Perhaps a dumb question...

Here's a simple explanation. A phono stage is needed to boost the relatively low voltage signal (about 0.3 - 3.0 mV) from a phono cartridge up to what is considered a line-level signal (about 100 - 200 mV). I don't remember all of the numbers but it takes about 40 to 70 dBs of gain, depending whether the cartridge is MM or MC, to boost the signal from a phono cartridge to a resonable level that most people can listen to confortably. So the phono stage is a pre-amplifier. Most pre-amps these days do not have a built-in phono stage and even those that do have one do not always have a very good one. The remedy is to use an external phono stage.

To reproduce the sound on an LP you must use either a built-in or stand alone phono preamp. Basically a phono pre-amp does two things. One, it converts the information embedded in the groove according to something called the RIAA (Recording Industry Association of America) curve. The curve is used during the cutting of LP's to compress the signal at its frequency extremes. Two, it amplifies the very low level signal produced by the phono cartridge. A cartridge's stylus riding in the groove generates a very faint signal. If you put your ear close to a record while it's playing you can hear it.

Back when turntables were the only game in town all stereo preamps, integrated amps and receivers had built-in phono pre-amps. When the popularity and availability of LP records waned manufacturers dropped the phono sections from their products. So nowadays systems with turntables use stand alone phono pre-amps.

I have had phono preamp for years but couldn't explain its function. The above explanations are very helpful, thanks audiogoners. There is no dumb question.

But if my receiver or amp has an input that says "Phono" as well as a knob that says "Phono output" or something like that, is it safe to say that it has a built in phono stage?

You better check your owners manual before you assume there is a phono stage built into your receiver. But anyway you'll know for sure when you plug it in. With no preamp you'll get crap out...