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Some simple facts

Posted by Jon Risch (B) on August 8, 2014 at 23:14:50

In Reply to: Whoa, cranial overload. posted by Cleet Torres on August 8, 2014 at 19:33:40:

RE #1, inductors don't inherently distort the wave form, only if the cores are saturated by any given current draw.
When building audiophile grade equipment, it is relatively easy to utilize inductors with cores substantial enough to not cause such problems.

This is why I specify a specific set of part numbers/manufacturer's for the series inductors in my DIY AC Line Filter.

If saturation is avoided, then having a pair of independent (NOT common mode connected) inductors assures the lowest differential and common mode interference feed through.

RE #2, the big deal with a CMC is that the core can be made much smaller, and thus save on costs. There is absolutely no strictly technical/performance benefit to the use of common mode chokes in an AC line filtering circuit, just a cost reduction aspect.

This is why virtually all commercial/retail filters use CMC's, since the costs are the lowest.

If you use separate independent inductors with a large enough current rating, then you don't need a CMC for any reason.

One thing that CMC's do tend to encourage, is the use of a VERY large inductance in the filter circuit. However, what a lot of people overlook is that placing 1 mH in the AC line actually degrades the peak current delivery due to the sheer AC reactance at the 60 Hz powerline frequency, and the higher harmonics that represent the true current draw waveform.

A 1 mH CMC will have an AC reactance of approx. 0.38 ohms at 60 Hz.
Nevermind the DCR of 0.05 ohms, all for naught as the reactance dominates.

Thus the current surges needed to replenish the power supply caps are being limited by this relatively large amount of AC reactance due to the inductance. With these types of current surges, they are a fraction of the total AC line sine wave, and the fundamental rep rate is 120 Hz, with the fundamental frequency of these current surge blips somewhere above 1,200 Hz. The reactance of a 1 mH CMC at 1,200 Hz is approx. 7.6 ohms.

Now you are really limiting the current surges.

Now I hope it becomes clear that the use of such a large inductance is counter-productive in terms of musical dynamics.
Thus, the use of a much more reasonably sized independent and separate pair of inductors does not require a large and costly core with a lot of windings. You don't really want it to be that large of an inductance to begin with.


Jon Risch

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