Class D

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

zappas OP


Ask yourself why all the Ncore data sheets have the 2 ohm power ratings in the data sheet right near the top. Some people never move forward and have to bring up a one time comparison of a $40,000 dollar amplifier against $10,000 amplifier that no one can verify.  Well not really $10K. They were clones of the Bel Canto, so unknown pedigree, and unknown performance. Now we are talking $40,000 something that has likely never seen a measurement that confirmed performance.

One day George will discover that EPDR is not the same as current delivery. That day is not today.

I think I may understand Equivalent Peak Dissipation Resistance which I had never heard until recently in one of the class d amplifier threads. Audio2design and Jaytor have nicely explained that.  It is very informative and interesting. Thanks