Would someone please explain how 'gain' functions?

Gain is the ratio of the output voltage of a component to its input voltage. In the case of a preamp or a line stage or other component having a volume control it is usually defined based on the volume control being set to its maximum position. It can be defined either as a numerical ratio (e.g., 100 times), or in db (db = 20 x logarithm(Vout/Vin); 100x = 40 db).

Usually a CD player or DAC or other digitally-based source component will provide higher output voltages than most phono stage/cartridge combinations, and so the resulting volumes will be different unless the line stage provides independently adjustable level settings for each input.

A factor that can dramatically affect perceived loudness is the dynamic range of the recording, and consequently how much dynamic compression it has been subjected to when it was engineered. Our hearing mechanisms tend to sense volume based on the average level of what is heard, rather than on brief dynamic peaks in volume or on passages that are significantly lower in volume than the average. So if everything else is equal (including the volume control setting) if a recording that has been compressed to a narrow dynamic range (i.e., so that a relatively small difference in volume occurs between the loudest notes and the softest notes) it will tend to sound louder than if the same music has been engineered with minimal or no dynamic compression.

Also, of course, recordings can differ somewhat in the overall volume levels they have been mastered at, even if the music and the amount of dynamic compression are similar.

Regarding phono stage gain, depending on the particular phono stage and cartridge, as well as on the gains and sensitivities of the downstream components including the speakers, there will usually be a relatively small range of phono stage gains that will result in an optimal balance of sonics, noise levels, volume control positions, and avoidance of any possibility of overloading either the phono stage or the preamp. Too much gain or too little gain can adversely affect some combination of these factors.

Regards,
-- Al

If you lose 6 db at the output, say, when switching from XLR to RCA connectors, is it a lot in real world ? Specifically talking about reel to reel decks.

@Inna, you may be aware of the rule of thumb guideline that a 10 db increase in volume corresponds subjectively to "twice as loud," and correspondingly a 10 db decrease corresponds subjectively to "half as loud."

To provide some additional perspective, for a preamp or integrated amp having a rotary volume control whose minimum position is approximately at the 6:30 o’clock position, and whose maximum position is approximately 5:30 o’clock, a 6 db change will probably correspond to around 45 degrees of rotation. Assuming, that is, that the control is not in the lowest 20% or 25% or so of its range, where volume changes usually occur more rapidly as the control is rotated.

Regards,
-- Al

My cheap phono pre-amp has a gain control and a little light that goes from green to red depending on the signal. I never know whether to set it so the red flickers from time to time or never.

@N80, I would think the manual should and hopefully would answer that.  Or if not, a call or email to the manufacturer would seem to be in order.

Regards,
-- Al