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RE: Sure, it quacks RFI unlike any traditional Class A/AB amp

Posted by Ralph (M) on May 30, 2018 at 13:54:39

In Reply to: Sure, it quacks RFI unlike any traditional Class A/AB amp posted by AbeCollins on May 25, 2018 at 13:34:08:

In electronics, a comparator is a device that compares two voltages or currents and outputs a Digital signal indicating which is larger.

Hmm, there's that digital word again!

Plenty of proof that Class D amps have digital circuitry and digital waveforms inherent in their design... along with high-speed switching noise that requires filtering to mitigate RFI. Can you prove otherwise?

Class D amps have digital circuitry, digital waveforms, and RFI that do not exist at all in Class A/AB amps.

My surmise appears correct- you seem to be confusing digital with switching and while the two share characteristics, they aren't exactly the same. And not the best choice of words in that definition. It would have been more accurate to say the output goes high (or low). That bit is important- how high does it go (because that makes a difference)? Does it have enough current to drive the level shifters that follow it? Will we need to add some drivers so it will? And why would a digital circuit need level shifters anyway? A digital circuit won't, but a switching circuit can!

You've not really addressed the issue of a triangle wave being used for the PWM. Nor have you addressed the fact that when such a waveform is used, that it is in fact setting the timing in the amp, and not a clock somewhere else. When the PWM technique is used, the resolution is not limited to 8 or 16 or 32 bits, its resolution is limited by the accuracy of the triangle wave. Some amps use a sawtooth rather than a triangle, and some a sort of rounded square wave where the leading edge is heavily curved, approximating a triangle leading edge. Sort of...

Those variants can work as long as the generator has stability, which is going to be mostly influenced by thermal conditions (an analog problem). But most modern amps are self-oscillating and their timing is set up by the oscillation- no clock. Think of the feedback network that makes that happen!

RFI can be generated by plenty of A and AB circuits- don't fool yourself. I've seen tube circuits oscillating at 10MHz because a stopping resistor wasn't included in the design. So the presence of RFI generation isn't the defining aspect; at most it might be a sign. I built an OTL in my youth that caused Barkhausen oscillations on channel 4 of the TV when the amp made over 50% of full power. I learned to limit the bandwidth of the amp after that...

IOW, while you are correct that its a whole nuther animal from classic A and AB circuits, its still an analog process, one that employs switching techniques, some of which can be handy for interfacing with actual digital circuits. I think we do agree though that some digital techniques are used, but it would be incorrect to say that the digital aspect dominates the design- it doesn't; its very design dependent!

We're well into a (over year long) design of or own, not using anyone else's modules and with an important class D patent pending. I suggested try building a class D of your own because hands-on is the best way to understand what's involved.

Here's a nice link breaking it all down. I'll save you the quote from Wikipedia, but if you care to, look that one up as well :)

• http://www.analog.com/en/analog-dialogue/articles/class-d-au   (Open in New Window)

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