Amp Channels for a Stack: Loads, Power and Counts
Working out amp channels for a stack comes down to three numbers per channel: the load impedance, the combined power rating of the drivers on it, and what the amplifier can deliver into that load. Pair identical boxes on a channel, keep every section on its own channel, and stay above the amplifier's minimum load.
The rules
- Identical boxes share a channel; nothing else does. Subs, kicks and tops each need their own channel so the DSP can filter and limit them separately.
- Know the minimum load for the mode you are using: stereo, parallel or bridged. Many amplifiers are rated to 4 Ω but not 2 Ω.
- Power is per channel. More drivers on a channel do not add amplifier power; they share it.
Loads and combined ratings
Identical drivers in parallel divide the impedance by the number of drivers; in series they multiply it; four in series-parallel return to the impedance of one. Power ratings add either way. Diagrams in impedance and wiring.
| Drivers on one channel | Wiring | Load | Combined rating |
|---|---|---|---|
| 1 × WM1850/8 | Single | 8 Ω | 1800 W AES |
| 2 × WM1850/8 | Parallel | 4 Ω | 3600 W AES |
| 4 × WM1850/8 | Series-parallel | 8 Ω | 7200 W AES |
| 2 × WM18X451 8 Ω | Parallel | 4 Ω | 3600 W RMS |
| 2 × WM18X451 4 Ω | Parallel | 2 Ω | 3600 W RMS |
| 2 × WM18X451 4 Ω | Series | 8 Ω | 3600 W RMS |
A pair of 8 Ω drivers in parallel is a straightforward 4 Ω load: that applies to the WM1850/8 and the 8 Ω WM18X451. With the 4 Ω WM18X451, run a parallel pair only on an amplifier rated for 2 Ω; otherwise wire the pair in series.
Matching power
A parallel pair of WM1850/8 is rated 3600 W AES between them, so the channel should deliver around 3600 W into 4 Ω, with headroom on top for peaks. Four on one channel at 8 Ω is 7200 W AES: very few amplifier channels deliver that into 8 Ω, which is why two subs per channel is the usual arrangement. The full method is in matching an amplifier to your subs, and ratings are compared in power ratings explained.
If the amplifier is smaller than ideal, a limiter set below its clip point keeps it clean. See limiter settings for 18s.
Worked example: a mono stack
Four scoops, two kick bins, two tops, all in mono so the whole dance hears the full mix:
| Section | Boxes and wiring | Channels |
|---|---|---|
| Subs | 4 scoops with WM1850/8, two parallel pairs (3600 W AES per pair) | 2 × 4 Ω |
| Kicks | 2 kick bins, one parallel pair | 1 |
| Tops | 2 tops, one parallel pair | 1 |
| Total | 4 channels: two 2-channel amplifiers |
The processor feeds the same mono signal to each output with its own crossover filters and limiter. On the crossover side, the XR204 4-way active crossover has 2 XLR inputs and 8 XLR outputs, enough for this rig with outputs spare. Expanding later means one more channel per pair of subs.
On cost: four WM1850/8 at £275 each are £1,100 for 7200 W AES of sub drivers, 15.3p per rated watt.
Bridging
Bridging combines two channels into one, but each side of the amplifier sees half the connected load. A 4 Ω pair on a bridged amplifier looks like 2 Ω to each side, which many amplifiers will not tolerate. Check the bridged minimum load in the manual first.
Always check the amplifier's rated power and minimum load for each mode, and use speaker cable heavy enough for the run length.
Questions
How many subs can I run on one amp channel?
Usually two identical subs. Two 8 ohm drivers in parallel give a 4 ohm load; a pair of WM1850/8 is rated 3600 W AES combined, so the channel should deliver around that into 4 ohms. Four per channel needs a channel that can deliver double that, which few can.
Can I run a pair of 4 ohm subs on one channel?
In parallel only if the amplifier is rated for 2 ohms. Otherwise wire them in series for an 8 ohm load.
Can I put subs and tops on the same amplifier channel?
No. Each section needs its own channel so the DSP can apply its own crossover filters and limiters.
What happens to the load when I bridge an amplifier?
Each side of the amplifier sees half the connected load, so a 4 ohm pair looks like 2 ohms per side. Check the bridged minimum load in the manual.