Tech Tips from Sweetwater
How
many devices can be connected to a line output? |
"How many Mackie SRM450 loudspeakers I chain together using
the THRU jacks?"
The number of SRM450 loudspeakers that can be chained together
at a time is limited by the output impedance of the mixer or equipment
that is directly feeding them.
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In general, to insure optimum
signal transfer, the net input impedance of a group of inputs
(ZT) connected in parallel should be no less than 10 times
the output impedance of the equipment acting as the signal
source. This general rule applies to your question because
connecting multiple SRM450's in the manner you describe is
effectively the same as connecting them through a series of
"Y cables."
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In other words, they will be connected in parallel with each other.
The net input impedance of a group of devices connected in parallel
can be found by donning a propeller-laden cap, and using the following
formula:
Where Z = the impedance of each device.
There is a shortcut for this when all of the devices have the same
impedance.
Where ZT is total Net Impedance, Z is the impedance of one device,
and n is the total number of devices.
To get the rest of the way to the answer to your question you need
to know the output impedance of the device that will be driving
the speakers. For the sake of finishing our example, let's assume
we will drive them with a Mackie VLZ Pro mixer. The main outputs
of these mixers have an impedance of 120 ohms.
Now, we know the net impedance of our load must be at least ten
times this, or at least 1200 ohms.
Each SRM450 has an input impedance of 20,000 ohms, or 20k ohms.
We can modify our formula in the following way to come up with an
answer.
This simplifies to
So, you can connect 16.666 Mackie SRM450's to one output of a VLZ
Pro mixer. Of course, unless you can find that 0.6 of a speaker
somewhere you'd probably want to limit it to an even 16.
On the other hand, these limits can be pushed a little bit in a
pinch. Also remember, that this is for just one of the outputs.
If you use both the left and right main output that will allow 32
total speakers to be connected.
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