Passive high pass filter - Marchand XM446XLR-A

Gentlemen, yes, you would need a capacitor on each of the two signal lines in the balanced signal pair. Preferably the two capacitors should have a reasonably tight +/- tolerance on their value (for example 5%), so that they match reasonably closely, although I doubt that the tolerance is particularly critical.

Usually, although not always, the specified input impedance of a balanced input reflects the sum of the input impedances between each of the two signal lines and ground. That appears to be the case for the RM-200 Mk2, since the spec is 30K balanced and 15K unbalanced, the unbalanced spec presumably reflecting the impedance that would be seen if a single-ended signal were provided to the XLR connector. Stereophile measured the actual balanced impedance as 29K, so you would calculate the capacitor value based on a 14.5K input impedance.

Regarding Mitch2’s Clayton amps, you would want to verify that the specified 100K input impedance reflects the sum of the input impedances of the two legs. If so, the value of each of the two capacitors would be calculated based on a 50K input impedance.

Good luck. Best regards,
-- Al

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.

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

Sounds like a plan, Tim!  Best of luck.

-- Al
 

Thanks Eric (bdp24). I’m wondering, though, if the change you referred to from 15K/30K to 30K/60K might have been introduced sometime after the Mk.II was initially marketed.

The Stereophile review I referred to indicates in numerous places that it is a review of the Mk.II version, and describes various changes and improvements that were made relative to the original version. The photo of the amp on the first page of the review also shows "RM-200 Mk.II" as being marked on its panel. As I mentioned, that review indicates a measured balanced input impedance of 29K (JA’s measurements of balanced input impedances always represent the sum of the two legs), and also indicates a spec of 15K ("one leg driven")/30K (balanced).

Also, Roger’s website still indicates 15K/30K, although he has not updated the amp's description to say "Mk.II."

Best regards,
-- Al