70V Line Speaker Wiring

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What is generally called a 70V Line speaker wiring system is also known as a constant voltage speaker wiring system.

This method of getting audio from amplifiers to speakers is similar in principle to power transmission methods used to distribute electrical power. The power company puts power on a line that carries it a long distance with thousands of Volts. The voltage is stepped down to hundreds of Volts at the building where the power is used. Because lower currents are required at high voltage, the resistance of the line does not cause a loss of power because of heat, and smaller diameter wires can be used.

Similarly, we can apply a high voltage audio signal to a speaker line as long as there is a transformer at the speaker end to reduce the voltage to appropriate values for the drivers. Usually this system is used to distribute audio to a large venue with many speakers. In some cases, a constant voltage system is used to save money on speaker wire.

The reason the North American audio industry settled on 70 Volts is so all audio wiring can be installed without conduit. Many electrical codes of the time stipulated that voltages of 100 or more Volts must be installed in conduit. Modern codes break audio and signaling voltages into various classes. Class 2 covers typical speaker wiring installations. The main implication is that primary wiring and Class 2 wiring must never be placed in the same conduit or junction box. Note that the voltage is not constant, as its name seems to imply, but does actually fluctuate with the audio signal. The tapped transformer Power to the speaker voice coil reaches its rating when the speaker line Voltage reaches 70.

Loudspeaker connection

In the constant voltage system, each speaker's transformer is often provided with multiple input taps so the volume level for different areas can be set appropriately. The connection between the speaker and the transformer are kept short, and often the transformer is built-in to the speaker or driver enclosure.

Each tap will be labeled as to the power provided to the speaker and often the impedance of the particular tap. Always connect speaker transformer primaries in parallel to the constant voltage line.

Amplifier connection

An amplifier with various taps on its output terminals that includes a 70V along with 8 Ohm or other low impedance taps generally has a built-in step up transformer. A low impedance amplifier can also be supplied with an external transformer.

Before solid state amplifiers became available in the 1950s, all amplifiers had output transformers. There is no other safe or practical way to remove the dangerously high plate voltages used on output tubes from the speaker line. The variations in the quality of the output transformer was (and still is) responsible for the differences of fidelity and prices charged for tube amplifiers.

Modern high voltage MOSFET amplifiers are now available capable of driving 70V lines with previously unattainable low distortion that can drive 70V drivers without the use of step up transformers and their attendant distortion.

Alternatives to constant voltage

Low Impedance Speakers

Use of low impedance speakers connected directly to the outputs of a modern solid state amplifier generally provides the best sound quality. The length and size of the speaker cable must be carefully determined. For our typical 400 Watt systems, 14 AWG wire is generally suitable up to 100 feet. For longer lengths consider using 12 AWG or larger. Always used stranded wire for audio.

Locate the amplifier near the speakers

Often the amplifier can be located near the speakers and a balanced audio line and power control signal can be carried over a smaller cable. We can provide adapters for this purpose. Considerations should include ventilation for amplifier cooling. Newer loudspeaker designs called powered speakers feature amplifiers integrated into the enclosure.

Disadvantages of 70V speakers

• Cost: The addition of transformers increases the cost of these speakers
• Frequency Response: Inexpensive transformers may have poor low and high frequency reproduction. Primary applications for constant voltage systems are low fidelity paging and background music systems.
• Insertion Loss: As much as 20% of the amplifier's power is wasted through the transformers. This figure should be accounted for and compared to when calculating the losses encountered through various sizes of low impedance wiring.
• Capacitance: To handle higher powers, transformers have to be scaled up in size. The added capacitance begins to attenuate high frequencies.

Transformers

Atlas PD-60AT 70 Volt Transformer wiring

PD60AT Drivers

Internally mounted transformers Atlas PD-60AT drivers have the following color code:

Red - Voice Coil Negative
White - Voice Coil Positive
Black - Common input terminal
Yellow - 16 Ohm input terminal
Orange- 60W input terminal
Blue - 40W input terminal 
Green - 20W input terminal
Gray - 10W input terminal
Violet - 5W input terminal
Brown - 2.5W input terminal

San Ming 70 Volt Transformer wiring

Internally mounted transformers in San Ming drivers have the following color code:

Top terminals - Voice Coil
Brown and Green - 40W inputs
Blue and Gray - 30W inputs
Orange and Red - 20W inputs
Violet and White - 10W inputs