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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.
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
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 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..

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.
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.
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!
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.
Consumer Versus Commercial

A Typical “Consumer” Home System
8 Ohm loudspeakers
CD Player
Integrated receiver
This System Has 2
Speakers
Connected, Usually
Up To 4 Maximum
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 Commercial
A Typical Commercial BGM and Paging System
Paging
Microphone
Commercial grade 70V
ceiling speakers
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.
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?
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
Typical 8W 70V step down transformer,
usually mounted on the loudspeaker
Primary
High Voltage, Low
Current From 70V
Amplifier
Taps Set The Amount Of Power The
Speaker Will Draw From The Amp
Secondary
Low Voltage, High Current To
Speaker
70V Amplifier With Internal Transformer
Shown
Internal 25/70/100V
Transformer
To 70V
Loudspeakers
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
Now Let’s Take A Look At The Major Components In A 70V
System….
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..

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.
Loudspeakers
4” Ceiling
Speaker
4” Ceiling Speaker
With Tweeter
Surface Mount
Speaker
High Power Ceiling
Speaker
“Pendant” Type
Speaker
Paging Horn
High Power Stadium Horn
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!
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
One Loudspeaker
Wiring A 70V Attenuator
Two Volume Controls
Questions?
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

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
Paging Horns - Used Indoors And Out, Very
Efficient, Voice Only
Surface Mount - Used Indoors, Voice & Music
Ceiling Mount - Used Indoors, Voice & Music
Choosing A Loudspeaker
Coaxial Indoor/Outdoor Speaker - Voice
& Music
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?
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!
Loudspeakers - How Many?
Here Is The Hard Way To Calculate.
Loudspeaker
Ceiling
a
Ceiling
Height
Ear Height
Floor
6” Speaker in a 10ft ceiling
r = (10-5) tan 110/2
r= 7.14
a
r = ( h - L) tan
2
where:
r is the radius of the coverage area
h is the ceiling height
L is the listener height
a is the loudspeaker coverage angle
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
Each Type Has Its’ Own Pros And Cons
Let’s Start With……
Edge To Edge Ceiling Layout
-6dB Down Point
For The
Loudspeaker Or ¼
The Power
Notice The Large Gaps
In The Coverage?
14’ On Center
Minimum Overlap Speaker Layout
See the
overlap
between
adjacent
speakers?
10’ on center
Edge To Edge Layout
Lots Of Overlap
Between
Speakers
Our customers may not want a system
with this much density, nor would they
want to pay for it!
7’ On Center
A Typical Ceiling Speaker Layout
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
This One Goes
To “11”!
SPL Design Goals
Typical 8” Ceiling
Speakers Here,
Tapped At ¼ to1W.
Paging Horns
Required Here
Be Careful If SPL
Is Above 100dB!
Now Let’s Look At 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
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. 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

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
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..

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!
Wire Calculator
Run
200ft
Z
Zsource
21
0.01
AWG
Sq mm
R per foot
Loop Resistance
dB Loss
Status
8
8.36
0.000628
0.2512
-0.1
Okay
10
5.26
0.000999
0.3996
-0.2
Okay
12
3.31
0.001588
0.6352
-0.3
Okay
14
2.08
0.002525
1.0100
-0.4
Okay
16
1.31
0.004016
1.6064
-0.7
18
0.823
0.006390
2.5560
-1.0
20
0.517
0.010152
4.0608
-1.6
22
0.325
0.016142
6.4568
-2.4
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!
Questions????
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
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 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!
Typical Impedance “Sweep” Of An 8
Ohm Loudspeaker
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..
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 <8 Ohms, Where We Expected
Around 625 Ohms!
What Could The Issue Be Here?
Impedance Bridge Continued..
8 Speakers, All
Tapped At 1W
120W Commercial
Grade Mixer Amp
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!
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!
-Available From Several Manufacturers
-Matches Loudspeaker Loads To Amplifier Outputs
-Step Up Or Step Down Voltage Or Impedance
Questions?
Thank you
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