“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. 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.” 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:
1 ----------------------- = Total Net Impedance, or ZT 1 1 1 1 --- + --- + --- + --- Z1 Z2 Z3 Z4 etc...
Where Z = the impedance of each device.
There is a shortcut for this when all of the devices have the same impedance.
1 1 --- = --- x (n) ZT Z
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.
1 1 ------- = --------- x (n) 1200 20,000
This simplifies to
1 1,200 --- = --------- n 20,000
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.