Audio in the Class D age. - Page 1
Forum section: electronics / general

author: hamster_nz
author_rank: Super Contributor
timestamp: July 01, 2018, 09:22:51 am
content: In the past I used to build audio from kitset. A 300W Sub amp for the lounge, a 20W stereo amp for the study, that sort of thing.Yesterday I rediscovered a playing-card siized 25W+25W class D amp module, and decided to attach some HiFi speakers, a Chromecast Audio, and power it up with a switchmode bench supply.Somewhat unexpectedly, It sounded perfectly fine, even playing at quote a reasonable volume with quite a lot of bass/subbass. So much so that I might replace the big Pioneer DSP receiver with a small amp from Ali Express, and let the Spotify age wash over me.But what really surprised me was how little power you really need. Even at a a good volume the PSU was only showing 12V @ 0.15A - About 1W per channel! At bachground levels it was about 0.03A.So I am left wondering - outside of Pro sound reinforcement work, or really really inefficient speakers, do you really need high power (100W+) amps for domestic audio anymore?Back in the old linear amp days you did because you needed the headroom as once the amp started working hard you could hear it get rough, but that seems to be a thing of the past.Has anybody tried the same sort of thing? If so, do you agree?Or maybe my ears are past it?

author: Beamin
author_rank: Super Contributor
timestamp: July 01, 2018, 09:59:51 am
content: I thought the class D amps were only used as sub amps because they were basically SMPS inside and not good for high frequency. A/B was done for the rest. Isn't it a class A amp that uses full power all the time regardless of volume?

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: July 01, 2018, 12:12:32 pm
content: "bachground levels"Does that rate as a Floydian slip?A problem with linear amplifiers is that having an excess of power/headroom does make things very inefficient at lower levels. It's basically the same as dimming a lamp with a resistor. Class D does overcome this.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 01, 2018, 12:14:53 pm
content: Modern digital amplifiers have come a long way. TI has a number of chipsets that will do HD audio.

author: Whales
author_rank: Super Contributor
timestamp: July 01, 2018, 12:35:36 pm
content: Quote from: Beamin on July 01, 2018, 09:59:51 amNope, class D is perfectly good for normal audio range. Everything from little fellas to rackmount stage units. Having more components (class D) is often more economic than big heatsinks (A or AB). From the things I've worked on: the crossover point into class Dtendsto be around a few watts.QuoteA non-constant amount of power is always being wasted (but probably close enough to "constant" in most circumstances).Wikipedia has a summary of amp classes

author: bd139
author_rank: Super Contributor
timestamp: July 01, 2018, 12:39:08 pm
content: I don’t have any non class D audio amps now. They sound pretty damn excellent.Best win was a £20 head unit for the car. Bluetooth receiver, 4x class D amplifer channels. Sounds absolutely amazing.

author: tooki
author_rank: Super Contributor
timestamp: July 02, 2018, 04:45:38 am
content: I have a strong suspicion that the vast majority of audio is now reproduced on Class-D amps, since all our mobile gadgets use them, as do modern TVs, car stereos, computers, and Bluetooth speakers. (And probably all low-cost stereos and HTIBs). I think the transition happened without most people even realizing.And that’s OK: the audiophools would have us believe that audio reproduction has gone to hell in a handbasket over the past 50 years, when in fact what these cheap Class-D amps, computationally expensive compressed digital audio codecs, etc replaced were really far worse. (Yes, the high end of 50 years ago was pretty damned good. But that’s not what most people had!!) A 10¢ entry level PAM Class-D amp is better than the two-transistor amp that might have been used in a pocket radio from 1975.

author: bd139
author_rank: Super Contributor
timestamp: July 02, 2018, 06:54:51 am
content: The high end 50 years ago wasn’t that good either as the signal source was crap. Things have only got better. Even the music has got better.

author: b_force
author_rank: Super Contributor
timestamp: July 02, 2018, 07:47:35 am
content: Quote from: NiHaoMike on July 01, 2018, 12:14:53 pmDigital?I didn't know you could amplify 1s and 0s?There is nothing digital about a PWM amplifier, it's purely in the analog domain.But for some reason the marketing department had to give a very bad name to it from an technical point of view.Anyways,Over 90-95% of the market is Class-D.Even the newest modeling guitar amplifiers are all Class-D, PA is all Class-D and all other professional audio equipment.The average performance nowadays is better of similar to the simple chipamps (power opamps) that people used in the old days.If it wasn't that some patents completely block the competition in the Class-D feedback technology, there would be many performing even on order of magnitude better.

author: luiHS
author_rank: Frequent Contributor
timestamp: July 02, 2018, 08:43:35 am
content: I only use amplifiers in Class D, with TPA chips from Texas, for designs that need some small sound amplifier (20W maximum), without great quality pretensions, although they sound quite good.If I really need a Hi-Fi amplifier for quality music equipment, then I build amplifiers in Class A with tubes, 300B if there is enough budget, or also with 6550, KT88 and EL34.The main advantage of a Class D, is that they are very efficient, they barely heat up, they do not need heat sinks and now you have chips that integrate everything for very little price. Texas Instruments has a good range, I am using TPA3121 and TPA3111 in some designs.

author: Beamin
author_rank: Super Contributor
timestamp: July 02, 2018, 10:41:00 am
content: Quote from: NiHaoMike on July 01, 2018, 12:14:53 pmWhy do they call them digital amplifiers? Digital I think logic and ones and zeros not really sound which is analog and sine waves. All digital sound needs a D/A converter at some point. Even if it's PWM its not really digital or logic level. Sounds like marketing wank like putting quantum in front of everything. Before that it was atomic everything.

author: nfmax
author_rank: Super Contributor
timestamp: July 02, 2018, 11:08:58 am
content: Quote from: Beamin on July 02, 2018, 10:41:00 amThe marketeers will of course be able to put 'quantum' in front of everything and nothing simultaneously...

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2018, 12:34:02 pm
content: Quote from: Beamin on July 02, 2018, 10:41:00 amThey accept a digital signal like I2S and the conversion to analog is done by the amplifier itself at full speaker power. A very scaled down version even exists as the chips in some USB-C headphones.

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2018, 01:29:39 pm
content: Quote from: Whales on July 01, 2018, 12:35:36 pmYes, class D amplifiers are much better than lots of the supposedly classic class AB amplifiers.It's long being the case that, as long as an amplifier isn't overdriven, most of the distortion occurs in the speakers and room acoustics, to the point where the little distortion due to the amplifier is inaudible. Swap a quality class AB amplifier, for a decent class D unit and no one will be able to hear the difference, given everything else being equal: speakers, volume setting, room acoustics etc.Quote from: NiHaoMike on July 02, 2018, 12:34:02 pmYes the digital to analogue conversion is often integrated into the amplifier, so I suppose there is some truth in calling it a digial amplifier, even if the output is obviously analogue.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2018, 04:18:02 pm
content: Quote from: Hero999 on July 02, 2018, 01:29:39 pmOnly halfway converted to analog at the output of the power stage - the output filter takes care of the remaining half. Thus why such devices are sometimes called "power DACs", although I don't see that term being used much nowadays.

author: Bassman59
author_rank: Super Contributor
timestamp: July 02, 2018, 04:52:02 pm
content: Quote from: b_force on July 02, 2018, 07:47:35 amDespite old farts who think that the only amps worth using on big PA subs is a Crest 8001 or a Macrotech (and they're wrong, and they are wrong about digital mixing consoles too), everything in PA nowadays is class D. When you hang 20 self-powered boxes, the weight saved by using Class D amps is significant. And with self-powered boxes, you don't have to truck heavy racks of power amps, so the cost in transport goes down.

author: The Soulman
author_rank: Super Contributor
timestamp: July 02, 2018, 05:33:58 pm
content: Quote from: Bassman59 on July 02, 2018, 04:52:02 pmI'm a old fart, imho for some PA applications class-d still isn't good enough.*Using a PA capable of 126dB, at 70dB (speech reinforcement in a theater) not to many bits lefts at that point.*Damping factor at mid and high frequency's is typically very poor due to the low speed of the control loop, giving a lessaccurate reproduction, again especially notable in speech reinforcement.Popular amplifiers such as Camco Vortex and Crest Pro 200 series are based on switch mode power supplies withclass-h output devices, the best compromise imho and not much heavier than any powersoft or labgruppen.At home I have a stack of older modified Rotel class a/b amps, not replacing them anytime soon..Ps, pro 8001 and friends are still going strong around the world taking daily abuse for decades.

author: BrianHG
author_rank: Super Contributor
timestamp: July 02, 2018, 06:45:17 pm
content: Unless you are into really high power full bandwidth performance audio, class D, within it's first 30% of it's spec power output are phenomenal. Distortion begins to grow exponentially around above 35% on the top tier TI class D amps, however, usually playing the type of modern music recordings which are already clipped and distorted during the peaks, and owning speaker whose designs today are not focused on full power playback with definition inside these peaks, those Aliexpress Class D amps should be fine for you.If you demand refinement and undistorted definition at the high power output right inside the peaks, class D might not be for you unless it's one of those above 5k$-10k$ specifically ground up custom designed amps. If you listen to modern music, don't waste your money here and just get the top class D amp from Aliexpress and you will be happy. The peaks in modern music are already recorded distorted and if you cant hear that now, you wont hear it with your Aliexpress amp at full volume.

author: b_force
author_rank: Super Contributor
timestamp: July 02, 2018, 08:15:36 pm
content: Quote from: NiHaoMike on July 02, 2018, 12:34:02 pmNo they don't?I think some people need to get their definition straight about what digital means.The fact that something is switching doesn't automatically mean it's digital.On the other hand the state of magnetization levels can be called digital.

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2018, 09:00:14 pm
content: Quote from: b_force on July 02, 2018, 08:15:36 pmYes, PWM is analogue.The DAC side will still be a low logic level, probably 3V, then boosted to the full voltage with a MOSFET driver, to power the output stage.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2018, 09:35:24 pm
content: Quote from: b_force on July 02, 2018, 08:15:36 pmhttp://www.ti.com/product/tas6424l-q1Put I2S in, get PWM at full speaker voltage out. Pretty much the definition of a "power DAC".

author: BrianHG
author_rank: Super Contributor
timestamp: July 02, 2018, 09:56:28 pm
content: OK, to get thing straight, there are 2 main methods to a class D amp, with some additional variations which use feedback from the speaker drive, or, feedback from the power supply.The all analog one uses a comparator front end, comparing an analog carrier reference triangle waveform with your source analog audio in. The output oscillates digitally feeding the mosfet switch bridge which is filtered then drives the speaker. See attached photo. In this design, your output sound quality lies strictly on the analog input error correction opamp, the analog triangle waveform generator, and the analog comparator making the PWM signal to drive the mosfets.There also exist an all digital variety. This one takes digital in, processes that signal through a few digital filters to compensate for the output low pass filters, and then converts that 16bit/24bit digital mathematically into a 1 bit waveform at a programed switching carrier frequency with added noise to fill in missing resolution due to the counter frequency of that 1 bit PWM counter. Basically a power PWM DAC. This design has the advantage no loss at all except for the speed/resolution of the 1 bit pwm counter at low volume and your chosen PWM carrier frequency. Also, some of these all digital class D amp ICs also sample the power VCC so they may compensate the audio waveform digitally to correct for un-regulated power supplies.

author: james_s
author_rank: Super Contributor
timestamp: July 02, 2018, 10:06:47 pm
content: Some of these modern amplifiers are truly impressive. I have a little board not much larger than a postage stamp that cost under $2 including shipping and is rated 2x10W. Now I'm no audiophile but I connected it to a pair of decent vintage hifi speakers and was astonished at how good it sounds. I mean I'm sure the rating is inflated but even so, it produced proper room-filling volume, and could go louder than I'd likely ever want to listen to. It's small and cheap enough to integrate into just about anything.

author: b_force
author_rank: Super Contributor
timestamp: July 02, 2018, 11:25:38 pm
content: Quote from: james_s on July 02, 2018, 10:06:47 pmI made several designs myself. (And still have some new ideas)But the smallest vs power ratio was a 1kW amplifier on a board that was 100x50mmIncl all kinds of protection.

author: hamster_nz
author_rank: Super Contributor
timestamp: July 03, 2018, 12:03:13 am
content: Quote from: james_s on July 02, 2018, 10:06:47 pmPhew - I am glad that it wasn't just me.