70V Systems

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70V Systems

• Steve Brooks
• Atlas Sound Tech Support Manager

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What Are We Going To Cover In This Session?

• A Few Terms Used In This Industry.
• How 70V Systems Work, With a Brief Comparison Of
  Consumer Versus Commercial 70V Audio Systems.
• What Are The Major Components In A 70V Audio
  System?
• What Steps Does One Take In Designing A 70V Audio
  System?
• A Brief Discussion On Loudness, With Some
  Examples.
• The Impedance Bridge, Why You Need One.

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Lets Review Some Of The Terminology

• Constant Voltage -
• A Name Given To A General Practice Begun In The
  Late 1920 And Early 1930s Governing the
  Interface Between Amplifiers And Loudspeakers
  Used For Public Address Systems.
• Early Sound System Engineers (Geeks Today!)
  Looked At this Countrys Electrical Power
  Distribution System And Used Their Practice Of
  Distributing Power, Not Voltage.

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Constant Voltage

• The Key Here Is That Power Voltage x Current.
• Example - I Want 100W Delivered to My House.
  Here Are Two Ways To Do This.
• 10V x 10A 100W (Low Voltage x High Current)
• Large Cable Needed Here To Carry The High
  Current!

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Constant Voltage

• Or, I Could Achieve The Same Result By
• 100V x 1A 100W (High Voltage x Low Current)
• I Can Reduce The Wire Diameter And Still Get 100W
  To My House!

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More Terminology..

• Commercial Audio System - An audio system used in
  commercial spaces, typically used for paging,
  signaling, Voice Evac and background music.
• Distributed Audio System - See Commercial Audio
  System.
• BGM - Background Music. Music that is played in
  an office or store at a low level, designed to
  enhance worker productivity or the shopping
  experience.

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Terminology..

• 25/70/100V Audio Systems- Refers To The Maximum
  Output Voltage Of The Amplifier In The System.
• - 25V Systems Are Typically Found In Schools
  And Prisons. Considered Low Voltage, Conduit
  Typically Not Required.
• - 70V Is The Most Common, May Or May Not
  Require Conduit For The Speaker Lines.
• - 100V Is Used Where Very Long Speaker Runs Are
  Necessary, The Higher Voltage Results In Lower
  Current In The Speaker Line, Resulting In
  Smaller Gauge Wire Needed.

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Where Did 70 Volts Come From?

• Sometime In The Late 1940s, UL Stated That All
  Voltages Above 100V Peak Will Be Installed In
  Conduit.
• To Get Around This And Reduce Install Costs, We
  Look At the RMS Or Average Of 100V.
• Vrms .707 X Vpeak
• .707 x 100 70.7V!- No Conduit!

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Terminology..

• Transformer Taps- Refers To The Amount Of Power
  To Be Delivered To A Commercial Loudspeaker,
  i.e. 1W, 2W, 5W. The Higher The Number, The
  Louder The Speaker Will Be.
• Auto Former - A Specific Type Of Transformer,
  Used On Attenuators (Volume Controls) In A
  Commercial Audio System. Also Available As A
  Stand Alone Product, Used For Stepping Up/Down
  Voltage And Impedance.

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Consumer Versus Commercial

• A Typical Consumer Home System

8 Ohm loudspeakers
This System Has 2 Speakers Connected, Usually Up
To 4 Maximum
CD Player
Integrated receiver
Assuming This Is A 100W Receiver And It Is
Delivering Maximum Power, We Would See About
28VAC At The Speaker Terminals. Remember This
Number.
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Consumer Versus Commercial

• A Typical Commercial BGM and Paging System

Paging Microphone
Commercial grade 70V ceiling speakers
CD Player-BGM Source
This Amplifier Is Rated At 120W. At Maximum
Power, We See 70VAC At The Speaker Terminals.
Commercial Grade 70V Mixer Amplifier
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Whats The difference?

• The Home System Can Only Have A Few Speakers
  Connected.
• The Home System Cant Work With Long Speaker Wire
  Runs Without Using Very Large Diameter Cable, Due
  To Voltage Drops In The Cable.
• The Commercial System Can Have Lots Of Speakers
  Attached To The Amplifier.
• The Commercial System Can Use Smaller Speaker
  Wire.

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So, How Do We Accomplish This?

• By The Use Of Step Up And Step Down
  Transformers
• We Step Up The Voltage/Impedance (Audio
  Signal) Which Lowers The Current Coming Out Of
  The Amplifier, And Step Down The
  Voltage/Impedance (And Raise The Current Back Up)
  To The Loudspeakers!
• Power Companies Do This Everyday, They Step Up
  The Voltage On The Power Lines For Distribution,
  Then Step It Back Down At Your Home!
• Lets Take A Closer Look At Transformers!

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Loudspeaker Transformers
Typical 8W 70V step down transformer, usually
mounted on the loudspeaker
Primary
Secondary
Low Voltage, High Current To Speaker
High Voltage, Low Current From 70V Amplifier
Taps Set The Amount Of Power The Speaker Will
Draw From The Amp
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70V Amplifier With Internal Transformer Shown
Internal 25/70/100V Transformer
To 70V Loudspeakers
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One Final Word About Transformers

• Besides Stepping Up/Down The Voltage, They Also
  Step Up/Down The Impedance Of The Amplifiers
  Output And Speakers.
• By Raising The Impedance, We Can Now Attach Lots
  Of Speakers In Parallel, Without Causing The
  Amplifier To See Too Low Of A Load At The
  Speaker Terminals.

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Next Up

• Now Lets Take A Look At The Major Components In
  A 70V System.

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Amplifiers/Mixer Amplifiers
70V Power Amps
Small Mixer Amps
Large Mixer Amps
Power Amps And Mixer Amps?? Whats The Difference?
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Here Is The Difference..

• Power Amplifiers Typically Dont Have A
  Microphone Preamp And Accept Line level Signals
  Only. Usually Have Multiple Channels.
• Usually Have A Separate Mixer Connected To The
  Inputs.
• --------------------------------------------------
  ---------------------------------
• Mixer Amps Have A Preamp and A Power Amp Section
  And Typically Accept Multiple Line Level And
  Microphone Level Signals.
• Mixer Amps Usually Have Tone Controls
  (Bass/Treble) Included.

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Loudspeakers
4 Ceiling Speaker With Tweeter
High Power Ceiling Speaker
4 Ceiling Speaker
Pendant Type Speaker
Paging Horn
Surface Mount Speaker
High Power Stadium Horn
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Attenuators

• Since We May Need A Way To Control The Loudness
  Of the Speakers, We Install Attenuators.

70V Attenuators Are Wired Into The Speaker Line,
And Are Rated On How Much Power (Speaker Qty. x
Tap Setting) They Can Deliver To The Speakers
Connected!
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While We Are On The Subject

• How Do We Wire Attenuators To Control The Volume
  Of Just One Speaker On A 70V Line?

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Wiring A 70V Attenuator

• One Loudspeaker

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Wiring A 70V Attenuator

• Two Volume Controls

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Questions?
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Designing An Audio System

• Some Things To Consider..
• SPL - How Loud Does It Really Need To Be To
  Overcome The Ambient Noise Levels?
• Coverage - Where Does The Page Need To Be Heard?
• Frequency Response - Paging or BGM or Both?
  Background Music or Foreground Music?
• Power - How Large Will Our Amplifier Need To be?
• Important! What Does The Customer Want Or Expect?

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Selecting A Loudspeaker

• Will They Be Installed Inside or Outside?
• Will They Be Installed Flush or Surface Mounted?
• What Is The Ambient Noise Level The Speaker Must
  Overcome?
• How Far Will The Speaker Be From The Listener?
• What Type Of Sound Quality Is The Customer
  Expecting?

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Choosing A Loudspeaker
Paging Horns - Used Indoors And Out, Very
Efficient, Voice Only
Surface Mount - Used Indoors, Voice Music
Ceiling Mount - Used Indoors, Voice Music
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Choosing A Loudspeaker
Coaxial Indoor/Outdoor Speaker - Voice Music
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How Many Loudspeakers Do I Need?

• We Have Chosen The Amplifier And The Loudspeaker,
  Now, How Many Will I Need?
• Always Measure The Dimensions Of The Spaces
  Needing Audio.
• - Room Length?
• - Room Width?
• - Room Height To Ceiling?
• - Will Occupants Be Seated Or Standing?

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What Is One Of The Most Important Things To
Consider When Designing An Audio System?

• The Customers Ability To Pay For It?
• Can I Get 60 Day Terms From My Vendors?
• Proper Loudspeaker Spacing?
• Does The Customer Get The Keys To The Equipment
  Rack?
• Answer.

Proper Speaker Spacing!
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Loudspeakers - How Many?Here Is The Hard Way To
Calculate.
6 Speaker in a 10ft ceiling r (10-5) tan
110/2 r 7.14
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Dont Have Time To Do The Math?

• Most Speaker Manufacturers Provide A Ceiling
  Speaker Calculator On Their Website!

Make Use Of It!
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We Should Look At Three Accepted Ceiling Speaker
Spacing Scenarios

• Each Type Has Its Own Pros And Cons
• Lets Start With

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Edge To Edge Ceiling Layout
Notice The Large Gaps In The Coverage?
-6dB Down Point For The Loudspeaker Or ¼ The Power
14 On Center
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Minimum Overlap Speaker Layout
See the overlap between adjacent speakers?
10 on center
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Edge To Edge Layout
Our customers may not want a system with this
much density, nor would they want to pay for it!
Lots Of Overlap Between Speakers
7 On Center
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A Typical Ceiling Speaker Layout
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How Loud Does The System Need To Be?

• The System Only Needs To Be Loud Enough!
• Typically, We Shoot For Minimum 6dB Above Ambient
  Noise Levels, 10dB Is Better.
• It Is Much Easier To Turn Down The System At The
  Amplifier Than It Is To Go And Re-Tap All The
  speakers Up!

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Lets talk About Loudness
This One Goes To 11!
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SPL Design Goals
Typical 8 Ceiling Speakers Here, Tapped At ¼
to1W.
Paging Horns Required Here
Be Careful If SPL Is Above 100dB!
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Now Lets Look At Wire
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Loudspeaker Wire

• 70V Speakers Are Always Wired In Parallel, () To
  (), (-) To (-).
• Use Stranded Wire, Do Not Use Voice And Data Wire
  (CAT5). Solid Core Data Wire Is Too Small And Can
  Be Unreliable When Twisted
• Typically, 2 Conductor, Stranded Of Sufficient
  Capacity For Speaker Lines
• 18 Gauge, 2 Conductor Stranded Is Most Common For
  Small To Mid Size Systems. Shielded Cable Is
  Unnecessary!

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Calculating The Proper Size Wire

• If You Are Unsure, Or Have An Install With Many
  Loudspeakers And Long Speaker Runs
• Then
• We Need To Determine The Wire Size For This Job.
  Its Best To Calculate The Total Speaker Load
  That The Amplifier Will Be Required To Drive AND
  The Total Wire Length.

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Calculating The Load Impedance

• The Formula For Calculating The Total Load
  Impedance Comes From Ohms Law.
• ZtE² / P
• Zt Is The Total Impedance You Are Looking For
• E² 70.7 X 70.7 or 5000
• P The Number Of Speakers X The Tap Setting

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Lets Look At An Example

• Example
• A system has 50 speakers, all tapped at 5W each.
  So, 5 X 50 250, this is P in the equation,
  and we already know E² 5000. So, divide 5000 by
  250, and the answer is..20 O impedance!
• Zt E² / P
• The Total Speaker Load That The Amplifier Sees
  Is 20O.

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Now We Factor In The Speaker Wire..

• Load Impedance 20 ohms
• Speaker run is 200 feet, and wire specified (14
  gauge) has a resistance of .0025 ohms per foot.
  So, multiply .0025 X 200 X 2 1 ohm. The 2 in
  the equation is for the loop length, out and
  back.
• Add The 1 Ohm To Our 20 Ohms 21 Ohms
• Looking at the wire gauge chart, I see that we
  will have -.4dB loss, which is acceptable with
  this wire and speaker load!

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Wire Calculator
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Speaker Zones

• How Many Zones Will There Be?
• Are The Zones Volume Zones Or Content Zones?

Each Content Zone Requires A Separate Amplifier!
Each Volume Zone Requires A 70V Attenuator!
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Questions????
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Selecting The Proper Amplifier

• What Are The Power Requirements?
• We Know That Our Total Speaker Load Is 21O (Zt)
• So How Do We Know How To Choose The Correct
  Amplifier Size?
• As A General Rule Add 20 To The Total Load And
  The Closest Available Amp Size (Always Round Up)
• 5000 Divided By 21 Ohms 238W
• 238W 20 47.6W ( 238W) 285W
• Choose An Amplifier Rated At 300-350W / Channel

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The Impedance Bridge

• What Is It, And Why Is It Often Missing From Most
  Toolboxes?
• The Impedance Bridge Is A Meter For Measuring The
  Impedance Of A 70V Speaker Line.
• It Measures Impedance At A Specific Frequency
  Using An AC Signal.
• Gives A Reading In Ohms Or Watts.
• VOMs Do Not Do This!
• The Bridge Cannot Measure Voltage Or Current, And
  Is Typically Not Used Everyday By Installation
  People.

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The Impedance Bridge
If One Calculates The Expected Impedance
(Remember Taps X Speaker Count?) And Then
Measures The Loudspeaker Line, The Numbers Should
Be Close. If There Is A Big Difference, Its Time
To Figure Out Why. The Impedance Bridge Connects
To The () And () Connections Of The Speaker
Line. It Measures The Impedance At One Or Several
Frequencies, Much Different Than Using A Standard
VOM Set For Resistance!
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Typical Impedance Sweep Of An 8 Ohm Loudspeaker
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Using The Impedance Bridge

• Lets Go On A Service Call And Do Some
  Troubleshooting With Our Impedance Bridge.
• We Have A Paging System That Was Working Well
  Until Yesterday. The System Has 8 Speakers
  Tapped At 1W, A 70V Mixer Amp and Paging Is Done
  With A Desktop Microphone. The Customer Says The
  Amp Is Running Hot And The paging Is Distorted.
• We Check Out The Equipment And Wiring In The Rack
  And Everything Appears OK.
• Next Step..

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Impedance Bridge Continued..

• We Calculate The Load
• 8 Speakers X 1W Tap 8W,
• Impedance 5000/8 625 Ohms.
• We Disconnect The Speaker Line And Connect The
  Bridge. The Bridge Measures lt8 Ohms, Where We
  Expected Around 625 Ohms!
• What Could The Issue Be Here?

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Impedance Bridge Continued..
8 Speakers, All Tapped At 1W
120W Commercial Grade Mixer Amp
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Impedance Bridge Continued..
8 Speakers, All Tapped At 1W
Connect Bridge To Speaker Line
Someone Installed An 8 Ohm Speaker On The 70V
Line!
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One Last Tip For You!

• You Have A Customer Who Wants To Upgrade Their
  Current SystemAdd More Speakers, Maybe Some
  Volume Controls In Each Room And They Are On A
  Budget.They Have A Consumer Grade Stereo
  Receiver That They Want To Re-Use!
• How Can We Convert A Consumer OR Non 70.7V
  Amplifier To Run 70.7V????

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Use An Auto Former!
-Available From Several Manufacturers -Matches
Loudspeaker Loads To Amplifier Outputs -Step Up
Or Step Down Voltage Or Impedance
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Questions?
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Thank you