Class D Technology

Hello atmasphere,

     As I understand class D technology as a layman, timlub could be referring to 1 of 2 different possible types of distortion:

1. Distortion caused by the output transistors not turning on and off quickly enough- Most current class D amps utilize MOSFET transistors that have a limit to how fast they can switch from completely off to on and vice versa.  Any lag time between the transistor being fully in the on or off state is termed 'dead time' and can cause signal distortion.

2. Distortion caused by the carrier frequency being too low- This is a possible form of distortion that georgehifi claims is audible but has never provided any valid evidence to support his claim.  Basically, he claims the ultra-sonic frequencies (typically in the 300-600 kHz range), that class D amps modulate to carry the audio signal and subsequently completely filter out the frequency leaving just the signal, are too low and somehow affect or distort frequencies in the uppermost audible frequency range.  He claims carrier frequencies need to be above at least 1 mHz in order not to affect or distort frequencies in the audible range.

     I've never subjectively perceived any distortions or sonic artifacts on the many class D amps I've owned or heard.  However,I know some amps are beginning to utilize the newer, ultra-fast switching GAN output transistors and I believe at least 1 company, Technics, offers a class D amp model with a carrier frequency above 1 mHz.

Tim

@noble100 

OK- I just wanted to be clear on what was meant by 'switching distortion' since that's not an engineering term at all!

Artifacts from dead time show up as harmonic and intermodulation distortions, so a class D amplifier can easily be compared to other amps on that basis. IOW, on its own 'switching distortion' has no meaning. 


With regards to the switching frequency, with the current state of affairs with switching devices, its actually easier to make a low distortion amplifier when the switching frequency is less than 1MHz. This is because the amplifier has to meet certain standards for RF radiation- various countries have specs for how much RF noise can be tolerated. The faster you go, the difficulty in getting the amp to behave in this regard goes up on an exponential curve! Its quite a feat that Technics was able to switch so fast, but in reading their ads for the amp, I suspect a bit of marketing hype is also involved.


The problem with a low switching frequency is probably that of bandwidth, but class D amps can have a way of getting around that, since it is possible to add enough feedback to allow the amp to compensate for phase shift that would otherwise plague an amplifier with bandwidth from only 20Hz-20KHz.


In a nutshell its really dangerous to make blanket statements about 'all class D amplifiers' just as it is about 'all tube amplifiers' or the like- there are often many exceptions.

Hello atmashere,

     I'm always learning useful and interesting stuff from you.

Thank you,
   Tim

Hello all.  I'm not able to be on here as much as I used to, but I will answer when I can. 
@atmasphere   Hi Ralph,  You are correct, I don't necessarily use engineering terms,  but that may be because I am not an engineer. 
I have posted in the past that As much as I do know and practice about speakers, I've spent my adult life working on, building, modifying and designing speakers,  I found that I was only touching the surface with amplifiers.  I was out of place in some more technical conversations... So I decided to learn.  6 or 7 years ago, I started buying bad amplifiers,  repairing and rebuilding and in general just learning about amplifiers.  I have repaired or rebuilt maybe a dozen old classics and today am fairly competent doing most repairs or modifications on amplifiers. Compared to those builds,  my class D quest has been fairly simple.  I have been purchasing class D modules and doing changes.  With these modules, I built with linear supplies and switching supplies,  both when hefty enough supplies were used would produce quite good results.... I started mostly just upgrading parts on these boards to very high quality parts.  I have done very few circuit mods,  but I have done a couple.   Many of these amps do a good job.  

Bottom line is until we can get rid of the nasty 3rd harmonic distortion in class d our ears will always prefer the cozy 2nd harmonics that tubes generate!😮