MACKIE FRS1700 - Manual (Page 15)

15
Owner’s Manual
Owner’
s Manual
The recommended conductor gauges are listed for
AWG (American Wire Gauge) and Metric WG (Metric
Wire Gauge). Note that smaller AWG numbers = larger
conductors and smaller Metric WG numbers = smaller
conductors. The Metric WG is equal to ten times the
nominal conductor diameter in millimeters.
Longer Lengths
For cable lengths over 200 feet / 60 m at 8 ohms,
and over 100 feet / 30 m at 4 ohms, the conductor sizes
needed for less than 0.5 dB power losses are rarely
practical for physical and cost reasons.
As a practical compromise for these situations the
recommended conductor gauge is 10 AWG or 25 metric.
Speaker Impedances
A speaker’s impedance varies with frequency.
For example, it may be 4 ohms at 500 Hz, and 6 ohms
at 120 Hz. What you need to know is the average
(or nominal) impedance across the speaker’s frequency
range. This will be printed somewhere on the cabinet,
or in the specification section of the missing manual.
If you’re just dealing with one speaker per channel,
then make sure that the average impedance is greater
than or equal to 2 ohms.
In bridged-mono mode, make sure that your speaker
impedance is greater than or equal to 4 ohms.
If you’re driving an assortment of speakers, you have
to make sure that the total impedance does not go below
these same levels. There are two basic ways of linking
multiple speakers: series and parallel. The following
sections show how to make the connections and how to
work out the total impedance:
Series
“Series” means that the positive amp output
connects to the first speaker’s positive terminal, the
first speaker’s negative terminal connects to the second
speaker’s positive terminal, the second speaker’s
negative terminal goes to the amp’s negative output.
Series connections are not normally used in PA
applications because it ruins the amplifier’s ability to
damp (control) the speakers. The other snag: if one
speaker goes out, they all do. The audience may not
appreciate this, and things could get ugly fast.
Doing load calculations with series configurations
is easy — just add the loads. For instance, two 4-ohm
speakers in series, equals 8 ohms (4 + 4 = 8).
Parallel
“Parallel” means that the positive amp output con-
nects to the positive terminals of all the speakers,
and the negative amp output connects to the negative
terminals of all the speakers. If one speaker opens in a
parallel configuration, the others will still work, but the
load will change. That lets you breathe a little easier
(the show will go on), except that you may have a dead
speaker and not even know it.
Calculating parallel loads is also easy, as long as each
speaker has the same value — just divide the value by
the number of speakers.
For example, four 8-ohm speakers, connected in
parallel, will equal 2 ohms (8 / 4 = 2).
If the parallel loads aren’t all the same, things gets a
little more complicated, but nothing that you can’t do
with a simple calculator. The total impedance (Z
T
) is
given by the following formula, where Z
1
, Z
2
and Z
3
are
the impedances of your speakers.
There are other, more complicated configurations,
like series-parallel (using a combination of series and
parallel links to arrive at a desired load) and parallel
configurations of unmatched loads (usually not
recommended). But rather than get too deep into
this, let’s just summarize the basics, as they apply
to you and your amp:
• The lower the speaker impedance, the more
power can be put out by the amplifier.
• Driving lower impedance speakers makes the
amplifier work harder and heat up quicker.
• Do not connect a total impedance of under
2 ohms per channel in stereo and mono modes.
• Do not connect a total impedance of under
4 ohms in bridged mono mode.
• Connecting speakers in series or parallel can
drastically alter their frequency response.
• Consider using multiple amplifiers rather than
overloading one.
• Reduce the low-frequency output by engaging
the low cut filter.
• Never plug the amplifier outputs into anything
except speakers (unless you have an outboard
box designed to accept speaker-level levels).
• Be careful in bridged mono mode, as both
speaker wires are live. In this mode, do not
connect the speaker wires to any external
device which is grounded.
ZT =
1
1
Z1
+ 1
Z2
+ 1
Z3
+ . . .