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I went from Class D to Luxman A/AB - And most of what you think is wrong

Hi everyone,

As most of you know, I’m a fan of Class D. I have lived with ICEPower 250AS based amps for a couple of years. Before that I lived with a pair of Parasound A21s (for HT) and now I’m listening to a Luxman 507ux.


I have some thoughts after long term listening:
  • The tropes of Class D having particularly bad, noticeable Class D qualities are all wrong and have been for years.
  • No one has ever heard my Class D amps and gone: "Oh, wow, Class D, that’s why I hate it."
  • The Luxman is a better amp than my ICEPower modules, which are already pretty old.

I found the Class D a touch warm, powerful, noise free. Blindfolded I cannot tell them apart from the Parasound A21s which are completely linear, and run a touch warm due to high Class A operation, and VERY similar in power output.


The Luxman 507 beats them both, but no amp stands out as nasty sounding or lacking in the ability to be musical and involving.


What the Luxman 507 does better is in the midrange and ends of the spectrum. It is less dark, sweeter in the midrange, and sounds more powerful, almost "louder" in the sense of having more treble and bass. It IS a better amplifier than I had before. Imaging is about the same.


There was one significant operational difference, which others have confirmed. I don't know why this is true, but the Class D amps needed 2-4 days to warm up. The Luxman needs no time at all. I have no rational, engineering explanation for this. After leaving the ICEPower amps off for a weekend, they sounded pretty low fi. Took 2 days to come back. I can come home after work and turn the Luxman on and it sounds great from the first moment.


Please keep this in mind when evaluating.


Best,

E
erik_squires
kijanki's avatar
kijanki
4,394 posts
 
When class D amp is switching between two discrete levels not the voltage but duty cycle is a quantity of interest and it is linear (no discrete steps).  It becomes converted back to corresponding voltage thru averaging (filtering).  Voltage gain of such amp is a function of switched voltage amplitude.  
georgehifi's avatar
georgehifi
10,312 posts
 
For example, I’ve read that GaN transistors are capable of switching so fast they literally eliminate dead-time and any resulting distortion caused by it. I would imagine that any good amp designer would believe that substituting these for the typical FETs used would result in significant improvements in the sound quality of any class D amps, or really any solid-state amps, that they’re utilized in.


This is so correct, but so far it seems that only one has utilized this ability to take the higher switching speed up to 1.5mHz available with GaN (2.5 x higher than present), and that is Technics with their SE-R1, it does mean they had to use heatsinks to get it.

Also it’s not that the higher switching speed alone make for the good sound it gets, it’s also that the "low order output switching noise filter" that all Class-D’s have to use, can now also be taken up 2.5 x higher, this then has no effects now down into the audible range, like the filters today do, which can cause phase shifts down to 5khz, and why many complain about something not right in the upper mids/highs.

Cheers George
kijanki's avatar
kijanki
4,394 posts
 
Also it’s not that the higher switching speed alone make for the good sound it gets, it’s also that the "low order output switching noise filter" that all Class-D’s have to use, can now also be taken up 2.5 x higher, this then has no effects now down into the audible range, like the filters today do, which can cause phase shifts down to 5khz, and why many complain about something not right in the upper mids/highs.

That's true, but it might open another bag of worms.  In current designs filter leaves about 1% of switching noise on the speaker cable.  It is only small percentage but of very high value, reaching 100V - making it approx. 1V peak switching noise on the cable.  It can be easily seen with the scope.  The only reason why it does not radiate is the fact that 500kHz wavelength is 600m.  Antena becomes very ineffective below 1/10 of the wavelength - in this case 60m.  There will be still some, very small electromagnetic radiation but it should not be a problem.  Increasing switching frequency, let say 10x will make this antena "dangerous" at 6m, producing some radiation even with regular speaker cables.  It is possible to filter it better, but it would lower bandwidth resulting in phase shift in audio band, that we want to avoid.  Perhaps compromise is somewhere in-between, increasing switching frequency only 2-3x while still enjoying improved linearity thru reduction of dead time.
georgehifi's avatar
georgehifi
10,312 posts
 

I believe after a trusted friends opinion on how the Technics SE-R1 sounded, that you may as well stay with what's used now with the old Mosfet technology and 600k switching speed, if the GaN's higher switching speed transistors are used WITHOUT utilizing it's higher 1.5mHz switching speeds and filtering that their capable of.
It would be a waist sonically to use them and not fully utilize their performance advantages.
(Bit like never going over 2000 revs in a Ferrari)

Cheers George  
erik_squires OP
15,657 posts
 
Not if you talk to a designer of a class D amp :)

Your need to go on ad nauseum without actually addressing the points I made is actually kind of like watching a fetish in public.
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