-20 DB Volume Measurment

The preamp and amplifier do not communicate with each other. The amplifier gains the signal from the preamp and the preamp does a negative voltage gain on the signal (the volume pot reduces the voltage from the source then it’s amplified it to a lower voltage than the source before sending it out to the amplifier. Which is why a preamp does more harm than good with only one source attached).

For example, a CD player puts out a constant 2V signal. This is the reference voltage for the preamplifier and is designated as 0dBFS. That means that any voltage below 2 Volts will be a negative dB. So when the volume setting is at "-20dB" the 2 Volt signal from the CD player is output at 0.2 Volts. Turn the volume to "-10dB" and the preamp puts out 0.6 Volts.

The amplifier takes the preamp signal -- no matter what it is -- and gains it up by the rated dB gain. If the amplifier clips (say) at 1.3 Volt signal input, then that would be (1.3 / 2) fraction of the CD player voltage which would be "-4 dB" on the preamp volume setting, so a 16 dB range of turn from the "-20 dB" position is the amount of volume knob travel available before clipping.

I'm sure others will cover the technical aspect adequately. I'd just like to note that in your scenario, it's dangerous to assume you have 20dB of headroom available on your amp (dangerous if you try to push it in that direction). It doesn't work like that, especially in the modern era where digital sources have extremely high output signal levels (many balanced DACs pushing out 4 - 5 Volts), and combined with any preamp gain this typically means you can easily drive your amp to clipping long before you tap your preamp's maximum gain. 

+1 @mulveling 

Send to Amir and find out what the coefficient is.

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