Fundamental Rules for Successful Installations

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
  • Example - Power companies send 200,000 volts through "small" wire. It drops to 120 volts in your home

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?

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

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

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; the opposite is true when the impedance is divided (raised)
  • Any mixture needed is ok with the amp
  • Any low impedance speaker can be transformed into a 70 volt speaker

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 tthe 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

Autoformers

  • Used to change power and impedance ratios
  • Can turn 8 ohm a
  • 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
  • AtlasIED model AF140 can be your secret weapon

Transformer Subsitutions

  • Use a 25 volt transformer on a 70 volt system and you will deliver 8 times the power to the secondary
  • Use a 70 volt transformer on a 25 volt system and you will 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