In the case of a high impedance guitar output (7,000 to 15,000 Ohms or more) driving a relatively low impedance input of a mixer (2,000 to 10,000 Ohms), it’s like connecting a garden hose to a fire nozzle.
The hose just can’t produce enough flow (current) for the size of the opening to maintain the pressure (voltage).
When a signal needs to be split and sent to more than one destination, the impedances of the destinations provide additional paths for the electrical current. This has the effect of reducing the overall impedance presented to that signal.
In terms of our garden hose analogy, we’ve now added a second open nozzle which provides an additional path for the water (less resistance to flow causes reduced pressure in the entire system and each opening has a little less spray).
As a general rule of thumb, it’s wise to try and maintain an input impedance of at least 10 times the amount of the source impedance.
For example, if we are going to connect the output of a mixer to several amplifiers, calculate the total load provided by the amplifiers by using the formulas below.
If that total is approximately 10 times the output impedance of the mixer, then simple passive, parallel splits (like “Y” connections) will usually work fine. The same general principle applies to splitting microphones too. (There can be other issues involving ground loops and isolation.)
The formula for calculating the total load presented by a number of different parallel impedances is:
If there are only two differing impedances, use the following:
or “the product over the sum.”
If there are parallel impedances of the same value, then just divide that value by the number of impedances.
—Two 10,000 Ohm loads = 10000/2 = 5,000 Ohm total impedance.
—Three 20,000 Ohm loads = 20000/3 = 6,666.66 Ohm total impedance.
If a microphone has its signal split to 2 mixers which have a 5,000 Ohm input impedance each, the total load to the microphone is 5,000/2 = 2,500 Ohms.
If a mixer output with an impedance of 100 Ohms is split to 4 amplifiers, each with an input impedance of 20,000 Ohms, the total impedance of the load is 20,000/4 = 5,000 Ohms. This is well within the 10:1 load impedance ratio and illustrates how a mixer’s output can be passively split several times to a bank of amplifiers without the need for an active distribution amplifier.
Al Keltz is a technical writer and general manager of Whirlwind.