FerdinandII wrote:
I was just trying to disabuse the OP of the following statement he made:
"...gain Controls reduce the voltage by impedance matching..."In order to dispute this false concept, I chose to go back the the basics, ala Thévenin.
This concept kept creeping back into the discussion above and it needs to GO and never come back.
tomp wrote:
FerdinandII wrote:
You need to just forget the concept of impedance when looking at this issue. It is irrelevant.
V=IRThe line level output is (generally) a voltage source.
Actually, I am right.
There is a problem in the Thevenin equivalent circuit you posted. Thevenin equivalent circuit must have a single voltage source and single series resistor. Like this:
That is a closed loop.
Kirchhoff's Voltage Law: The sum of voltages around a loop is zero
Suppose, preamp puts out 5v at 1000ohms and the amp input load is 10kohms
Voltage drop across First resistor = 5/(1+10) = 0.45v
Voltage drop across second resistor = 50/11 = 4.54v . Amp’s gain stage will see 4.54v with 10k input impedance
If the amp’s impedance is reduced to 2kohms
Voltage drop across First resistor = 5/(1+2) = 1.66v
Voltage drop across second resistor = 5x2/3=3.33v. <— Amp’s gain stage will see 3.33v with 2k input impedance
Amp’s’ gain factor being constant, lowering the voltage seen by its gain stage reduces its output voltage. As Roscoe and others suggested, gain controls are potentiometers. Pots control the volume by manipulating the preamps input impedance.