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One aspect of my day job is the design of LC filters. It's difficult to maintain smooth response in power supplies, but over the years, I've come up with a few tricks. Most techniques aren't directly applicable to the HV supplies we work with here, but what they all have in common is dampening. So, one thing we can do with the filter you posted (first image below) is to reduce the effective Q of the last L. That's primarily responsible for the nasty 200 Ohm spike at 56 Hz. Looking at the second image, it's a duplicate of yours with one exception - a 200 Ohm resistor has been added across that last L. This reduces the 56 Hz peak to only 40 Ohms, or 1/5 the original impedance.

The primary risk in doing this is the possibility that the resistor might allow some percentage of ripple and noise to bypass the choke. We can check that by comparing the frequency response of the filter with and without the resistor. That comparison is shown in the last two plots. Ironically, 60 Hz performance actually improves with the parallel resistor. In the plot without the dampening resistor, the 60 Hz response is pulled up by the 56 Hz resonant peak to -13.5 dB. That's pretty poor IMO. Rejection at 120 Hz (perhaps the most important frequency) is -53 dB. The last plot shows response with the dampening resistor. 60 Hz rejection has improved to -25 dB due to the absence of the peak. We pay for this with a slight degradation in the 120 Hz number, now 3 dB worse than it was. That can easily be recovered with a slightly larger cap at either the last position or the one before it. 70uF in either location will do the job.


























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