What does "70(25)-Volt" mean? Are they interchangeable?
Most constant voltage distributed sound systems use a 70 volt signal. This is deemed "low voltage," thus not held to the more stringent safety standards as the higher voltage you find in electrical outlets (120 volts). Licensed electricians are required to install higher voltage circuits, but most 70 volt systems may be installed by low voltage contractors who know something about sound, and generally charge a fair rate for their labor. As industry and regulatory agencies picked 70 volts (100 watts peak has a RMS of 70.7 volts) as a safe standard, some municipalities still require conduit and installation by a licensed electrician for even 70 volts. Using the same principle (high voltage/low current) to distribute sound to a lot of speakers over long wire runs,25 volts is a slightly less efficient way to achieve similar results. The advantages that 25 volt systems offer include no need for conduit and freedom from local safety codes. Because of the additional safety of the 25 volt standard, almost all educational facilities require the 25 volt standard. Additionally, the 25 volt standard is almost exclusively specified in correctional facilities, as well. Whether it's 25 or 70 volts, system layout is the same. 70 volt attenuators work with 25 volt signal. Atlas Sound offers a dual voltage transformer that has primary taps for both 25 and 70 volts, the LT-72. This allows dealers who install both types of systems to order and stock a model that is useful no matter what the application.
Why isn't there a need for shielded wire in 70-Volt installations?
Shielded wire is big in commercial audio before the power amp. Once line level signal is amplified to speaker level (25 or 70 volts), shielding is not needed. Shielding is used to re-direct electromagnetic interference (EMI) and radio frequency interference (RFI). While these types of interference can have a devastating effect on relatively low mic and line level signals, there is virtually no way EMI/RFI can muster the energy to make a magnet/coil circuit in a speaker produce any sound.
Why are volume controls only necessary when sound levels need to be changed in various areas on the same system?
They may be "nice to have" but not necessary when environmental and acoustical conditions are equal through-out an entire listening area. Filling a big box with sound may require no need for local level control. For example, a large call center with cubicles may not need different listening levels from one place to another because all listening needs are equal through out. The volume control knob on the pre-amp or mixer amp may be all that is needed. Conversely, an architectural firm with 30 different listening areas spread through out an old renovated warehouse in the loft district would require different level controls in each area. One area may have hardwood floors and four glass walls and a high wooden ceiling, while another may have carpet with peg-board, brick, and drapes with a drop ceiling.
What is a tap setting? Why do you have them? Can they be changed?
The tap setting allows the installer to determine how much power that speaker will receive. Multiple tap settings allow the same speaker to be set at two different levels in the same application. For example, the same speaker maybe be used through out a restaurant that has two different ceiling heights... 8' and 12 '. If the speakers were all hooked up to the same volume control, what's comfortable for listeners under the 12' ceiling may be too loud for those under the 8' ceiling. Turn it down to make it comfortable for the folks under the 8' ceiling, and those under the 12' can barely hear. Why not give separate volume controls for each ceiling height? The manager may want the front dinning area on one volume control knob and that area has four speakers at 12' and four speakers at 8'. The ability to tap the 12' high speakers at a higher tap setting allows the installer to send more power to the speakers that have a greater distance to go before sound reached the listener. This will yield even sound pressure level at ear height no matter what the ceiling height. Taps are frequently changed during the test phase of the installation or soon after the installation is completed. The installer may decide that the speaker in the lobby is to loud, so he has to get up on his ladder, disconnect the positive lead on the primary side of the transformer, and attach it to a lower rated tap. Tap settings are typically designed to either double or half the power that is sent to the secondary side of the step-down transformer, and then to the speaker. This coincides with the 3dB rule that states, "If you double the power, you get a 3db gain in SPL (Sound Pressure Level)." And, conversely, if you divide the power by two you'll realize a 3dB loss in SPL.
Why are transformer color codes important?
The color codes allow the installer to glance at a mess of wire leads and grab the two that he needs. Otherwise, the identification of each tap or lead would involve a search for a potentially smeared number on the side of a transformer, up in a dark ceiling. When the installation involves hundreds of speakers, colors make the process much easier. Also, trouble shooting or re-tapping during testing is easier with colored leads.
Are the color codes an industry standard?
Colors vary from transformer to transformer. For example, the 1 watt tap on the HT-47 is blue, but the 1 watt tap on the LT-70 is yellow. They can even vary on the same transformer, based on the voltage there're driven by. For example, the brown lead on the LT- 72 will send 4 watts to the output of the transformer when driven by 70 volts, but only a 1/2 watt when driven by a 25 volt signal.