Specifications

16
Other design considerations
Speaker transformer saturation
Speaker transformers tend to be fairly small and can vary widely in quality. Many are
thus prone to core saturation at low frequencies, which occurs when the magnetic
field induced in the transformer’s iron core by the audio signal waveform reaches the
limit the core can handle. Even if the instantaneous current increases in the primary
windings, the magnetic flux in the core cannot, so it “clips,” in a way. High frequencies
generally reverse the direction of the magnetic flux well before saturation occurs,
so they are of much less concern.
Saturation causes audible distortion, but in rare yet extreme cases it can also damage
the power amplifier driving the line: as the transformers go out of saturation, their
magnetic fields collapse and induce a large voltage spike across the line. That voltage
spike travels along the line back to the amplifier, which has to absorb it. Interestingly,
this phenomenon is much more likely to happen on a lightly loaded line than on a
heavily loaded one.
Besides using larger, more expensive transformers of higher quality, an effective way
to prevent saturation is to filter out the frequencies most likely to cause it without
adversely affecting the audio quality. The CX models have user-selectable built-in
high-pass filters; the “V” models offer 12 dB-per-octave rolloffs at 50 or 75 Hz, while
the “non-V” models offer 33 or 75 Hz. The input filters can also be switched off, but
we recommend that only if there is adequate filtering in the signal path before the
amp. The amplifiers’ filters are adequate saturation prevention for virtually all good-
quality speaker transformers. However, if any speaker transformers on the line do
not have low-frequency responses at least as low as 75 Hz, you must insert
corresponding high-pass filtering in the audio signal path before the amplifier.
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INSTALLATION TIP:
Once the sound system is installed and operational, turn the amplifier off and take an impedance measurement
across the distributed line at the amplifier output, using an audio impedance meter ( not an ohmmeter). Record
the measurement for later use. If you ever have to make a service call on the system, measure the impedance
again and see how it compares to the recorded figure; it’s a quick and easy way to see if anything in the
distributed line system has been changed. Likewise, measure and record the impedance anytime you’ve
changed a transformer tap,
added or removed a loud-
speaker, or made any other
adjustment to anything on
the distributed line.
Amp
18.5
Z meter