The JPB-2016 Common Mode Speaker Line Filter is used to eliminate undesirable
heterodyne tones caused by capacitive coupling of multiple Class D power amplifier
outputs. Heterodyning is the combination of two or more frequencies to create sum
and difference tones, in this case within the audible range. Heterodyne tones can
occur when output cables from more than one Class D amplifier are run close together
over a long distance, even with no audio signal present. Ashly Class D amplifiers
currently
include all nX, NE two-channel, NE multi-channel, Pema, SRA,
and TRA models. The JPB-2016 is wired using Neutrik® speak-ON NL-2 or NL-4 type
connectors.
Theory:
The carrier signals on each output channel of an individual multi-channel Class-D
amplifier are typically synchronized, or frequency “locked” to a single
common carrier frequency (~ 300 KHz.). Therefore heterodyning between multiple
channels of the same physical amplifier cannot occur. NOTE: The heterodyning
mechanism is actually created by similar, but not exactly the same high-frequency
carrier signals feeding back into the outputs of each respective amplifier. Those
signals then interact inside the Class-D amplifier output stage which will then
generate the heterodyne tones. i.e. If one amplifier running a 300 MHz carrier leaks
into the output of another amplifier running at 309 MHz, the amplifiers can multiply
those two signals together creating both sum and difference tones, with the
difference tone being only 9 KHz, which is within the range of hearing. (Sounding a
little like an old AM radio.)
The JPB-2016 common-mode speaker line filter rejects high-frequency signals
coming back into the power amplifier output, not the reverse. Because both + & -
speaker lines are acting as antennas, and the 300 MHz carriers capacitively couple
equally into both sides of the speaker line, one good way to reject an incoming high-
frequency carrier is to place a common-mode filter at the output which will have
virtually no effect on the differential-mode signal traveling from the amplifier to
the speaker, but will block common-mode interference coming back into the
amplifier.
Because different “Model Series”, and brands of amplifiers will most
likely operate at different carrier frequencies, at least greater than 20 KHz apart,
the difference tones that high cannot be heard. The heterodyne tone problem is most
often observed when running multiple individual amplifiers of the same exact model
series, and then running their speaker lines closely together within the same
conduit or wire bundle for long-distance runs which will increase the capacitive
crosstalk coupling between the speaker lines.
This is because running the speaker wires close together creates a capacitor. The
longer the run the higher the capacitive coupling from one line to another, and
thereby one amp to another. This effect has been observed with closely bundled
speaker lines running distances as short as 75 feet but more typically occurs at
lengths of 200 feet or more. NOTE: In all but the shortest speaker lines, separate
Class-D amplifiers should NEVER share a common multi-core speaker cable. Multi-core
speaker wire will maximize high-frequency crosstalk (i.e. at 300 MHz) as distance
increases, potentially exacerbating the problem of heterodyning between the
amplifiers. Multi-core speaker wire will work perfectly fine in the audio range, and
will not create the potential for heterodyning when wired from a single Class-D
multichannel amp (2, 4, 8, channels etc), however using multi-core speaker wire with
separate Class-D amplifiers where the carrier frequencies may not be synchronized
greatly increases the risk of heterodyne tones