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History of 70 Volts
• 100 volts peak = 70.7 volts
• 70 Volt is the highest "safe" voltage authorities permit.
• [Voltage X Current = Power] Power remains the same as long as the voltage/power ratio remains the same
• 100 watts of power = 10 amps X 10 volts or 100 watts of power = 100 volts X 1 amp
• Raise the voltage = Lower the current
Why Use 70 Volts?
• Lots of speakers on one amp
• No need to home run each speaker
• Higher voltage allows use of smaller wire
• Impedance matching speaker selectors restrict the number of speakers possible
• Speakers can easily be added and removed
• Economical
• No need to calculate impedance, just total power
• Easy to design!
Disadvantages of 70 Volts
• Limited frequency response
• Considered high voltage by codes
What Parts Make up the Basic Design?
• Source components
• Mixer Amp
• Wire
• Volume Controls
• Attenuators
• Step Down Transformers
• Speakers
• Autoformers
• Transformer Substitutions
Types of Source Components
• CD players
• Juke boxes
• VCR audio
• Digital message announcers
• Cassette decks
• Tuners
• Satellite receivers
• Telephone systems
• Microphones
Mixer Amp
• Mix sources, i.e., Microphone and music
• Has pre-amp & power amp circuitry
• Provides level control for each input
• Provides master level control for output
• Usually provides both mic and line level inputs
• Usually provides several outputs
Wire
• Most commercial systems require 18 gauge 2 conductor WITHOUT a shield
• Stranded wire is necessary, solid will not work
• Wire gauge depends on power load, as voltage increases, the resistance of the wire becomes less significant
• Higher voltage allows smaller gauge wire
• Wire connections are all parallel
Volume Control
• Pots - rheostats used to adjust levels for just one speaker; low power handling and poor durability;show variable resistance to amp
• L-Pads - two pots in one switch that show a constant load to the amp while changing resistance to speaker; same drawbacks as pots
• Autoformers - clean, durable way to adjust level; adjust level; changes impedance seen by the amp. Highest power handling
Attenuators
• Autoformers, not transformers; one winding with multiple connections to various points in the winding
• Available in 10, 35, 100 watt versions
• Wattage rating refers tthe total load they can handle
• Available in standard or Decora
• Colors include white, ivory, almond, & stainless steel
• Require large boxes or plaster rings
• Able to control any load at or below it's rated value, i.e.., A 100 watt attenuator will work with just a one watt load, or with 100 speakers tapped a one watt
Step Down Transformers
• Transforms 70 volt signal down to 8 ohms, or high voltage and low current to low voltage and high high current
• Attached the speaker in most cases
• Usually have one common lead and multiple primary leads on the primary side (input)
• Usually two leads on the secondary (output), but may have several, i.e.., 4 ohms, 8 ohms, and 16 ohms
• Step-down transformers are what the amp sees, not the speaker
• Frequency response varies depending on materials used to build it and size of core and windings
Speakers
• Can be cone drivers, horns, or enclosed assemblies
• 8 ohms is most common; other impedance may be 4 or 16 ohms
• If the speaker impedance is doubled (lowered), the speaker will draw twice the power
• If the speaker impedance is divided (raised), the speaker will draw twice less power
• Any mixture needed is ok with the amp
• Any low impedance speaker can be transformed into a 70 volt speaker
Autoformers
• Used to change power and impedance ratios
• Can turn 8 ohm amp into 70 volt amp
• Can jump 25 volt output to 70 volt output
• Can drop 70 volt output to 25 volt
• May be used as step-down transformer for high powered speakers
Transformer Substitutions
• Use a 25 volt transformer on a 70 volt system and deliver 8 times the power to the secondary
• Use a 70 volt transformer on a 25 volt system and deliver 1/8 the power to the secondary
• Substitution is acceptable as long as secondary voltage does not exceed the transformers' rating
• A 4 ohm load (speaker) on an 8 ohm secondary tap will draw twice the power
• An 8 ohm load on a 4 ohm secondary tap will only draw half the taps power
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