Impedance is where good systems get damaged. It is not complicated, but the consequences of getting it wrong are immediate and expensive. This is the practical version, without the maths you will never use.
Confirm your amplifier's minimum rated impedance and never go below it. Two 8-ohm cabinets in parallel give 4 ohms, four give 2 ohms. For distributed systems with many speakers, use 70V or 100V line instead.
What impedance actually means here
Impedance, measured in ohms, is how much the speaker resists the current the amplifier pushes. Lower impedance means the amplifier delivers more current and more power, and runs hotter.
Most PA cabinets are 8 ohms. Some are 4. The number printed on the back is nominal; real impedance varies with frequency, which is why amplifiers need margin.
Parallel and series wiring
Wiring two identical cabinets in parallel halves the impedance: two 8-ohm boxes present 4 ohms. Four present 2 ohms, which most amplifiers cannot drive safely.
Series wiring adds impedance: two 8-ohm boxes in series present 16 ohms. That is safe but wastes amplifier power, so it is rarely used in PA.
Staying inside your amplifier's limit
Find the minimum impedance in your amplifier specification, commonly 4 ohms per channel or 8 ohms bridged. Never go below it.
Going below causes the amplifier to overheat, trip protection or fail outright. Protection circuits are a last resort, not a design allowance.
When to use 70V or 100V line
For distributed systems with many speakers over long cable runs, constant-voltage line removes impedance arithmetic entirely. Each speaker has a transformer with selectable wattage taps.
Add the tap settings, keep the total under the amplifier's rating, and the system works. This is standard for retail, hospitality and paging.
| Cabinets (8 ohm each) | Parallel | Series |
|---|---|---|
| 1 | 8 ohms | 8 ohms |
| 2 | 4 ohms | 16 ohms |
| 3 | 2.67 ohms | 24 ohms |
| 4 | 2 ohms | 32 ohms |
Nominal versus actual impedance
The 8 ohms printed on a cabinet is a nominal figure. Real impedance varies continuously with frequency and can dip well below the nominal value at certain points, often near the port tuning frequency.
This is why amplifiers need margin. An amplifier that is marginal into a nominal 4-ohm load may struggle when the actual impedance dips to 3 ohms on a bass note.
It is also why manufacturers specify a minimum load rather than an exact one. Design to the nominal figure but choose equipment with headroom.
Wiring, connectors and common mistakes
Speakon connectors are the professional standard because they lock, cannot be touched live, and carry high current safely. Jack connectors are acceptable at low power but can short while being connected.
A common error is daisy-chaining cabinets without tracking the total load. Three cabinets from one amplifier channel puts you at 2.67 ohms, which many amplifiers cannot sustain.
Check polarity on every cable. One reversed cable in a pair causes low frequency cancellation that sounds like a faulty speaker.
Calculating a distributed system load
On 100V line, add the tap settings of every speaker. Twenty speakers at 10 W each equals 200 W, so a 300 W amplifier gives comfortable margin.
Never load an amplifier to 100 percent of its rating continuously. Aim for 70 to 80 percent so it runs cool and has headroom for peaks.
Record the tap settings in the system documentation. Future changes then take minutes rather than requiring a survey.