Class D

I fail to understand one thing - people spend "their own money" to buy the class D amp they like. So what bothers George? Don't know what he is trying to achieve jumping on all the Class D threads and making a mess. The only reason I can think of is - his light attenuator must not be compatible with Class D and hence he does not want people to adapt them. George, can your attenuator drive Class D amps? I might look into Purifi or GAN based amp if I upgrade in the future. So what is your agenda? 

I smell another fuser

So what bothers George?

People, shillers and manufactures saying Class-D already great and is better than the best linear amps, which it’s far from yet.

So what is your agenda?

That Class-D has trouble with low <2ohm impedances, and that it’s switching frequency is too low for it’s output filter to remove completely without introducing large phase shift down into the audio band.

And if you accept it for what it is, nothing will be done to fix those inherent problems.

The Technics SE-R1 and maybe the SU-R1000 has done something about the switching frequency, but still the 2ohm load capabilities are not yet fixed, if you check independent tests.

can your attenuator drive Class D amps

you are a **** why bring that up if you know the answer already. If they have >33kohm input impedance buffers it can

But no need, I go direct into my Class-D’s from my dac
I own and listen to a pair of direct dac signal feed Hypex NC500 modules (with no buffers) with very big linear supplies, into my second system, which speakers are a nice benign 6ohm load.

It may not double power into 2 ohms though, but that’s not important

That more fake than anything!
Most Class-D’s, independent tests, can’t better the 4ohm wattage with the 2ohm load, if they don’t turn off first, let alone double it. That’s how "strangled" they are for current.

With regards to that bit about ’oscillate’: most modern class D amps already oscillate as part of their modus operandi. You can’t somehow make them oscillate *more*; the idea is rubbish.

No you are. You know exactly what was meant by that, don't play the arrogant card, just another ***** side step to protect an upcoming Class-D from you

:) sheesh.


In case you didn't mean that as a joke (because its pretty funny):

Think about it- if an amp has an output impedance (Purifi for example) that is lower than a foot of 14gauge wire, how is it really going to be affected by a 2 ohm load? The answer is it isn't. Now the power supply might not support the current the amp would demand at full power, but all that means is you would clip the amp at some point. When that happens, simply turn down the volume. Why is this so hard to understand??

The idea that you can make a self-oscillating class D amp oscillate when in fact they already oscillate for a living is rubbish. Maybe you can explain that??


When an amplifier is oscillating, another way to put that is its Phase Margin has been exceeded. This simply means that there is so much phase shift at some high frequency that the negative feedback applied is now positive feedback- and presto! The amp oscillates. If you don't believe me on this, read the works of Norman Crowhurst. You can find his books on Pete Millet's excellent resource site:http://www.tubebooks.org/technical_books_online.htm

Now I doubt you'll be doing you're homework, but I provided the link so you or others can, so you or others can follow along. So try this on:

In a self oscillating class D amp, the phase margin is exceeded ***on purpose***. The oscillation is used as the switching frequency. Its got a certain elegance, but now that you brought it up, one of the advantages is that its quite literally impossible to get it to oscillate any more than it already is. So its impervious to load conditions in that regard.

I don't know what his agenda is, but given he spreads grossly false information and refuses, absolutely refuses to educate himself such that he understands the statements he makes, I don't get it at all.

Come on George several messages above I went into great detail about what EPDR is and how that impacts a Class-D amplifier.  I went into great detail about hard current limiting and explained why the watts don't double at 2ohms. I think I made it simple enough that many people can understand.

Tell me, at this point, who do you think people are going to believe, someone who goes into great detail to explain the process, relates it to actual amplifiers, and uses actual numbers to back up their arguments, or someone who can't do any more than link to an article on Stereophile that they don't even understand?

I will address one other thing on Class-D amplifiers, thermal shutdown.  Class-D amplifiers will often shut down early in amplifier tests when the current load is high and continuous. Why is that?  This is actually a design choice.  The market, at least initially for Class-D, is compact, light weight, efficient amplifiers. Sure, there is a portion of the market who is quite happy with an 80lb monoblock Class-D, but for the most part, that is not the market, even in audiophile land. People want a powerful, efficient, and still compact amplifier. That puts limitations on the size of the heatsink.  Most Class-D amplifiers don't use any more than the small aluminum plate heatsink they are built with.  As well, for good circuit operation, the switching devices are small and surface mount to keep parasitics low. Unlike a linear amp, you can't mount those transistors at the end of inches of wire or PCB trace if you want good performance.  Now sure,  you could design a system, easily, but more expensive, where you could effectively couple to a large heatsink and keep the circuit compact, but that would greatly increase implementation difficulty and cost. I did a prototype once using a heat pipe assembly designed for a PC.

Now I think we all know that the ratio between the peak wattage and the average wattage is quite low. 20:1 or even higher is typical. So even though you may be hitting 500W peaks, 10-20 watts may be your average.  Most Class-D amplifiers, knowing their target market, take this into account. Class-D amplifiers are already efficient, so creating a design that allows very high peaks, does not require a design with high quiescent power draw and dissipation. Since it is not needed, it is not included.  That works great for music, but does not work great when someone is trying to run continuous tones for maximum wattage testing.