Class D is just Dandy!

Erik,
Buried on page 5 of the NC400 spec sheet is the figure relating to output coil inductance. Typically, 10uH. This equates to an output impedance of 1.25 Ohms at 20kHz and SIGNIFICANT phase shift within the audio band.

Page 8, graph 9.1 is particularly telling. -0.5dB @ 20kHz is nothing to write home about. It also suggests serious phase shift problems well down into the audible spectrum.

I remember seeing more info somewhere about this,but I did'nt save it.

This is related to the nc400 only but I did'nt stress about it too much I just continue to enjoy them.I do like my 600 m's more than the 400, but I'm using my nc400 right now.

I hope this points u in the right direction,I way spend some time later looking for more info.
Kenny.

Hi Kdude,

That output coil is perfectly in line with the output coil in many SS amps.

Based on my simulations, this ends up being no more than about 10 degrees of phase shift at the top end. Far better than ICEPower. :)

Best,

E

George,
I have looked at at the Technics,very interesting,now we need other designers too go in that direction and evolve.

They can’t yet, as that technology needs to be taken up by the major semiconductor manufacturers Motorloa, Sanken, Fairchild, Toshiba ect.
Then the small Class-d manufacturers will be able to get them as well at a decent price.

Remember the guy http://epc-co.com/epc/Applications/ClassDAudio.aspx  that invented these special Gallium Nitride transistors that Technics use in that amp, also invented the Mosfet years back, and look where that ended up, every big manufacturer makes them now.

So hopefully it won’t be long, then all other Class’s of amps will become defunct, and there’ll just be Class-D.

Cheers George

+1 kdude66  This equates to an output impedance of 1.25 Ohms at 20kHz and SIGNIFICANT phase shift within the audio band. 

Just what I’ve been saying all along, would be great if you can post a link to this.
This is why it important to increase the switching frequency by hopefully x5, which then takes the filter up higher by the same amount, away from the audio band. And why the above transistor development (in my last post) is so needed to get it up there.

Cheers George

A little confirmation bias there, George.

The phase shift in the nCore, if ONLY due to the output coil is minor, about 10 degrees, and almost none from 10kHz or below. We don’t actually know the output phase shift without a direct measurement. We can only estimate what a 10 uH coil would do.

The size of the output coil is in the same order of magnitude of most SS amplifier output coils. Maybe 2-3 times bigger.

Also please note the effective nCore 400 output impedance is not 1.25 Ohms at 20 kHz but around 0.003 Ohms. Probably due to feedback being outside the coil.

These are minor quibbles. In fact, the interesting part about the Technics amplifier is that it solves this not by a better output section but by measuring the frequency and phase differences with the actual speaker load and then compensating for it at the input. This is pure frequency and phase equalization. You could do this with any current Class D amp plus a miniDSP if you chose to.

The ICEpower amps on the other hand have much bigger phase shifts. No idea what it’s attributed to.

Of course, time marches on, and progress will be made but what we have now is really very very good.

Best,


E