Class D music amplifier loading? - Page 1
Forum section: electronics / projects

author: Faringdon
author_rank: Super Contributor
timestamp: June 23, 2022, 11:15:08 am
content: Hi,Do you agree that for a Class D amplifier being used at a thrash metal concert at full power...the power draw of the Class D would be going from no load to full load multiple times within any 5 second interval?And that it would never be on constant load?Consider for Bass Amp and 6string guitar amp.A Bass guitar is from 40Hz to 4000Hz......T(40Hz) is 25ms......so in fact, for a Bass Class D amplifier at 40Hz (low E string), the power draw of the class D amplifier must go from no load to full load within that 25ms period?Surely this must be so.....its impossible for music (even explosively loud thrash metal) to avoid regularly returning to "no load", since the whole situation of music is the difference in the loudness over millisecond intervals?...so over a 1 second interval, the class D must have multiple no load intervals?

author: langwadt
author_rank: Super Contributor
timestamp: June 23, 2022, 11:30:53 am
content: instead of making silly "do you agree" statements based on nothing, why don't you just load some of your imaginary "trash metal" into an audio program and look at the waveform .....

author: Circlotron
author_rank: Super Contributor
timestamp: June 23, 2022, 11:35:15 am
content: With a class D amplifier you can get negative power consumption during part of the cycle e.g. when the amplifier damps the moving speaker cone the kinetic energy is turned back into electricity by the voice coil and magnet and this energy is then returned to the PSU filter caps. Same deal as regenerative braking in an electric car.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 23, 2022, 12:09:18 pm
content: They're mostly powered by single phase mains, anyway; the input power returns to zero a hundred or more times a second already.Actually I don't know that the really big wall-of-sound installations are single phase, they might have some industrial grade systems to help out there.Tim

author: Zero999
author_rank: Super Contributor
timestamp: June 23, 2022, 12:34:29 pm
content: Other than the usual power fluctuation throughout the mains cycle, no. There will be lots of filtering, with big smoothing capacitors and a large installation will probably be run off a power factor corrected power supply.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: June 23, 2022, 12:45:28 pm
content: Quote from: Faringdon on June 23, 2022, 11:15:08 ami'd assume that you mean "no power", rather than "no load". and then again, even a trivial sine-wave tone w/o offset will hit a "no power" condition twice per cycle. do the math: p(t) = u(t)*i(t).

author: CaptDon
author_rank: Super Contributor
timestamp: June 23, 2022, 01:07:33 pm
content: Your question is without merit unless you define a few things. At zero crossing of the speaker voltage the amplifier will be drawing very little power from its well filtered supplies. Looking at your question from an audiophool standpoint, yes, a real heavy hit on the kick drum will dim the lights in the venue until the amplifier's power supply can charge the filter capacitor bank back up to finish voltage. In the animal clubs we often see the lights dimming to the beat of the kick drum. Why do you specify 'Class-D Amplifier'? If I were looking at line current draw during a concert I would see that line current going up and down with the volume of the music with a delay of the charge cycle of the energy storage filter capacitors and I wouldn't be able to tell what class of amplifier you were using. I just serviced one of those THREE THOUSAND WATT pieces of shit D.J. amplifiers yesterday and those marketing hype turds can't hardly do a mere 300 watts RMS per channel steady tone both channels driven without going into heavy flat-topping and sounding like shit!!! You can't buy 3000 or even some of the 7000 watt amplifiers for under $250 and think your getting anything more than a 300 watt RMS turd!!!!

author: Faringdon
author_rank: Super Contributor
timestamp: June 23, 2022, 01:45:04 pm
content: Thanks, i am referring to the power draw directly at the vin terminals of the class D Amp.....as opposed to at mains input.

author: dmills
author_rank: Super Contributor
timestamp: June 23, 2022, 02:17:32 pm
content: Class D amp power input is actually 2 quadrant, and particularly in a split rail half bridge design can have issues with bus pumping.And yes, it tracks output power at the audio rate.PFC is 'interesting' as the load varies faster then the mains envelope, and is often just not a thing because it is too difficult to make work properly.Incidentally, good class D power stages are essentially analog sigma delta modulators and have substantial PSRR, bad ones are essentially PWM and have no PSRR to speak of.The trick with the power supplies is that they need to be short term rated for full power into a reactive load at an unfortunate phase angle, but only need thermal design for ~1/8th full power, doesn't save you much on silicon or peak flux but does save a hell of a lot of cooling.

author: langwadt
author_rank: Super Contributor
timestamp: June 23, 2022, 03:00:19 pm
content: Quote from: CaptDon on June 23, 2022, 01:07:33 pmdo you often listen to steady tones? Music is more like 10-20dB crest factor

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 23, 2022, 06:01:45 pm
content: OP: What exactly is your point though? I can't even figure out what this is all about or why this matters.

author: Faringdon
author_rank: Super Contributor
timestamp: June 23, 2022, 09:31:32 pm
content: Thanks, its to assess the transient loading of a power supply meant for Class D amp supply.The "reactive loading" of a class D amp is interesting too......as this could have interesting meaning for the feedback loop compensation of the supplying SMPS.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 23, 2022, 11:48:37 pm
content: I see. Hence this "load" term you used that could have sounded confusing if we were not considering you were referring to the amp's power supply, rather than the amp itself.Generally speaking, a good audio amplifier should be able to handle pretty extreme transients. That will be completely related to its dynamic range. So its power supply will have to accomodate this. Certainly."Banks" of large capacitors on the power rails are often used to help with the transients, especially for the higher power amps.Now for sure, the transients are typicaly more extreme on instrument amps such as bass amps, while heavily mastered music will usually have a more tamed dynamic range.I have designed a few audio amps, but never powered them with SMPS, so I could not help much with this particular point.

author: rsjsouza
author_rank: Super Contributor
timestamp: June 24, 2022, 12:32:15 am
content: Quote from: SiliconWizard on June 23, 2022, 11:48:37 pmIndeed. That is usually why amplifiers use unregulated power supplies on their PA stages, since an ultra low-impedance supply is required to dampen the excess current draw - put a regulator and you will kill it with distortion.SMPS is harder to get the lower noise, although I haven't kept up with the latest developments in the MHz-designs.

author: Circlotron
author_rank: Super Contributor
timestamp: June 24, 2022, 01:20:23 am
content: A related thought - if we use a half bridge pwm class D amp to supply a sinewave into a resistive load, what shape will the (smoothed) current draw from the supply rails look like? It won’t be sinusoidal.

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 24, 2022, 02:09:05 am
content: Depends on your modulation and filtering. I've designed an ultrasonic class D amplifier that had sinusoidal current and voltage after minimal filtering by using a modulation scheme that erased all harmonics up to the n-th (where n depends on your maximum switching frequency).

author: Circlotron
author_rank: Super Contributor
timestamp: June 24, 2022, 06:15:49 am
content: Quote from: KaneTW on June 24, 2022, 02:09:05 amSomething to do with this? ->https://www.tinaja.com/glib/msintro1.pdf

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2022, 06:32:25 am
content: Quote from: SiliconWizard on June 23, 2022, 11:48:37 pmI have designed a an audio amplifier and ran it off an SMPS. Well I say designed, I cheated and used the old TDA2009A and a ready made 24V PSU. It was a PA/intercom system with long cables, driving efficient 15R ceiling speakers. The only issue I had was mains hum, picked up via the Y capacitor, because I used a PSU without an earth connection. A toroidal RF suppression inductor and some capacitors connecting the secondary to mains earth solved the problem.Quote from: dmills on June 23, 2022, 02:17:32 pmHow much does it normally regenerate and push power back into the power supply? I take it that it's more of an issue for large speakers such as a sub woofer?QuoteI would have thought large filtering capacitors would solve that issue, as well as power bus pumping.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: June 24, 2022, 11:08:41 am
content: Quote from: Circlotron on June 24, 2022, 01:20:23 amif the amplifier can be considered being reasonably lossless (as it is class-d), pin(t) = pout(t) = uout(t) * uout(t)/R. with the DC bus voltage VBUS being constant, ibus(t) = pout(t)/VBUS. thus, for sinusoidal output voltage (and output current), the DC bus current into the output stage of the amplifier has sin^2 shape.

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2022, 11:19:02 am
content: Quote from: Le_Bassiste on June 24, 2022, 11:08:41 amIn reality the impedance of the speaker is frequency dependent, rather than just a plain resistive load, which complicates things.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: June 24, 2022, 11:21:01 am
content: Quote from: Zero999 on June 24, 2022, 11:19:02 amso what? as long as voltage and current are both sinusoidal, the power ist sin^2 shaped, no matter what impedance (or z=f(f), or cos phi, for that matter) the load has.

author: KaneTW
author_rank: Frequent Contributor
timestamp: June 24, 2022, 02:13:35 pm
content: Quote from: Circlotron on June 24, 2022, 06:15:49 amLooks similar enough. The technique I used is called "selective harmonic elimination PWM" or "programmed PWM" -- you can find a whole bunch of papers on them.

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2022, 03:46:04 pm
content: Quote from: Le_Bassiste on June 24, 2022, 11:21:01 amMusic is not sinusoidal and the impedance of the speaker is frequency dependant.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 24, 2022, 05:46:50 pm
content: Quote from: Zero999 on June 24, 2022, 11:19:02 amYes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 24, 2022, 06:35:15 pm
content: In short, design for expected worse case load step. In practice, bass drum kicks, explosions, that sort of thing, but any full amplitude tone burst will do.That's it. Nothing is worse, and speakers don't have nearly enough Q at anywhere near low enough of a frequency for regeneration to be a problem. And you've got more than enough capacitance to handle it going up or down. If the supply controller is linear (synchronous LLC innit?), it doesn't give a crap.Tim

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: June 24, 2022, 11:44:14 pm
content: Quote from: Zero999 on June 24, 2022, 03:46:04 pmdid i point out that the impedance is frequency dependent? yesis music non-sinusoidal? yes, in almost all cases.was that the point in circlotron's question? no. however,you can generalize what i said above to p(t) = u(t)*i(t), with i(t) = u(t)/Z, giving p(t) = u²(t)/Z.as you see, no matter what impedance you throw at the amplifier's output, the square relationship of output power and output voltage will hold.

author: Circlotron
author_rank: Super Contributor
timestamp: June 25, 2022, 11:50:36 am
content: Quote from: Le_Bassiste on June 24, 2022, 11:44:14 pmAfter thinking about it a bit further, the power in the load varies according to the square of the load voltage. No surprises there. And seeing the supply rail is a fixed voltage, the current supplied by the supply rail will be directly proportional to the load instantaneous power. So, yes, it seems that the current drawn from the power rails will be sin^2 if the resistive load voltage is a sine. I was getting a bit distracted by the fact that the pwm + filter is similar to a buck regulator so input and output current could differ.

author: Circlotron
author_rank: Super Contributor
timestamp: June 25, 2022, 11:55:39 am
content: Quote from: T3sl4co1l on June 24, 2022, 06:35:15 pmBut seeing speakers are a reactive load, they present a low power factor, if you will. They can suck current on one part of the cycle andreturnsome of it later in the cycle. How low that power factor can get, I'm not sure.

author: langwadt
author_rank: Super Contributor
timestamp: June 25, 2022, 12:24:52 pm
content: Quote from: Circlotron on June 25, 2022, 11:55:39 ambut unless there is DC offset or a very asymmetric waveform it all evens out over a few cycles

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 25, 2022, 03:28:12 pm
content: Quote from: Circlotron on June 25, 2022, 11:55:39 amThat's what I said, the Q is low, like maybe 2 at the most. Well, piezo tweeters could be more, near or above resonance; but at low frequencies yeh, voice coil resistance is a dominant fraction of the total at any frequency. In other words:Quote from: langwadt on June 25, 2022, 12:24:52 pm...Like that.Note that impedance generally goes as sqrt(f) due to skin effect (the steel pole the voice coil moves over, is a diffusion resistance at essentially any frequency of interest), and, the peak at f_s should be small, well matched in a properly designed speaker box.I have a plot here, that I have no idea what it's from; evidently a fairly large woofer, probably 16 ohms nominal, probably in open air (hence the relatively high peak). If it's reasonable, then we can interpret it this way...Note that, even as prominent as the peak looks, it's still only 3x baseline, or about as much Q.Bizarrely, the seeming asymptote, isn't: notice the plot is semilog, so that's actually logarithmic impedance, not sqrt!Tim