• Constant voltage systems (70V, 100V lines)

    Disadvantages


    Input and output transformer losses
    Transformers lose some of the power as heat. Speaker transformers can lose up to 1 dB, although amplifier transformers are more efficient. Still, the efficiency loss is probably not significant if we make a comparison with a low impedance installation (i.e. not 70 or 100V) using the same wiring gauge.

    Transformer saturation
    Transformer saturation is a significant issue: besides affecting the safety it also alters the frequency response, since it is the lowest frequencies that saturate the transformer and therefore, when the power is increased, the frequency response is modified in such a way that the bass is reinforced (since the impedance at these frequencies is lowered), to the point that a speaker with a line transformer may be perceived as having "more bass" than the same model of box with a transformer. Transformer saturation also generates harmonic distortion. To avoid saturation, line amplifiers often incorporate a high-pass filter that increases the lower limit of low frequencies. Amplifiers that generate 70 or 100V directly without the aid of a step-up transformer do not have these disadvantages, although speaker transformers usually limit the frequency range in the high frequencies in any case to some extent.

    Less sound quality
    The sound quality of these installations usually relegates their use to installations with small speaker, in particular in-ceiling speakers.

    Cost
    On the other hand, contant voltage systems can be more expensive as the transformers of the speakers and the amplifier (unless you don't need a step-up transformer to get 70 or 100V) add an extra cost. Depending on the application, one may need to consider whether the higher cost a system without transformers exceeds that of a system with transformers and thinner wiring. Also, transformers will add weight to amplifiers (if fitted) and speakers, which might be an inconvenience in some installations.

    The characteristics of each installation will determine whether a high impedance system (70 or 100V) or a low impedance system (without transformers) is more suitable. In installations where there are no long cable distances, an installation speaker system of a sufficiently high impedance (e.g. 16 ohms) may allow us to connect a sufficient number of boxes in parallel (e.g. eight per channel) and will normally be preferable, for cost and sound quality, to a 70 or 100V system.

    Calculation

    As mentioned, the calculation of a 100 or 70V system is very simple.

    In the illustration above we have used three speakers on a 100V line. In one of them the input power selector is at 60W. In another it is 20W. The last speaker has no selector but a transformer with a fixed (input) power of 15W. Therefore for this installation we would need an amplifier channel of at least 95W (60W+20W+15W). There is a rule of thumb that adds 20% to the power of the amplifier to compensate for possible transformer saturation and other causes that could demand more power from the amplifier than the nominal and constitute a safety risk. In this case the amplifier we would need would be 114W (95W plus 20%) or more.

    Connection

    The connection is very simple. We connect the first speaker to the amplifier, and from there we only have to wire the rest in parallel, bridging from one speaker to another. To make this operation easier, higher-end speakers usually have two pairs of connectors to use one as input and one as loopthrough output, otherwise two cables have to be inserted in one connector. Some models have removable connectors (Euroblock, Phoenix), which allow the wiring to be pre-wired, reducing installation time.
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