1)why is the DRB connected in series to find the input impedence of an amplifier n why not in parallel with the input.??

2)why is the o/p voltage reduced to half to find the i/p impedence??

3)why are the DRB knobs increased from minimum position while finding the i/p impedence and why decreased from maximum for o/p impedence.??

regards: vishal

 

What is a DRB?

To measure the input impedance you add a resistance in series with the input of the amplifier and slowly increase the resistance when a fixed signal is feeding it. When the output of the amplifier becomes half then the resistance is the same as the input impedance of the amplifier because they make a voltage divider.

It is difficult to measure the output impedance of a modern audio amplifier because its negative feedback makes the output impedance as low as 0.04 ohms or less.