. Only after confirming those factors should you then consider whether it will provide the tonal qualities you want. The reason I say this is because you will often see people (or advertisements) saying to use a certain preamp to get a killer tone--but do you need it to properly drive a power amp? Is it only intended to feed the input of an instrument amp head? Does it have a balanced output that can be sent as a DI signal to a PA mixer? Is it functionally different than the preamp built into an active instrument? A preamp is a wonderful tool, but so is a shoe--if you get the wrong size of shoe, or shoes with high heels when you need to go running, then that is the wrong shoe for the job and is no longer useful to you.