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RE: Is The Last Cap In The PSU Still Important...

Lew,

It is my impression that the original poster was questioning the need for a high quality 2nd cap in the raw B+ supply upstream from the CCS/Shunt reg. It is my understanding that the cap you mentioned on the K&K board is there for frequency compensation.

To clarify things consider that all voltage regulators are based on the voltage divider principle. In its most basic form a voltage divider could be constructed from two resistors connected in series. With a voltage reg. one of these "arms" is replaced with an active devise. A series reg. is inserted between the B+ and the load. A shunt reg. between the load and ground. The K&K Salas shunt reg. uses active devises in both "arms", an active current source in the "upper arm" and the shunt circuit in the "lower". The impedance of the CCS is on the order of several tens of millions of Ohms, the Salas shunt circuit is on the order ten or so milliOhms (according to what I read on the DIY forum).

This represents one hell of voltage division and noise reduction. So, the question is: is the upstream PS all that relevant? In my experience yes. Changes upstream do matter and can be heard. Why is this? I don't really know. I do know that the impedance of the CCS drops at higher frequencies due to devise capacitance (see the two part current source article by Walt Jung in the May/June 2007 issues of AudioXpress where he measures the performance of various current sources). Even more relevant here is the follow-up article published in the letters section of the April 2009 issue where Mr. Jung discusses the cascode MOSFET CCS like the one used in the K&K reg. How to reduce this capacitance? Well, other than using MOSFETs with less capacitance, not much. I have discovered that mounting both devises on the same heat sink introduces some undesired capacitance (about 10pF by my measure). Since the bottom devise has only a few volts across it, it usually doesn't need heat sinking and can be located off the heat sink. Also, the IXSYS 8N100D2 is a 1000V devise. The higher the voltage rating, the higher the capacitance all things being equal. You can substitute a 8N50D2, a 500V devise, on the bottom. BTW Kevin suggested these changes to me and they made a significant difference in my system. Your milage may vary.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln


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