70V Audio

What is This Seventy Volt Thing ?
                                           by Allen Barnett

As a commercial sound technician, quite often I am asked to explain the typical 70 Volt Sound System.  In this short article I will attempt to do just that.  But first -- let's take a moment to better understand what we already know about home audio and typical 8 ohm speakers.

Most visitors here are somewhat familiar with home and auto sound systems.  Just about all of these systems use common speakers - 8 ohm impedance. Most available speakers for home and auto systems are typically 8 ohms impedance.  Your home amplifiers  and car radios expect to see standard 8 ohm speakers.  The voice coil of the transducer connects directly to the terminals. Always,  the output terminals on the amplifier show where to connect each speaker and the count is equal to the maximum number of 8 ohm speakers you can connect. Sometimes, in the manual or on the information sticker - you will see some indication that the lowest impedance connected should be  'not less than'  perhaps 2 or 3 ohms.  This varies with your brand and type of equipment.  It is important  to know this specification for your home or auto amplifier ... before you connect too many speakers and 'blow it up'.  The standard rule is one speaker per terminal - no more.

If you decide to connect more than one set of speakers to your home stereo amplifier or put more than one speaker on a terminal, ... you must be sure that the connected load does not exceed the  'not less than'  design parameter of your amplifier.  Most modern amplifiers incorporate some type of internal electronic protection circuit which will cause the unit to shut down when your wiring shorts out - or if you connect too many speakers.  Much of the older equipment uses a common 'one shot' wire fuse - or no protection at all - and could be 'blown' quite easily.  The methods to connect multiple speakers to a standard 8 ohm power amplifier is not the topic of this article.

Commercial Grade 70 Volt Systems are almost always specified when the system designer knows that many speakers will be needed.  Long wire runs,  ever increasing material cost, local fire and building codes and the overall ease of installation are important factors in commercial sound distribution.  Remote volume control and individual speaker level adjustment [along a single wire run] are common requirements in commercial sound.  Eight ohm - home type stereo equipment was not intended for,  does not carry the proper UL ratings -  and does not fit the bill to drive multi-speaker systems.  But, 70 Volt does.  Common home type speakers cannot be used in multi-speaker commercial applications [unless a transformer is added to each speaker unit] -- and 70 volt speakers [with their included transformers] were not intended for home or auto use.. However - in most cases you can safely connect 70 volt speakers to home type equipment -- but they will not be very loud at all. Home type equipment does not produce enough output voltage for 70 volt speakers to move the cone to their design level db output.

Here is the difference.  All 70 volt commercial amplifiers [ANY brand - ANY wattage] will produce 70 volts RMS peak voltage at full output [just at the point of clipping].  A 10 Watt amplifier produces 70 volts [with 10 Watts available] - a 250 Watt unit produces 70 volts [with 250 Watts available].  Each of the above will show you [up to] 70 volts audio/AC on peak signals - so long as it is not overloaded.  Remember though - music is amplified up to a level of 70 volts -on peaks-.  The instantaneous average voltage level you could measure [on music or page] may be much less.

Good commercial design loads the amplifier to slightly less than its rating. Each speaker unit connected to a 70 volt amplifier must incorporate a line transformer to lower the voltage to a level that the actual 8 ohm speaker cone can use.  This transformer is integral to a box speaker or mounted to the speaker basket assembly of a typical bulk speaker.  The transformer has several wires [or taps] - such that we can select the level that we need.  The transformer is marked with wattage indicators.  Typical values may be 1,2,4,5,10 Watts etc., - usually doubling as they go up.  Remember [from above] - a one watt speaker tap connected to a 10 watt amplifier will be just as loud as if it were connected to a 250 watt amplifier.  This is because each 70 Volt amplifier  has the capability to output up to 70 volts [AC as audio] at peak, ... hence the term - constant voltage amplifier.  The wattage you need is determined by the db you want -- see specifications of your speaker. NOTE:  One full watt on a 70 volt line is most often much louder than you may think.

Here's the result of some actual math:  1 watt at 70 volts means that you have 4900 Ohms Impedance.  That's right, ... the transformer / speaker combination measures about 5000 ohms on the 1 watt tap.  If we selected the 2 watt tap the impedance would go down to  2500 ohms  etc.  With this high of a load impedance, the wire loss due to resistance between speakers on relatively small wire is negligible.  Wire size of AWG 18 is adequate for small and even large 70 volt projects.  Seventy Volt is really simple to understand when you realize that the transformer is the trick.  All speaker units simply connect in parallel and total wattage required at the amplifier - simply adds up. System design determines how many db are needed at each speaker - and how much db [sound pressure] a speaker will produce increases with the tapped wattage.

Typical - 8 ohm home type speakers MUST NOT be connected to a 70 Volt System.   A single 8 ohm speaker on a 70 Volt line will Kill Your System.  In fact - that single 8 ohm speaker if connected on a 70 volt system will attempt to draw over 600 Watts. Most speakers are not rated for 600 watts - nor are the other in-line components of a 70 volt system.  Something will surely fail.

Perhaps one of the first commercial uses for 70 volt type systems was with the drive in theater.  Imagine this, ... a thousand speakers connected to the same amplifier.  Anyone want to draw me a series/parallel sketch on that ?  Anyone want to go install it -- or try to keep all of the speakers working ?  Another similar example is our power distribution grid.  Your local power company puts up high-voltage power lines and installs a separate transformer for each user.  High voltage is kept at a constant level - and is conducted through a step-down transformer to a voltage you can use. The power company is able to use a relatively small wire size to support many users - all common to a single 'high-voltage' line   High voltage at low current = high current at low voltage !

There is much more to good commercial sound than knowing just a little about the 70 volt system, ... and there are many accessory items that make-up the complete package.  The 70 volt distribution system design is the staple of the commercial audio industry.

The most important tool for 70 volt testing, repairs or simply finding out what is going on is a special audio impedance meter. All 70 volt sound technicians should have one at their disposal before working on your system.  The most popular is perhaps the TOA - ZM 104. This meter is connected to your speaker line [in place of the amplifier] - puts a tone on the line and reads impedance [Z]. The formula for calculating Wattage is:  P=E square divided by Z.  For 70 volt systems E square will be 70x70 or 4900.  So you take the reading from the Impedance Meter - say 120 ohms Z and divide into 4900. For 120 ohms Z - Power in Watts required will be = 40.8. You cannot use a regular volt ohm meter [DC-VOM] for this measurement as speakers do not work on DC.  You must use an Impedance Meter [which puts the speakers into operation] to read your 70 volt values. Here is a picture of the TOA ZM-104 meter [list price - near $400.]

TOA  ZM-104 Meter

This is an Excellent Tool - designed for many years of reliable field service - and truly professional results.

A  'must have tool'  for 70 volt system troubleshooting.

Need one ? -- call me !


Bonus ... Here is a picture / cut sheet for a typical 70 volt transformer.  This will most often be mounted directly to the speaker 'basket'.  Common [Black] and Tap Lead [Orange]  will couple the 4 or 8 ohm speaker to a 70 volt amplifier -- at 2 watts. etc. Be sure to separate and insulate un-used tap leads.  Use Common [BK] and only one other for proper coupling.

CONCLUSION -- the 70 volt system/method allows you to send one watt of audio from the amplifier to the speaker over a 5000 ohm transmission line with minimal loss within the cable. The 8 ohm direct coupling method would require a much higher current within the cable - and higher current means loss of signal and power. Multi speaker systems need to be 70 volts.

ABTEC carries a complete line of the products you need for 70 Volt projects. Whether you are designing a small system for your store, office or warehouse, -  or need to replace a failed amplifier or other component -- products available through ABTEC meet commercial usage requirements, are UL approved and can be installed by you - or qualified local technicians.  All 70 Volt systems are subject to certain engineering considerations [as the article above suggests]. We will be happy to work with you to supply whatever you need. Simply contact us by phone or email.  ABTEC is located in Atlanta, GA Phone 770-963-9846 or email using button below.


 

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