Electronics > Beginners
Ferrite common mode choke
made2hack:
Hi all,
I'm building a small audio amp using an off the shelf TPA3116 board (2x50w + 1x100w). I'm powering it from a 12VDC SMPS.
Should I loop the Mains AC power wires in a ferrite torrid? Or is the "noise" coming off the SMPS much larger than any other noise coming off the mains AC? Or should the choke be used somewhere else?
The idea is to attenuate any stray signals from affecting the audio signal.
Regards,
Zero999:
The only reason for adding a choke on the mains side would be to stop the noise generated by the SMPS from going back into the mains. If it's probably designed and complies with all the relevant standards, it shouldn't need an additional input filter.
It might be necessary to add a choke and filter capacitors to the 12VDC to prevent noise generated by the power supply from interfering with the amplifier.
made2hack:
Ok cool, so better on the 12VDC output towards the amp then!
T3sl4co1l:
Unless it's a too-small module intended to be placed within a system that's ultimately filtered -- it contains a filter (and CMC) already.
Consider the PSU as a three-terminal device. The two mains leads are one, the two output leads are another, and the chassis ground (or ground screws for the loose PCB kind) is the third.
If ground isn't present, or connected*, then the two leads (mains and output) are a through circuit, and a choke can be put on either one, equivalently. It's better to put it on the mains side, because the wire can be smaller = more turns on a given core.
*Even if you don't physically connect it, notice there is still a stray capacitance to the surroundings.
If it is connected, then there's probably some "Y" capacitance to it which improves filtering, and the noise level can be different on both ports. In that case, extra choking impedance in series, or bypass capacitance in parallel (to ground), will help, and you have a choice of which port to quiet down more.
If you can't demonstrate you need it, though... what's the point in adding it at all? Just a "feel good" act? But it's just superstitious then. That doesn't make me feel good, at least...
You don't need expensive instruments to demonstrate some sort of effect. Is the amplifier susceptible to RF rectification? Is anything nearby or attached, susceptible? Mind, while this gives a testable hypothesis, it is only a tiny step above guessing in the dark. While technically possible to reason your way out, it requires an unnatural amount of observation and discipline to succeed. You may find it helpful to use a radio (an AM or shortwave radio would be best) to get some idea of efficacy.
Tim
made2hack:
Thanks!
What about interference from cell phones? I take it that a CMC won't have any effect and I should look to shroud the amp board in a metallic case separate from it's surroundings?
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