is input and output impedance necessary on an audio attenuator?


Let’s say you have a CD player and a power amplifier, between these two do you have a dual pot to both positive and negative signal wire, no grounding at all. Or you could say a variable resistor in the middle of the cable.

Would the variable input resistance do anything negative?
Or should the impedance be constant even under change in the attenuators resistance?

In: 0

Most likely, the resistor would do little to nothing, depending on values. To change the level you would need to create a voltage divider, which you can do with the variable resistor if one end of the elements is tied to ground (for it to act like a normal level control, it would be the left terminal looking at the pot from the front), the other (right terminal) is connected to the CD player and the wiper (middle terminal) connected to the power amp in. All grounds connected together.

Without the ground, most likely nothing much would happen. The output impedance of your CD player is pretty low and the input impedance of the power amp is pretty high, so not much current flows between the two, therefore very little voltage drop across the resistor in between.

Since you have a volume control on the CD player, why do you want to add a resistor in-between?

As you describe it, you’d be degrading the performance of your audio system.

In an ideal audio system, you have a voltage divider where the impedance of the cable is zero and the impedance of the audio equipment is infinite. This yields a situation where all of the power of the system is in the audio equipment and none of it is in the cable.

What you’re doing with your variable resistors is increasing the impedance of the cable, reducing the amount of power that is delivered to the audio equipment. You’re also going to be introducing noise into your signal (potentiometers are fairly noisy components even above the fact that you’re setting up a bad voltage division).

If you’re trying to implement a volume control, the general rule is that you adjust the signal where it is most powerful rather than where it is least powerful because low power signals are far more subject to noise.