I am designing an ultralow noise audio board.

On the first PSU PCB (just talking after bridge) I have a CMC to remove switching noise on diodes to undetectable levels (with plenty of capacitance for ripple and inductor to remove high frequency).

This PCB has wires going to main board. I am thinking to use a second CMC on this board so the two CMCs will be in series

My question is:

Should I worry about any impedance characteristic of CMCs? Can I use the same CMC at both places or should I use a lower impendance CMC (lets say 700R) as first CMC while using a 2.2k as a second CMC?

  • 1
    \$\begingroup\$ If carefully designed it can prove quite profitable to have two different CM chokes in series. You can broaden usefull bandwidth with a "small" choke good at HF helping a second "bigger" one good at LF but poor at HF due its parasitic capacitances. I'd just once more stress it all has to carefully designed to damp possible high Q resonances. \$\endgroup\$ – carloc Sep 11 '18 at 10:03
  • \$\begingroup\$ Dave talks about Capacitance Multipliers in this video, might be useful if looking to experiment with other ideas. \$\endgroup\$ – rdtsc Sep 11 '18 at 10:14
  • \$\begingroup\$ If you go very high in frequency and place the two chokes close to each other, the capacitive coupling will be different compared to one large choke but in general I would not worry to use two CM chokes in series, especially in CLCLC configuration. \$\endgroup\$ – winny Sep 11 '18 at 10:45
  • \$\begingroup\$ A common mode choke doesn't remove bridge rectifier current switching noise because that is a differential signal i.e. current enters and leaves in opposite directions and that is unaffected by a CMC choke. \$\endgroup\$ – Andy aka Sep 11 '18 at 11:13

There is rather more too it then that, high Z common mode chokes (Large value inductors generally really) tend to have resonances at surprisingly low frequencies, so details matter.

What does spice say (after you have added all the parasitics!), and what numbers are you trying for? Design is a numbers game, once you decide what noise over what bandwidth you can live with and what the supply impedance limits are, you can start designing, but decide what your criteria are first, otherwise how will you ever know when you have it done?

I have designed some high end pro audio kit for a living, and trust me DC-20KHz with a 110 odd dB of dynamic range? Meh doesn't really take a special power supply to pull that off, board layout is a ton more important (I use XP Power 2 by 4 inch switchers, right combo of universal input, cheap and reliable, do it right and you cannot see any residual in the FFT of the output).

  • \$\begingroup\$ I am damping all inductors by using rather large litics in front. My question is can I use CMCs no matter what impedance in series ? \$\endgroup\$ – Johan B. Sep 11 '18 at 9:49
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    \$\begingroup\$ You can do anything you like, question is will it help to hit whatever performance numbers matter to you, and where is the common mode current that you are trying to impede coming from anyway? Adding impedance to a power supply only sometimes makes it perform better (for whatever definition of better matters to your application), so while the answer is 'yes', that is qualified by 'but it may or may not do what you want'. \$\endgroup\$ – Dan Mills Sep 11 '18 at 9:56

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