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