Preamps... understanding volume...

Amps are limited, in absolute terms, by

• The voltage rails in the power supplies. These provide the maximum + and - V swing.
  
• The current available from their own power supplies, before those rails sag
  
• Thermally limited in the amount of power they can output before they shut off or melt.This is why peak power may be much  greater than continuous, as well as limit the ability to deliver into low impedance loads.
  

Amps are also a fixed voltage gain. Often around 28 dB. So:
Output = ( Input + 28 dB ) OR ( MAX)
whichever is less.

I was typing from a bar, so couldn’t put a lot of detail. The simplest active preamps I know of are perhaps the Conrad Johnson PV10s, but they have a lot of similarities with others. The structure is something like this:

Input --> Source selector --> Fixed gain, high impedance buffer --> Volume knob --> Output


The combination of the fixed gain (+ x dB) , and volume (- y dB) loss determines the final output level relative to input.

One thing to note, while these systems are super simple, and short, they also are sensitive to the volume knob setting, like you see on figure 1 in this review:


https://www.stereophile.com/content/audio-research-reference-6-line-preamplifier-measurements


This can be improved with additional buffer stages, at the cost of more devices in the audio path.


Along with gain comes noise though, and honestly, we have too much gain in most preamps, especially tubes. We’d be better off reducing the gain, and using the volume knob closer to 12 o’clock.

Anyway, the point is, active preamps almost always have a fixed gain stage, but the output is almost always lower than that.

Best,

Erik

Ozzy is right. 

It may help to understand that usually the first stage the signal sees is a high impedance, fixed gain stage. For historical reasons, this gain is often way more than needed.

The volume knob comes after.