HamCall (October 1991)/HamCall (Whitehall Publishing)(1991).bin/bcast/audvid/70volt.txt

Text File | 1990-10-14 | 10KB | 154 lines

             NOTES ON 70-VOLT AND DISTRIBUTED SYSTEM PRESENTATION
                           NSCA, SEPTEMBER 10, 1985

     The so-called 70 volt line distributed loudspeaker system wiring scheme
offers a flexible means of operating multiple loudspeakers connected to
singular amplifier lines.
     The definition of the "70 volt" system is one in which 70 volts (70.7
volts) represents the maximum operating VOLTAGE delivered from the driving
amplifier, regardless of the particular power level capability of that
amplifier.  A "70 volt" speaker transformer with power level taps of 1, 2, and
4 watts, will draw 1, 2, or 4 watts, depending on the tap selection, when the
line voltage fed to the transformer's primary reaches 70.7 volts.
     The 70 volt and other constant voltage (e.g. 25 volt, 50 volt, 140 volt)
systems were devised to provide an economical means of driving many speakers
over long signal lines with low loss.  Higher voltage on the line allows use
of less current in the wire, which in turn causes less voltage drop and power
loss in the wire itself and allows use of smaller less expensive wiring.
     It is not necessary to achieve 70 volts in the speaker lines to
successfully operate a 70 volt system, but following the same logic that
applies to any amp and speaker combination, the square of the voltage divided
by the number of ohms representing the total system load will determine how
much power will actually be distributed through the system.
     An analogy of distributed system operation can be made from everyday
house wiring to illustrate how a distributed system works: in a house there is
an electrical conduit carrying 120 volts all over the house to wall outlets.
A 20-amp circuit breaker feeds the line.  At any outlet you can plug in a lamp
to give you as much light as you need in that particular location, however,
since the line is supplied by a 20 amp breaker, you can only plug in 2400
watts (120 volts X 20 amps) of total load before you run out of power and trip
the circuit breaker. You can use twenty-four 100-watt lamps or fourty-eight
50-watt lamps or a hundred 24-watt lamps and so on, to use all of the
available power, but you might also only use one lamp in each room drawing
only a few hundred total watts, which will leave power to spare.  The
distributed system is a constant voltage system.
     An amplifier capable of developing 70 volts into a load of 8 ohms can be
used to provide 600 watts in a 70 volt system.  This much power might be used
to drive 200 ceiling speakers each with their transformer taps set to 3 watts,
or half of all the speakers set to 4 watts and half set to 2 watts to create a
loud zone-quiet zone arrangement where the two zones differ in sound level by
3 dB (3 dB is half/twice power and a just noticeable difference in speech
sound level).
     Substituting an amplifier with a maximum 50-volt / 4-ohm (600 watts) load
capability rating and doing nothing else, would drop the available power to
this system to 300 watts and provide each speaker in the system with half the
power indicated by its transformer tap setting. Since this substitute
amplifier is rated to drive a 4-ohm load where the original amplifier was
rated at 8 ohms, another 200 speakers--a doubling of the original number--
could be added to the system and would be driven at the same power level as
the original 200 units, or half the rated tap setting value, allowing the full
600 watt potential of the substitute amplifier to be realized.
     The Ohm's Law-based equations provide an easy way to determine just how
much voltage, current or power is involved in particular system designs or
what the total loading on a distributed line will be based on the wattage taps
used and number of speakers connected to the line.  JBL tech note, Volume 1,
Number 2: "70-volt Distribution Systems Using JBL Industrial Series
Loudspeakers," gives tables and other valuable information to aid in
distributed system design.
     Loudspeaker sensitivity and impedance rating play a big part in overall
system efficiency.  Speakers of different impedances draw different amounts of
power from a constant voltage (e.g. the 70 volt system) source.  For example,
let's use two commercially available speakers, A and B.  The pertinent
specifications of the two devices are as follows:
SPEAKER A: Sensitivity = 97 dB SPL, 1 W, 1 m and impedance = 8 ohms.
SPEAKER B: Sensitivity = 86.5 dB SPL, 1W, 1m and impedance = 6 ohms.
     Speaker transformers have insertion loss that is due mostly to resistive
losses in the transformer, so the transformer loss itself can be calculated as
if the loss element is a resistor.  If we know that some typical transformer
has one dB of insertion loss when working into its rated load impedance
(usually 8 ohms), then we can calculate backwards and find the transformer's
equivalent resistance to be 2 ohms.  We know this from the fact that a
transformer that has 1 dB of loss delivers 4 watts to a speaker when its 5-
watt tap is used.
     A speaker with lower impedance will draw more power from a constant
voltage source, and if the source had negligible resistance itself, then the 4
watts available to the 8-ohm speaker A would become 5.3 watts but it's not
quite that simple.  If we place speaker A in a series circuit with our typical
transformer, we find that the speaker drops 4 watts and the transformer drops
1 watt to make up the 5-watt total.  The current across this combination is
0.707 ampere, which means the voltage drop across the 10-ohm load (8 ohms for
the speaker and 2 ohms for the transformer's resistive loss) is 7.07 volts.
     Substituting speaker B across the same constant voltage produces 0.884
ampere of current through the load (now 8 ohms total), and causes 1.56 watts
to be lost in the transformer and 4.69 watts to be delivered to the speaker.
     The difference between the 4.69 watts for speaker B and the 4 watts for
speaker A is only a little over one-half dB.  What might have seemed to be a
potential advantage is eaten up by the transformer, and worse, the lower
impedance speaker B is now pulling 6.25 watts from the line, which is 25% more
power and will mean that you can only connect 80% of the number of of speaker
A's you would have been able to connect to the line before you exceed the
amplifier's available power.
     The issue of speaker sensitivity is much more important when many
speakers are used and the "dB's for dollars" problem can eat up profits
quickly.  Speaker A offers 97 dB SPL for 1 watt at the standard 1 meter
distance.  Speaker B offers 86.5 dB SPL for the same watt.  This means that to
achieve the same sound level at the same distance, speaker B will require more
than 10 times more power than speaker A.  Another way to look at it might be
that it will require at least 3 times as many of speaker B to provide the same
sound level as speaker A, and in some physical situations up to 10 times as
many of speaker B.
                        OHM'S LAW-DERIVED EQUATIONS
                        ---------------------------
   TO FIND WATTS:                        TO FIND AMPS:
(volts squared) divided by ohms       volts divided by ohms
(amps squared) X ohms                 watts divided by volts
volts X amps                          square root of (watts divided by ohms)

   TO FIND OHMS:                         TO FIND VOLTS:
volts divided by amps                 amps X ohms
(volts squared) divided by watts      watts divided by amps
watts divided by (amps squared)       square root of (watts X ohms)

              JBL AMPLIFIER / AUTOFORMER OUTPUT CHARACTERISTICS

       JBL        Stereo        Bridged       Maximum        Maximum
    Amplifier     Voltage       Voltage       Power/         Power/
      Model       Output        Output        Channel      Mono Bridge
    ---------     -------       -------       -------      -----------
      6215        17 VAC        27 VAC         45 W           90 W
      6230        25 VAC        50 VAC        150 W          300 W
      6260        35 VAC        70 VAC        300 W          600 W
      6290        50 VAC       100 VAC        600 W         1200 W

JBL 70 volt
Autoformer/         Input        70 volt      Impedance     Impedance
Transformer #     Impedance      Output       Matching *      Ratio
___________      _________      _______      __________    _________
   9375            4 ohms         100 W        8-32 ohms       1:8
   6217            4 ohms          45 W        111 ohms        1:28
   6237            4 ohms         150 W        36 ohms         1:9
   6267            4 ohms         300 W        16 ohms         1:4
   6297            4 ohms         600 W         8 ohms         1:2
* can be used for step up or down.

              JBL AUTOFORMER/TRANSFORMER SPECIFICATIONS
             -------------------------------------------
  Frequency Response : +,- 0.5 dB, 25 Hz to 20 kHz
                 THD : Less than 0.5 % , 25 Hz-20 kHz at rated power.
      Insertion Loss : Less than 0.75 dB
         Connections : Screw/Solder Lugs
            Mounting : Mounting Brackets Attached

 Model     Power     H   x   W   x  D      Weight     Shipping Weight
-------    -----    ---    ----    ---     ------     ---------------
 9375      100 W    3.5    4.13    3.0        6              7
 6237AFMR  150 W
 6237XFMR  150 W    4.0    3.25    3.5        7              8
 6267AFMR  300 W
 6267XFMR  300 W    4.5    3.5     3.75       9             10
 6297AFMR  600 W
 6297XFMR  600 W    4.5    3.75    3.75      10             11

NOTE: Suffix AFMR denotes autoformer, XFMR denotes dual winding true
transformer.