Distorsions in class D

[IVX]

Hi,
maybe several reasons..
i'd just checked my MC7 simulation for 700khz D amp, 10khz, 60khz bandwidth:
A) THD_max step-1ns_cutoff by 1Kohm to 1nf (-.5db at 60khz)_FFT only for second sinusoidal period
to avoid initial, non-periodic transients.@.005%, actually it's only 3th harmonic.
B) same, but without cutoff@.006%,
how mentioned Pabo, EWB have much more filtering troubles.
C) like case B) but max step-10ns@.006%
D) like case B) but max step-100ns@.07%, funny- 3th is still .005%
E) like case B) but max step-1us@.6%
F) like case A) but max step-1us@.3%
G) like case A) but entire time range, ie non-periodic transients are included@110%,
if the initial click isn't seen even..this rather does exist then doesn't.
Usually, for class D, i've from .7 to .25 MC7/reality THD ratio, and more power makes lower ratio because i didn't try to simulate decoupling/stargrounding components, actually, my statistics is pretty poor so far. My first D amp had half bridge, .5-1us dead time, 50-100khz, PWM, without feedback..i don't remember exactly, it was THD 5-10%@100hz 100w 4ohm or so. Same circuit with feedback loop before filter had .5% etc..

[subwo1]

That kind of makes sense, I'm trying to visualize this effect..basically your saying the feedback would kind of amplify the deadtime effect..thereby increasing distortion..instead of cancelling it? Hmmm.....hard concept to grasp..at first. In this sense feedback worsens distortion rather than improves, hence the importance on minimizing, and I assume matching deadtimes.

I think it may increase distortion but probably stay less than non-feedback circuitry, for the most part, unless I am not seeing things right. best regards

[classd4sure]

Originally posted by subwo1

I think it may increase distortion but probably stay less than non-feedback circuitry, for the most part, unless I am not seeing things right. best regards

No argument here at all. However what you stated was most valuble indeed. Certainly gives me a more in depth understanding on the importance of minimising dead times, other than the more obvious, at a glance, cross over distortion effect it produces.

Perhaps we should starve any chance of a feedback vs no feedback debate here by re-stating it as feedback can contribute to the effect of dead time induced distortion, with the cure still being the obvious one of having minimal dead time.

I understand the Mueta IC will incorporate some sort of adaptive dead time according to power level..

Is dead time generally greater for higher power levels? I guess in order to account for the harder to switch high power devices?

This leaves alot to be said for Crown's BCA topology doesn't it.
Hmmmmmm....... BCA-UCD hybrid ...here comes the bride!
Thanks,

Chris

[Bruno Putzeys]

Originally posted by classd4sure
This leaves alot to be said for Crown's BCA topology doesn't it.

The precision to which the two duty cycles (top and bottom) have to be matched is the same as for dead time. The effect of overlap (sum of duty cycles >100%) is also too much idle current. The effect of nonoverlap (sum of duty cycles <100%) is also similar: distortion.

BCA does not substantially solve the timing distortion problem. The only thing it does solve is EMI issues related with hard takeover (shoothrough current and/or recovery current). Also reliability problems that can occur with very-high-power stages and diode recovery are removed. The same improvement can be had with ordinary current steering diodes though.

Overall the effect isn't worth the extra complication. But, you know, class D folks like to invent "technologies" and attach their ego to it. To this end they gloss over the fact that they didn't really solve a problem (Mr. T, shrp1bit, deedee-ex, ...)
Unfortunately it keeps them from making real progress, because telling they've got something better means the original idea wasn't so wonderful after all.

[subwo1]

Super explanation, Bruno and the sentiment, "Overall the effect isn't worth the extra complication." cannot be overstated concerning design work, IMO.

[phase_accurate]

Unfortunately it keeps them from making real progress

... and others from huge profits

Apart from my cynism (you have to do things like that every now and then because it is sooo refreshing): A circuit that precisely measures and controls deadtime might be desirable.

OTOH deadtime induced distortion is not that severe as those suggest who don't like class-d. It would not reach the levels of an unbiased "linear" output stage.

Regards

Charles

[classd4sure]

Hello,

@Bruno, I knew I could lure you in here Welcome! Every good father has to put up a fight when his offspring is about to married off to some, undeserving ....pauper from the wrong side of the tracks.

I admitt, when I said that I was half joking ..(only half), and a very quick review of the BCA patent, which I hadn't bothered with before, dismissed that idea as quickly as it came.

Why? I realised what you said about the timing, not solving that issue at all, far from it. Why else? Ahhhh.....the complication...not even worth considering.

Why else again? Your UCD is about as perfect dead time wise as I can imagine so far, would be hard to improve on, and I've been trying, but I can't find even a single flaw. Shoot through seems non existant with it. Surely this must beat any sort of an adaptive dead time abomination.

I was reading up on ECL last night thinking here I go....I will find an improvement to be had...hahaha.....foolish me.

I am curious however, how well UCD could be made to work using a MAX9690. Functionally it seems identical, although it would limit some options we have with your comparator.

I'm not at all saying it would improve anything, but it's something a diy hack like myself could wire up in 5 minutes, with even less complication, and fewer parts.

Anyway, thanks for your informed response, of course you're 100% on target.

Regards,

Chris

PS: Sharp1bit.....still a good laugh....$20k LIE. tsk...wonder how many they sold.

[phase_accurate]

PS: Sharp1bit.....still a good laugh....$20k LIE. tsk...wonder how many they sold.

I once heard it and I must admit that it wasn't that bad at all soundwise. OTOH it is a lot of effort construction- and money-wise compared to the outcome.

But delta-sigma as such doesn't seem to be that bad at all. I did simulations on a DS amp and it was particularily good at suppression of IMD. But it would definitely take more effort to implement it than an UCD. OTOH for mass production it could be easily integrated.

Regards

Charles

[classd4sure]

Originally posted by phase_accurate


I once heard it and I must admit that it wasn't that bad at all soundwise. OTOH it is a lot of effort construction- and money-wise compared to the outcome.

But delta-sigma as such doesn't seem to be that bad at all. I did simulations on a DS amp and it was particularily good at suppression of IMD. But it would definitely take more effort to implement it than an UCD. OTOH for mass production it could be easily integrated.

Regards

Charles

Hi Charles,

MmmmmmmHmmmmm... From what I've seen everyone who's heard it admits the same, which personally I find encouraging that a bad design with bad specs can still sound nice.

My problem with it:

They went a little beyond "marketing hoopla" by saying it's a true digital amplifier, complete lie, and shame on alot of the reviewers who didn't catch that.

My other problem with it: $20k, that's insane, and for that much, it should sound alot better than "not bad".

It's ugly too!

Personally when it comes to such things, a little honesty regarding what makes it tick, and why, can go a long long way.
Buyer Beware.

Regards,
Chris

[subwo1]

Hi IVX,
Thanks for sharing your experiences with the simulations and real circuits in comparison form. I found it enlightening especially since I lack the chance to do such detailed analyses. I wouldn't mind studying the combined efforts of the DIYaudio community and construct a refined design which I theoretically understand so well that it is as though I have been bread boarding and experimenting with it myself.

Hi Chris;
yes, buyer beware.