Tim (Mitch2), to be sure it’s clear, I was saying that in most cases manufacturers specify balanced input impedances as the sum of the input impedances of the two balanced signal legs. I have seen a few exceptions, however, in which the specification corresponds to the input impedance of each leg. The fact that an unbalanced input is not provided has no relevance to that.
If the 100K spec on your amp is defined in the usual manner, you would want to base your calculation for each of the two capacitors that are required (per channel) on a 50K input impedance, as I indicated earlier. The 0.022 uf cap would then result in 144 Hz, not 72 Hz. And the 0.047 uf cap would result in 68 Hz, not 34 Hz.
In the much less common situation where the input impedance is defined on a per leg basis, meaning that each leg is 100K, your calculations would be correct.
Paralleling two resistors or two inductors or two capacitors results in a lower impedance than either of the two paralleled parts would present individually, at a given frequency. Since the impedance of a capacitor at a given frequency is inversely proportional to its capacitance, to have a lower impedance the capacitance of the paralleled capacitors must be greater than the capacitance of each of the two. Again, it’s the opposite for resistors and inductors. The impedance of a resistor is identical to its resistance (to the extent that it behaves in a theoretically ideal manner), and the impedance of an inductor is directly proportional to its inductance, rather than inversely proportional.
Best regards,
-- Al
If the 100K spec on your amp is defined in the usual manner, you would want to base your calculation for each of the two capacitors that are required (per channel) on a 50K input impedance, as I indicated earlier. The 0.022 uf cap would then result in 144 Hz, not 72 Hz. And the 0.047 uf cap would result in 68 Hz, not 34 Hz.
In the much less common situation where the input impedance is defined on a per leg basis, meaning that each leg is 100K, your calculations would be correct.
If I wanted something in-between, could I series the 0.022 with a 0.01 cap?No, you would want to parallel them. The values of capacitors in parallel add. The value of two capacitors in series corresponds to their product (multiplication) divided by their sum, which will always be less than the lower of the two individual values. It’s the opposite of what happens when resistors or inductors are connected in series or in parallel.
Paralleling two resistors or two inductors or two capacitors results in a lower impedance than either of the two paralleled parts would present individually, at a given frequency. Since the impedance of a capacitor at a given frequency is inversely proportional to its capacitance, to have a lower impedance the capacitance of the paralleled capacitors must be greater than the capacitance of each of the two. Again, it’s the opposite for resistors and inductors. The impedance of a resistor is identical to its resistance (to the extent that it behaves in a theoretically ideal manner), and the impedance of an inductor is directly proportional to its inductance, rather than inversely proportional.
Best regards,
-- Al