Can you explain the first paragraph please? I would like to learn more about your saying.
Impedances are never matched in audio circuits because then you would have a voltage divider and the output level would be reduced to half.
A preamp with an input impedance of 1.2k ohms is used for for a 150 ohm dynamic mic.
So you are saying that we can handle both a dynamic and an electret mic through just the same ciruitry (except of the feeding resistor for the electret mic)?
When a preamp has a low input impedance then it has less noise when nothing is connected to it. An electret mic has an impedance of about 3.3k ohms which is in parallel with the 10k resistor powering it so its preamp should have an input impedance of about 20k ohms.
What about the input of a transistor?)?
The input impedance of a transistor is more when its current is less and when it has a resistor in series with its emitter to ground.
I used a 10k resistor for it before (please take a look at Ron's modifiction).
I know. Ron's inverting opamp has an input impedance of 10k.
But why this is true ""The input impedance of an inverting opamp circuit is the value of its input resistor""?
That is how an inverting opamp behaves.
Is that due to this fact that an op amp has a very high impedance at its input then we can use a resistor at its input and consider it as a the input impedance of the opamp?
No.
The input impedance of a non-inverting opamp is extremely high.
The negative feedback to the inverting opamp's input causes it to have an input impedance that is the value of its input resistor.
Now that I am thinking I reach to another question, you know that I am using a mp3 player for the input too, we know that we can exchange the output of the mp3 player with a voltage source and a resistor in series with it. So do we need to consider that resistor as part of the input impedance too?
Yes.
The input to your modulator circuit is either an MP3 player or the output of the mic preamp.
The output impedance of the mic preamp's opamp is extremely low so the gain of the inverting part of your modulator is 1.
But you don't know what is the output impedance of the MP3 player. Then the gain of the inverting input of the modulator circuit changes with a different output impedance of an MP3 player.