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 CD
Player Integrated receiver 8 Ohm loudspeakers Assuming This Is A
100W Receiver And It Is Delivering Maximum Power, We Would See
About 28VAC At The Speaker Terminals. Remember This Number. This
System Has 2 Speakers Connected, Usually Up To 4 Maximum
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Consumer Versus Commercial A Typical Commercial BGM and Paging
System Paging Microphone CD Player-BGM Source Commercial Grade 70V
Mixer Amplifier This Amplifier Is Rated At 120W. At Maximum Power,
We See 70VAC At The Speaker Terminals. Commercial grade 70V ceiling
speakers
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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 High Voltage, Low Current From
70V Amplifier Taps Set The Amount Of Power The Speaker Will Draw
From The Amp Low Voltage, High Current To Speaker Primary
Secondary
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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.
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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 4 Ceiling Speaker With Tweeter
High Power Ceiling Speaker Pendant Type Speaker Surface Mount
Speaker Paging Horn 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? A. The Customers Ability To Pay For It?
B. Can I Get 60 Day Terms From My Vendors? C. Proper Loudspeaker
Spacing? D. 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 The Power 14 On
Center
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Minimum Overlap Speaker Layout 10 on center See the overlap
between adjacent speakers?
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Edge To Edge Layout 7 On Center Lots Of Overlap Between
Speakers Our customers may not want a system with this much
density, nor would they want to pay for it!
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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 Paging Horns Required Here Typical 8 Ceiling
Speakers Here, Tapped At to1W. 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. Zt=E / P Zt Is The Total
Impedance You Are Looking For E = 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 E = 5000. So, divide 5000 by 250, and the answer
is..20 impedance! Zt = E / P The Total Speaker Load That The
Amplifier Sees Is 20.
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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!
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 K now That Our Total Speaker Load Is 21 (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