Class D amplifiers.
 

Hi all, discussion on class D amplifiers has become more common recently on the forum. Am I the only one who a) has never seen or worked on one yet? b) doesn't understand how they work? I am used to the usual class A, B and C amplifiers which have been around for a long while, but can't get my head around an amplifier which can work well beyond cutoff, if that's actually how they work. Possibly use some kind of pulse width/amplitude system? Someone please enlighten me! Thanks in advance.
Alan.

Re: Class D amplifiers.
 

See here: https://en.wikipedia.org/wiki/Class-D_amplifier

It's not a new idea: there were attempts made in the 1930s to build such amplifiers using Thyratrons, but their switching-speed [or rather, the recovery-time after the gas had been ionised] was too slow.

Re: Class D amplifiers.
 

They basically chop up the analogue signal at a very high frequency, so are either fully on or fully off when operating, which is very efficient especially with a low voltage supply rail.

I recently bought some class D modules for pennies from China, but haven't got around to doing anything with them yet.

Re: Class D amplifiers.
 

The first time I came across Class D was in one of Uncle Clive's adverts, way on back.

Lawrence.

Re: Class D amplifiers.
 

Alan

Please see here for a decent technical description of class D amplifiers :-

http://sound.whsites.net/articles/pwm.htm

Things have moved on a bit since the above article from 2005; the sigma delta "power DAC" approach is now very practical, even for amplifiers used in mobile 'phone base stations.


John

Re: Class D amplifiers.
 

I remember building up one of the Sinclair kits. I think it was in the early 1970s soon after an interesting article on the topic had been published- Wireless World I think.

AFAIR, it made quite a cheerful noise into a cheap speaker, but it died as soon as I connected a speaker with a crossover network. I think that the impedance became too difficult for it so that the switching was no longer fast enough to avoid heating up the output transistors. I did resurrect it, and modified it to a less ambitious spec - it's still in a junk box somewhere.

I guess that today's ubiquitous digital power amps are close relations of Class D output circuits.

Martin

Re: Class D amplifiers.
 

TBH, despite nearly 50 years working with radio and audio gear, I'm not even sure how a Class 'C' amplifier works, though of course I know about Classes 'A' (single-ended) and 'B' (Push-Pull) amps.

Re: Class D amplifiers.
 

Not to mention class G and H!

Class-D are well entrenched in the professional audio world where high power (1kW rms +) without unwieldy heatsinks are required.
It's not uncommon to find a 10kW audio amp in a casing the size of a slimline DVD player.

Of course now, you're just as likely to find them in the output of a DAB radio or powering the earpiece of a smartphone.

Re: Class D amplifiers.
 

Hearing aids use class D for battery preservation.

Re: Class D amplifiers.
 

Thanks for the replies. I did think it involved switching of some sort but wasn't sure exactly how or why.
Alan.

Re: Class D amplifiers.
 

Class C only works at a single (RF usually) frequency with a resonant tank circuit load.

The device is biassed beyond cutoff and simply delivers tall narrow pulses around the positive peak half cycles of the drive signal to kick the tank circuit into resonance. A typical pi tank circuit also acts as a low pass filter to clean up the large harmonic content of the driving pulses.

It's no use for linear amplification it's just a fairly efficient way to generate a high power CW signal. If the supply voltage to the device is varied you get an amplitude modulated signal output.

Re: Class D amplifiers.
 

"Not to mention class G and H!" Isn't there a Class T or S? Some brainy folks like Doug Self or simply odd bods like Bob Carver have had some really whacky ideas and approaches.

A.

Re: Class D amplifiers.
 

A, B, C are well-defined. D is a reasonable bet, and then all the subsequent letters are a bit of a dog's breakfast. So many people and firms have been claiming letters for marketing purposes, sometimes the same letters. There is no regulating authority. If you want to claim a new letter for your audio amp or broadcast transmitter, just go ahead.

D probably means some sort of switch mode, but there are several variants with major differences so you need something more specific.

For later letters, forget them, go straight to the specifics.

David

Re: Class D amplifiers.
 

I have heard that Class "D" stands for disposable. :)

Mike

Re: Class D amplifiers.
 

Well I reckon that would silence most of today's audio devices!

Martin

Re: Class D amplifiers.
 

The very highest quality amplifiers are now class D. https://www.hypex.nl/ , particularly their N-core design.

Comprehensive spec sheet is here https://www.diyclassd.com/product/nc400/11

And measurements by Siegfried Linkwitz here http://www.linkwitzlab.com/frontiers_7.htm#G2

But basically 400W into 4 ohms, distortion at 100W 0.0003% (ie 3ppm) frequency independent, and at full power 0.002% (20ppm), output impedance 1 milli-ohm, SNR -125dB. Balanced input.

All from a circuit board in the form of a disc 88mm diameter weighing 180g. An astonishing design.

Oh - and thinking about Sinclair's 1960's class D. It was actually designed by a good friend and mentor (alas now dead) called Gordon Edge. Performance was limited by the lousy transistors that were available at that time, and relied on the loudspeaker to deal with switching transients - which meant that it was an excellent radio and TV jammer.

Re: Class D amplifiers.
 

Class E and F are defined for RF purposes (or other narrowband applications). Useless for audio.