High end Class D amps?

From an article on Audiophile Style - interview with Bruno Putzeys. 

@Sagittarius: Class D has achieved very low levels of distortion, but is it possible for class D amplifiers to continue their evolution into something close to a straight wire with gain, i.e. minimal phase shift in the audio band? (A similar question from maty).

 

Bruno: The 1ET400 module has the frequency and phase response of a 2nd order Butterworth filter cornering at 60kHz. If you look at the phase shift of that, it’s very nearly “linear phase” in the audio band. To take some rough numbers, it if you have a circuit that has a 0.2 degree phase shift at 200Hz, 2 degrees at 2kHz and 20 degrees at 20kHz, that’s the same as saying it has “0.001 degree per Hertz” phase shift. That’s another way of saying that the whole signal is simply delayed by 2.8 microseconds. If you plot phase shift on a linear frequency scale that’s immediately obvious because you get a straight line. Of course a simple delay doesn’t change the sound. It’s literally the same as starting your music a few microseconds later.

 

Lars: My dad used to say that if you left a CD in its case without playing it back, it’d just sit there accumulating massive amounts of phase shift as time went by.

 

Bruno: What that matters to sound is how much phase shift differs from a pure delay. Anyone who’s ever done phase measurements on speakers will remember that you have to remove the time-of-flight delay from the data, for instance by marking the leading edge of the impulse response. Otherwise the linear phase shift corresponding to the distance between the speaker and the mic completely clouds the picture. In the case of the 1ET400 module it’s just under 1 degree at 20kHz. There never was a phase shift problem in class D, it’s simply a trick of the light that happens when you plot the phase response on a log scale without removing the fixed delay.

I believe Bruno in that modern class D amps with the output filter in the feedback loop do not have an issue with phase shift in the audio band.  I actually thought the high frequencies on the Nord NC500 amps sounded very good (although being in my early 60s, high frequencies to me are not the same as they were 40 years ago). 

My issue with the Nords on the system I had then was that they lacked body in the upper bass/lower midrange. I tried both the Sonic Imagery and Sparkos discrete op amps in the buffer stage. The Sparkos had a bit more warmth and was closer to the sound profile I was looking for, but gave up too much detail. 

The Bel Canto Ref600M is pretty similar to the Nord in it's design approach, using a SMPS power supply and a custom input buffer. There aren't too many class D amps built with linear power supplies. The LKV Veros (mentioned earlier in this tread) is one of the few that does and has received good reviews, so I'm going to give it a try (partly because I'm comfortable building a linear power supply myself, and I don't want to just drop in a pre-built SMPS supply).

How many class D do I have to listen to? Is there a The class D which disproves the "myth"? Did I just happen to miss the good ones?

Here are some tips. The first thing is avoid amps using switch mode power supplies. The reason for this isn't that SMPSs work, they do and they can be quite low noise. The problem with them is they are cheap to buy off the shelf and expensive to have built custom. Off the shelf units tend to have current limiting protection circuits that cause the amp to suffer a loss of bass impact. So unless you are looking at an enormous amplifier manufacturer, they won't be able to afford to have a supply built custom.


This means that the supply has to be a traditional supply: power transformer, rectifiers and filter caps. The power transformer current and the capacity in the supply can have a profound effect on the performance of the amplifier, since class D amps can go from almost no current draw to really quite a lot at full power. This really means that the supply has to be well filtered, and the power transformer high current, else the power supply will sag at higher power (bass notes).

If the amp is zero feedback look for a filter frequency of 100KHz or higher. This is to reduce phase shift at audio frequencies. If they won't publish details like that, consider looking elsewhere.


If the amp employs feedback, find out how much. If details like that are not published, consider looking elsewhere. The feedback **must** be in excess of 35dB! Any less and the artifacts of the feedback itself will become audible (as they are in traditional amplifiers).


I understand your skepticism! The first class D amps I heard I really thought were a joke. But they have improved over the years, so much that about 5 years ago I personally started to take them seriously, but even then some are terrible (we have a small 30 watt amp we use for testing, but it sounds really boring; I've not looked into it to see why that might be the case, but its pretty obvious to everyone in the shop) and some really work quite well and everything in between.

@jaytor
Tube buffers are clearly the way to go.
https://atm-audio.com/class-d-amplifier/
https://vtvamplifier.com/product/vtv-amplifier-monoblock-purifi-audio-1et400a-amplifier-with-vtv-vacuum-tube-buffer/

Problems with any amps shouldn’t be fixed by using even more feedback as a fix, it’s a added by the best designers to clean thing up a little and usually just local not global in an already well designed amp.

All the greatest amp designers say it. An amp should be reasonably good spec’d, and to use it just as a clean up tool.
And the preference with the best is to use "local feedback" only by the best designers. Not global as being talked about here, and then even including the Class-D output filter in the global loop, what next throw the speaker wire and the speaker in the loop as well, like Trio/Kenwood tried all those years ago, utter sterilized disaster sound.

And using tube buffers with the capacitor coupling that’s needed. Is just colorized HF coverup softening job for the more serious Class-D sound problem.

Cheers George

@twoleftears  - I am thinking about trying an input buffer using tubes.

I don't think I would consider using the Korg NuTube like ATM does. This is a pretty high-distortion device - specifically designed to add distortion for synthesizer special effects. It also has a bad reputation for microphonics, and is hard to swap in and out. 

The 6DJ8/6922 tube that VTV uses is much better, but I'm not sure it's the ideal tube to use with the relatively low voltages I have available (approx +/-65v if I use the main supply for the Purifi module). I don't want to add another high voltage supply to drive a tube section, although I could consider a DC-DC converter as part of the buffer board design if I have to.

At any rate, I'd prefer to avoid the hassles of tubes, so I'm going to see what's possible with various solid state approaches first.