Amplifier power ratings are good for window shopping and water cooler chats. If you need to drive a real-world 4 ohm Load, you need the matrix.
Long ago, 8 ohms was established as a practical impedance for a loudspeaker. If made higher, it gets more difficult for the amplifier to force current through the loudspeaker’s voice coil to make it move. If made lower, the amplifier may run out of current, since the load impedance is the primary limiting factor for current flow. While many other impedances are possible, 8 ohms is a historically-supported value used by loudspeaker and amplifier designers. It also happens to be the resistance that will draw approximately 15 amps of current from a 120 Vrms household electrical circuit, so it represents the minimum numerical load value (maximum load) that 120 V 15 A utility power circuit can drive. Most amplifiers have no problem driving 8 ohm loads, but in special applications that use lower impedances, a closer look is needed.
The General Concept
A major decision for the amplifier designer is available current, both in amount and duration, and higher current means higher price. This financial reality provides a motive for limiting the value.
A Specific Application
Photo 1 – Caverness spaces need a lot of sound power for acoustic testing. The sound power level (LW) is an important specification for acoustical test loudspeakers. Shown is Hinkle Fieldhouse in Indianapolis, IN. Testing in this space is what motivated me to look for a better way to drive the the test loudspeaker. The “200 watt-per-channel” amplifier I was using could not drive the load(s) while maintaining the integrity of the sine sweep.
The loudspeaker is an Outline GlobeSourceTM (GS) radiator – a premium “dodecahedron-like” loudspeaker. The published rated power handling is 1600 W continuous, a rather nondescript rating since the 12 transducers are divided into 4 channels. There are 4 circuits of 3 loudspeakers each, so the one-number power rating presumably translates to 400 W per circuit, each rated at 4 ohms. Plugging these values into the power equation suggests that that each circuit can handle 40 Vrms, and since the objective is to produce as much sound power as possible I need to consider driving it to near that value.
Getting the Details
From these tests I now know that I need four amplifier channels that can simultaneously produce 22 VRMS into 4 ohms for at least 14 seconds with stable amplitude over that span. My space budget (it’s a portable system) is 2 rack units (RU). The power equation suggests a minimum amplifier power rating of 121 watts into 4 ohms will do the job.
Three major variables that must be addressed by amplifier designers are
- Output Voltage
- Output Current
- Output Current Duration
Apples vs. Oranges?
- The objective is to get the highest possible SPL from a specific loudspeaker.
- Its obscure power and impedance ratings prompted me to measure them for myself.
- This reveal the required amplifier characteristics to drive the load.
- Amplifier ratings don’t usually provide sufficient detail to know the amplifier’s performance under heavy load with sine wave.
- A CAF report does, and producing one gave me the exact information needed.
- I now have every reason to believe that my test rig can achieve the maximum possible SPL from the loudspeaker without damaging it in the process. The result is the best possible signal-to-noise ratio for acoustic testing in large spaces.