Understanding Impedance Matching

A brief explanation of impedance matching vs impedance protection
Impedance is essentially a measure of the load that a circuit (consisting of everything in the loop from the positive terminal of the amp to the negative- including cables, connectors, and speakers) places on an amplifier. The primary determinant of this load comes from the impedance rating of the speaker or speakers in that loop. If the impedance level of the circuit drops below the impedance rating of the amplifier, the amplifier will likely shut off or go into "protect" mode. Older amplifiers that don't include a protection circuit might even be permanently damaged.
In general, you want to match the impedance from the speakers to the impedance rating of the amplifier. The majority of stereo, full frequency range amplifiers are rated at 8 ohms, but most are also stable down to 4 ohms (check with the manufacturer of the amplifier to be certain). A majority of speakers, including all HTD speakers, are rated at 8 ohms so they can safely be connected to most amplifiers.

If you are using a speaker selector box, this device is wired to automatically connect all of the speakers in parallel. Most speaker selector boxes offer impedance protection by including resistors that prevent the impedance level from ever dropping below a certain level. This is NOT the same as impedance matching. And while you do not have to use impedance matching volume controls when using these impedance protection devices, one by-product of the protection circuit is lower volume. Impedance matching, on the other hand, allows for the best performance from both your amplifier and speakers.


But what happens to the impedance level placed on the amplifier when you want to connect more than one speaker to it? We offer two simple guides to help you determine and correct the impedance to a safe level. In both cases, the speakers must be connected in parallel. This essentially means connecting all of the positive (+) cables together and all of the negative (-) cables together. While speakers can be connected in series, we do not recommend it for reasons not worth discussing here.

When two lines of the same impedance are connected together in parallel, the impedance is cut in half.
The diagram below (click to enlarge) shows a parallel circuit connecting four pairs of 8 ohm speakers to one amplifier (note that one "pair" is represented by a single stereo speaker that accepts both the left and right inputs). The impedance level is corrected across the entire system using three impedance matching volume controls each set to x4. HTD impedance matching volume controls allow you to increase the impedance in multiples of x1, x2, x4, and x8. Whether or not you use 1 volume control, 2 volume controls, 4 volume controls, or 3 volume controls as in this example, the setting (x4) is dependent upon the total number of pairs of speakers in the circuit, not the number of volume controls.