Class D

+1 on @twoleftears suggestion.

As @audio2design points out, EPDR does not apply to class D amplifiers. However, the efficiency at which a class D amplifier operates is reduced as load impedance goes down since the output transistors will be conducting more current during their switching and on-phases for the same power into the load. 

Class D amps are theoretically 100% efficient if the output transistors had zero on-resistance, infinite off resistance, and switched infinitely fast. But, of course, this isn't the case. Typical class D amps are 90% or more efficient into 8 ohms (at max output power), but efficiency drops by approximately 40% into 4 ohms, and 40% again at 2 ohms.

Under normal circumstances, this isn't a huge problem because an audio amplifier in a normal home music environment is typically operating at a small fraction of it's peak power. But under heavy demand (or test conditions), the lower operating efficiency into lower impedance loads will eventually cause the amplifier to get too hot and shut down. The good news is that modern class D amps all have circuitry to protect the amplifier under these conditions. 

As @audio2design also points out, class D amps will also incorporate maximum current limiters which will also limit maximum power into low impedance loads. Since this circuitry operates almost instantaneously (checking current on every switching cycle), this is more likely to limit maximum power into low impedance loads since music peaks can often be many times higher than average power requirements. But these current limits are generally quite high. For example, the Purifi module, which is rated at just over 200w into 8ohms, has a 25A current limiter, but this is what ultimately limits the peak power into loads below about 2 ohms.

But, of course, this isn’t the case. Typical class D amps are 90% or more efficient into 8 ohms (at max output power), but efficiency drops by approximately 40% into 4 ohms, and 40% again at 2 ohms.

I am not sure you wrote this correctly. I think you mean losses go up 40%, or stated as efficiency, the reduction in efficiency goes up by 40%, i.e. 10% reduction becomes 14%?


Conduction losses are I^2 * R. At the same wattage, half the resistance, current goes up sqrt(2) = 41.4%, so losses must go up 100% due to conduction losses, but realistically total losses are going to be a quiescent component (in this case about 11 watts), plus a linear component and a squared component to come up with a very good model.


For reference as example, the NCORE 500 OEM module is about 95.2% efficient at 400W/8 ohms, about 92.6% efficiency at 400W/4 ohms, but drops down to about 87.5% at 400W/2 ohms. Losses are 20, 32, and 67 watts at 8/4/2 ohms, 400W output.


For the NC500, the current limit is 26A. That puts maximum possible RMS power at 2ohms = (26/sqrt(2))^2 * 2 = (26*26/2)*2 = 676W, not too far from the rated power of 550W as a ratio, and even closer if you add losses above to the 550W.

@audio2design Yes, that is what I meant. Sorry if my language wasn't clear. 

zappas OP

Just use your "common sense" when asking why? when ever Class-D is bench tested, why it is they never give the 2ohm wattage, or advertised and compared to the 8 or 4ohm wattage
And sometimes even stated that they try 2ohms but the amp switches off, before they could turn up the test signal level, to get some current flowing into the 2ohm load.

We heard an A/B of what the sound was like into Wilson Alexia with 0.9ohm EPDR bass loading, the 150w-8ohm Gryphon Antillion’s bass sounded absolutely magnificent, but with my 2 x more higher wattage 300w-8ohm Belcanto Ref 600Mono clones (NC500) with linear power supplies, were not anywhere near in the same league. And that can only be due to current delivery into the low impedance.

Cheers George