NOISE SUPPRESSION FILTERS - For ClariTy and other final amps （4）

Published:2011/8/10 21:53:00 Author:Phyllis From:SeekIC

By Ton Giesberts

Be sure to leave the windings leads long enough to allow them to be properly fitted to the circuit board. This will require a bit of ’cut and try’. The inductor is held reasonably firmly on the board by its four leads, but we recommend using a small sheet of plastic and plastic screw to secure it to the circuit board. These filters should be placed as close as possible to the amplifier outputs.

Next a few remarks about the capacitors. CI, C12, C15 and C16 are actually intended to filter out RF interference (in differential mode); they do not have any effect on common-mode signals. However, they are connected in parallel with the output of the amplifier, so they have intentionally been kept rather small in order to keep their effect as small as possible. They increase the filter capacitance by just less than 10 %.

There are also capacitors in the filter that do provide common-mode suppression: C9, CIO, C13 and C14. They are effectively in series for differential-mode signals, so they can practically be ignored for such signals. For common-mode signals, by contrast, they are effectively in parallel and provide a capacitance of 2 nF. The junction of C9/C10 and C13/C14 must be connected to the enclosure.

If you use our printed circuit board design, be sure to pay attention to the polarization of the filters for two-channel applications, since they are opposite to each other. For stereo use, swapping the phases of the outputs wouldn’t be obvious, but if the amplifier is used in bridge mode, it is essential ensure that the phasing is correct. For this reason, the circuit boards for the two channels are designed as mirror images of each other, so equivalent connections are close together and on the same side of the board.

Speaker cables

Finally, we mustn’t forget the cabling. Generally speaking, cables make excellent aerials, with a common-mode impedance of approximately 150 12. Particularly with the pulse-width modulated signals coming from our amplifier, the cables can be blamed for a significant portion of the interference radiated to the surroundings.

The simplest way to counter this is to keep the distance between the amplifier and the loudspeakers as short as possible. Another possibility is to slide a pair of substantial ferrite cores over the cables (at the amplifier end), which can increase the common-mode impedance by a factor of as much as several hundred. Another option is to run the loudspeaker cables inside metal screens connected to the amplifier enclosure.

Ferrite cores of various sizes can also be fitted to the other cables (mains cable and input signal cables).

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