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70V Systems Steve Brooks Atlas Sound Tech Support Manager InfoComm International Brings You AND Career Development ■ News Network Industry Awareness ■ Market Research Integrated Systems Events ■ Certification Stay informed! Sign up for…

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70v systems

70V Systems

Steve Brooks

Atlas Sound Tech Support Manager

infocomm international brings you
InfoComm International Brings You


Career Development ■ News Network

Industry Awareness ■ Market Research

Integrated Systems Events ■ Certification

stay informed
Stay informed!

Sign up for…

  • Executive Director Randal A. Lemke’s Executive Update
  • InfoComm International’s e-Newsletters:


what are we going to cover in this session
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.
let s review some of the terminology
Let’s Review Some Of The Terminology
  • Constant Voltage -
  • A Name Given To A General Practice Begun In The Late 1920 And Early 1930’s Governing the Interface Between Amplifiers And Loudspeakers Used For Public Address Systems.
  • Early Sound System Engineers (Geeks Today!) Looked At this Country’s Electrical Power Distribution System And Used Their Practice Of Distributing Power, Not Voltage.
constant voltage
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!

constant voltage7
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!

more terminology
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.
  • 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.

where did 70 volts come from
Where Did 70 Volts Come From?
  • Sometime In The Late 1940’s, 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!
  • 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.
consumer versus commercial
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.

consumer versus commercial13
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

what s the difference
What’s The difference?
  • The Home System Can Only Have A Few Speakers Connected.
  • The Home System Can’t 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.
so how do we accomplish this
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!

Let’s Take A Closer Look At Transformers!

loudspeaker transformers
Loudspeaker Transformers

Typical 8W 70V step down transformer,

usually mounted on the loudspeaker



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

70v amplifier with internal transformer shown
70V Amplifier With Internal Transformer Shown

Internal 25/70/100V Transformer

To 70V Loudspeakers

one final word about transformers
One Final Word About Transformers
  • Besides Stepping Up/Down The Voltage, They Also Step Up/Down The Impedance Of The Amplifier’s 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.
next up
Next Up

Now Let’s Take A Look At The Major Components In A 70V System….

amplifiers mixer amplifiers
Amplifiers/Mixer Amplifiers

70V Power Amps

Small Mixer Amps

Large Mixer Amps

Power Amps And Mixer Amps??

What’s The Difference?

here is the difference
Here Is The Difference..
  • Power Amplifiers Typically Don’t 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.

4” Ceiling Speaker With Tweeter

High Power Ceiling Speaker

4” Ceiling Speaker

“Pendant” Type Speaker

Paging Horn

Surface Mount Speaker

High Power Stadium Horn


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!

while we are on the subject
While We Are On The Subject
  • How Do We Wire Attenuators To Control The Volume Of Just One Speaker On A 70V Line?
wiring a 70v attenuator
Wiring A 70V Attenuator

One Loudspeaker

wiring a 70v attenuator27
Wiring A 70V Attenuator

Two Volume Controls

designing an audio system
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?
selecting a loudspeaker
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?
choosing a loudspeaker
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

choosing a loudspeaker32
Choosing A Loudspeaker

Coaxial Indoor/Outdoor Speaker - Voice & Music

how many loudspeakers do i need
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?

what is one of the most important things to consider when designing an audio system
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?


Proper Speaker Spacing!

loudspeakers how many here is the hard way to calculate
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

don t have time to do the math
Don’t Have Time To Do The Math?

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

Make Use Of It!

we should look at three accepted ceiling speaker spacing scenarios
We Should Look At Three Accepted Ceiling Speaker Spacing Scenarios

Each Type Has Its’ Own Pros And Cons

Let’s Start With……

edge to edge ceiling layout
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

minimum overlap speaker layout
Minimum Overlap Speaker Layout

See the overlap between adjacent speakers?

10’ on center

edge to edge layout
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

how loud does the system need to be
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!
let s talk about loudness
Let’s talk About Loudness

This One Goes To “11”!

spl design goals
SPL Design Goals

Typical 8” Ceiling Speakers Here, Tapped At ¼ to1W.

Paging Horns Required Here

Be Careful If SPL Is Above 100dB!

loudspeaker wire
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!
calculating the proper size wire
Calculating The Proper Size Wire

If You Are Unsure, Or Have An Install With Many Loudspeakers And Long Speaker Runs…


We Need To Determine The Wire Size For This Job. It’s Best To Calculate The Total Speaker Load That The Amplifier Will Be Required To Drive AND The Total Wire Length.

calculating the load impedance
Calculating The Load Impedance
  • The Formula For Calculating The Total Load Impedance Comes From Ohms Law.
  • Zt=E² / 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
let s look at an example
Let’s Look At An 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 Ω impedance!
  • Zt = E² / P
  • The Total Speaker Load That The Amplifier “Sees” Is 20Ω.
now we factor in the speaker wire
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
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!

selecting the proper amplifier
Selecting The Proper Amplifier
  • What Are The Power Requirements?
    • We Know That Our Total Speaker Load Is 21Ω(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
the impedance bridge
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.

VOM’s Do Not Do This!

The Bridge Cannot Measure Voltage Or Current, And Is Typically Not Used Everyday By Installation People.

the impedance bridge56
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, It’s 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!

using the impedance bridge
Using The Impedance Bridge

Let’s 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…..

impedance bridge continued
Impedance Bridge Continued..

WeCalculate 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 <8 Ohms, Where We Expected Around 625 Ohms!

What Could The Issue Be Here?

impedance bridge continued60
Impedance Bridge Continued..

8 Speakers, All Tapped At 1W

120W Commercial Grade Mixer Amp

impedance bridge continued61
Impedance Bridge Continued..

8 Speakers, All Tapped At 1W

Connect Bridge To Speaker Line

Someone Installed An 8 Ohm Speaker On The 70V Line!

one last tip for you
One Last Tip For You!

You Have A Customer Who Wants To Upgrade Their Current System…Add 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????

use an auto former
Use An Auto Former!

-Available From Several Manufacturers

-Matches Loudspeaker Loads To Amplifier Outputs

-Step Up Or Step Down Voltage Or Impedance