"Efficient" audio amp selection

Hi,

I've been designing a "network audio client" (aka "network loudspeaker") and now have to select a suitable amplifier to use in it.

1) The amplifier's presence is "optional" (e.g., when interfacing to external equipment via a "line out", the amplifier is superfluous)

2) The design is intended to support one or two audio channels (e.g., drive a single speaker -- BTL? -- or a pair WHICH MIGHT NOT BE IN CLOSE PHYSICAL PROXIMITY!)

3) Everything is PoE powered (though I will support PoE+ as well) so the power budget is extremely critical.

4) Size is important. E.g., a few cu in for the whole device. (note the implications on heat dissipation!)

5) "Louder is better" but LOUD CRAP isn't worth the effort (i.e., trying to get the most GOOD sound -- not the MOST sound)

6) Any controls need to be available thru software (gain, etc.)

7) Components need to be available in small quantities for hobbyists, etc. -- though I suspect I could find a middleman willing to make large buys and sell in smaller quantities (hardware and software designs will be released as "Open Source")

I've accommodated #1 by putting all of this on a "daughter card". Currently, the interface to that card is digital (i.e., for a "line out" version, another card with DACs and buffers would take its place). Of course, someone can always opt to change this later...

#2 Could be accommodated with two different daughter cards ("mono" and "stereo"). Or, operating a stereo amp in bridge mode (with suitable changes in load). A set of "mono" devices might be used, for example, staggered around a listening area (e.g., a 7.1 theater system) giving more power to each device and/or making wiring more practical (run network drops to each location instead of having to run drops to half of the stations and "string wire" along a baseboard to the "other" speaker in each pair).

Besides power budget, #3 also places constraints on how well behaved the device is when powering up/down -- since power can/will be applied/removed remotely. Clicks and pops need not apply! :>

The size requirement in #4 *could* be flexible but impacts the types of deployments possible. E.g., here, I plan to mount most of them in 1G Jboxes (possibly using the box itself to help dissipate heat). This eliminates all of the cosmetic "packaging" issues.

Sound level and quality (#5) is, of course, a subjective assessment. For high quality deployments, I imagine the amplifier will be removed and replaced by something (external?) of much better caliber. But, when present, it should be "usable" in normal (i.e., non-teenager!) environments. A two channel device might take the place of a "table radio". A set of mono devices (e.g., the 7.1 setup) could provide a rich home theater experience. The impact headroom has on that quality becomes a significant design issue (esp due to #3)

The controls (#6) need to be "remotable". I.e., a "network loudspeaker" mounted in a ceiling would be impractical to adjust if the controls were collocated with the speaker. Being able to "tell" the device what you want and having the device perform that action itself seems the only practical solution.

Personally, I could skip #7 () but that would be a bit selfish in light of the open nature intended. (building something out of unobtainium is hardly a good way to promote its use!)

So far, my searches keep bringing me to TI's class D offerings -- though none seems to be the perfect cherry. And, class D leaves me worrying about sound quality and RFI (generated) -- esp when the loads aren't close to the amp (e.g., the two channel case)

Pointers?

Thanks!

--don

Reply to
Don Y
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Put some high-frequency filtering at the output of the amplifier. Class-D can have perfectly respectable sound quality, if the switches are fast enough. You can confine the switching edges to the immediate vicinity of the amplifier with well chosen inductors and capacitors. In fact you'll have to if you don't want to screw up radio and TV reception in the area.

I don't know what switching frequencies TI is offering these days, so I don't know what your inductors are going to look like - keep in mind that a single layer wound inductor is going to have of the order of

1pF of parallel capacitance, while multilayer winding can be a lot worse. Find out - or measure - self-resonant frequency of any inductor you choose to use.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

I'm not familiar with PoE, what V, I, P is available?

If the PoE only does a watt, of course, this isn't a problem, but if you're allowed a bit more, like 10W, it'll start to get important.

Needless to say, efficient speakers will be a top priority -- 90dB speakers on a class A amplifier (~20% efficient, assuming continuous full volume of course) are *equal* to 83dB speakers on a lossless amplifier!

High efficiency speakers will tend to clash with your requirement of "high quality" audio, particularly if they are as small as this device. Speakers with high efficiency tend to be very resonant with poor frequency response. The very good ones have high efficiency AND flat response, but they are also large and expensive. If they have to be small, you might ask Apple or Bose about their devices (...or marketing..).

These are all, of course, solved problems available on the market, but you're going to pay for them.

That shouldn't be a problem. Even a simple design like this,

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has boom-box level quality. It would be greatly improved with a current source in the triangle generator, instead of the exponential slope, which produces a corresponding distortion in the output, visible on large signal, medium frequency outputs (i.e., between feedback and filter cutoff frequencies). Even as shown, this circuit is an excellent DC to LF amplifier for motor, driver or subwoofer applications, because the feedback loop pushes down the LF distortion.
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As built, RFI is quite low. Differential and common mode chokes, and ceramic caps, do a great job. >20MHz hash is invisible (a spec might argue differently of course), ripple is visible (~120kHz) but not substantial.

I can't imagine any popular monolithic solution would get any traction if it didn't have the distortion to sell it. The only thing I'd watch out for is style -- open loop, closed loop, voltage mode, current mode, carrier frequency, etc. These aspects will dominate performance and fault behavior. For instance, the above circuit is voltage mode, with no fault protection (well, I think I tested it into a 1 ohm load, but that yanked down the power supply instead..).

Hmm, only problem that comes to mind about this might be RFI. Mind where your currents are flowing, keep RF out of the board-to-board connection. I suppose the outputs will get filtering, which will help too. You probably already know all this.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

"Tim Williams"

** A class A amplifier is up to 50% efficient at full sine wave power.

This is true for single ended stages using an output transformer and push pull stages with or without a transformer.

Class B push-pull can go up to 78.5% efficiency at full level.

... Phil

Reply to
Phil Allison

Try an 8 inch high quality speaker in an infinite boffle; you will be amazed at the result.

Reply to
Robert Baer

I'm baffled by your boffle, seen that word twice recently.

Infinite baffle, aka acoustic suspension, nice. I don't like the boomy vented enclosures common these days. At least, not the ones in my price range.

Grant.

Reply to
omg

snipped-for-privacy@grrr.id.au>

** No really.

Infinite baffle implies a rather big enclosure while " acoustic suspension" implies a rather small one.

In the former, the bass resonance frequency is set mainly by the driver itself and in the latter mainly by the enclosure volume.

** Vented ( ie tuned port) boxes are not boomy.

Not if Messer's Thiele and Small have anything to do with it.

.... Phil

Reply to
Phil Allison

25% being the case for single ended, resistive loaded stages, and in practice, much lower being common (although I've had tubes up around 15-20% plate efficiency in this mode before, though the distortion was not what one would call high-fidelity).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

I'm concerned with the *bulk* that would involve (note the entire device wants to be just a couple of cubic inches). It's alarming how quickly that volume gets eaten up (electronics, connectors, etc.)

My original vision was for a single channel device bolted directly to the driver. I.e., it's bulk hides in the speaker's envelope; no long wires leading *to* the voice coil from the device, etc.

I.e., the two channel optimization may prove NOT to be a net improvement due to the other consequences it introduces.

Reply to
Don Y

The biggest constraint is available *power* (~12W for PoE, about double that for PoE+) as you can create whatever supplies you need from the input supply (at various efficiencies).

Of course, I have a low voltage supply available for the logic but that takes minimal power. A second supply for the amplifier (and any analog processing). Due to the overall power limitations, I can't imagine that second supply being more than 15-18V (single ended).

Since some instances may opt to power the amplifier locally (more power available, less demands on the PoE suppy, etc.), it would be good to pick a "nice number" to make the choice of local power supply easier to satisfy.

Exactly.

I'm not concerned with the choice of speaker. I.e., all of the value added, here, is in the hardware and software designs. If someone opts to tack a good/bad speaker onto one... as long as it fits their requirements, what do *I* care? :>

My personal needs vary in fidelity, durability, etc. E.g., the speaker used "for announcements" (someone is at the door; the garage door was just opened; time to check the roast; etc.) can be of low fidelity but must endure a harsh physical environment (i.e., without cone drying out). OTOH, those with multimedia application will tend to want to be of higher quality...

Ouch! Physically too large. I was hoping to tap into the market created by battery powered, hand-held devices -- though my power requirements tend to be an order of magnitude higher than most of those (and the voltage/power available accordingly)

What are the major tradeoffs with each? Remember the environment: aside from assembly, there should problem with "shorted outputs" unless a voice coil overheats/shorts -- in which case, the cost of the speaker and labor to replace it might be high enough to justify replacing the "amplifier" as well.

I'm concerned with the efficacy and size of filters on the output. Since space is *really* tight, even high frequency devices can be (relatively) large.

Reply to
Don Y

TPA2018

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

Ok, so it could be kind of all over, implementation dependent. That'll make loudness difficult to tack down, but maybe that's not such a big deal after all. The PA speaker might be a cheap, tinny, relatively efficient type, or outdoor compatible; the hi-fi types might be low efficiency, but that might not be a problem for the casual iPod user who wants something for quiet listening without those horrible earbuds.

That circuit could be miniaturized quite a bit as-is; notice it's discrete on a two layer board (the best I've ever made by hand, if I do say so myself.. the layers lined up perfectly!). A couple drivers, or a whole monolithic chip, should leave enough room for your receiver and DAC business.

Yeah, it's not quite consumer material, but if you have to use BGAs and such to fit the package, a dead circuit is pretty much throwaway to anyone.

As for amps, I don't know much about them beyond my own experience. Even open loop can give reasonable fidelity, but protection circuitry is extra. Closed loop cleans up the response, asymptotically, but usually leaves distortion at higher frequencies, relating to the open-loop artifacts that feedback isn't fast enough to clean up. (As with any audio, you get the best results with the best of everything, and only then add NFB to make it electrically perfect.)

Current mode is the best method for switching supplies in general, because having first-order control over inductor current prevents any possible fault problems. Unfortunately, speakers aren't current mode (well, most of them*..), so a voltage-mode loop is required, which means the overall response will be inductive at high frequencies, coming down to resistive or voltage sourced at low frequencies. On the plus side, current control is usually a first order system, so its dominant pole can be almost as high as the clock frequency, impacting overall frequency response minimally. With the inner loop keeping average current accurately at the setpoint, linearity should be high, even if the PWM method is poor.

*A friend of mine built a variable impedance amplifier for the audiophile circle. You can dial in the boominess at will. Some specially made speakers respond quite well to high impedances, most are ridiculously terrible!

If you can put the boards behind the speakers, this will be fine -- the voice coils or magnets might get a bit toasty from the eddy currents, which will hit you with quiescent current, but if it's not too much, no big deal. This does rule out stereo amplifiers unless you're going to have a panel connector for the other one.

I grilled a turkey last night, over charcoal of course (anything else is wasted time!). Soaked in brine and all that. (I'd use the grill to claim hardcore Wisconsinism, but it's been downright muggy this year.) This turkey sandwich is *delicious*. Oven roasted, never again ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

Presuming so much as 6 watts to the speaker (normally plenty to way loud) it is worthwhile to invest in quality 4 to 6 inch drivers, US $50 and up each. No, i am not kidding, good drivers co$t plenty. With external power you can get less expensive higher power drivers and compensate them in electronics (al la Bose)

Militating strongly for class D. Keep switch frequency above 200 kHz but below 500 kHz, and place then in a metal (foil or even film) shielded = box.

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Reply to
josephkk

though

I am reasonably sure that one channel devices is the way to go. Whether it is mono from the doorbell, stereo, or 7.1. Too many problems, too little advantage.

Reply to
josephkk

The biggest problem comes with tabletop/countertop/bedside deployments -- where the speakers tend to be *relatively* lose together. You don't want to have to run *two* drops to the same place.

Or, in our case, the pair of speakers above the kitchen sink (separated by about 4 ft).

(The "boombox" option can typically be addressed by adding components to the "enclosure" housing the speakers)

Reply to
Don Y

Yup. As it should be, IMO. Let the end user trade dollars for performance. Folks with "tin ears" or who just listen to talk radio will care little for the quality of the driver.

Exactly. E.g., we often listen to "background music" in the kitchen while preparing a meal. We don't need head banging volumes but would prefer resonable *quality*.

Yup. And, my point was, you aren't dicking with those connections (intentionally) often. Not like "time to move the hifi as we rearrange furniture". Nor are you likely to encounter UNintentional changes ("oops! I tripped on the speaker wires and pulled them out of their bindings!")

I.e., you might stress the network connection but not the

*internal* ones!

Hmmm.. I thought you always wanted to keep the damping factor as high as practical (of course, the impedance of the speaker also varies)

Yes.

Even a connector implies a *cable*. Then, I think the filter becomes necessary.

Some years ago, a friend roasted a pig (in his driveway). It was, without a doubt, the best pork I'd ever eaten! THough I admit some trepidation in taking those first bites: "Do I

*really* think this thing has been prepared and cooked properly???!"

Unfortunately, its not the sort of exercise you can repeat without dozens of hungry mouths available!

Reply to
Don Y

Whether

Understood. Small change in the module boundaries; one NIC, two (maybe three) audio channels; for those (few?) locations. I had focused on the distributed ceiling speakers. Oops.

Of course, maybe you do not need stereo at boom box locations. Only = place for 7.1 is the home theater. (?)

Different requirements result in different system specifications and thus modularization. Do what supports your needs.

?-)

Reply to
josephkk

Kinda wimpy -- 1(3)W into 8(4) ohms.

That seems to be typical of the sorts of devices I encounter... as if intended for a small, battery powered PMP, etc. (e.g., the 6V power supply)

I guess the rationale is that you will "tolerate" class-D for those extreme low power applications where nothing else is possible. I seem to be operating at a point a bit too high for most of these devices -- but still constrained on power so unable to use AB style monolithic amps.

Reply to
Don Y

It *really* feels "wrong" to support two channels. Where do you stop? 2? 2+1? 4? etc.

And, when the amplifier is NOT required (e.g., when an external amplifier is available and all you need is "line out"), then you have to think about "is two actually *enough*"?

Depending on the device(s) chosen, I *could*, conceivably, stack multiple "output cards" (since any package that has to present "line out" jacks would need a larger enclosure). Likewise for power amps.

Of course, the more channels you support, the tougher the processing requirements (so you start needing more resources ahead of the amp/DAC/etc.)

But, that's what makes it all *fun*!

Yup. I may have to resign myself to different hardware designs to address the differing deployments. But, I should be able to scale the software easily (which is the hardest part of such a project, anyway!)

Reply to
Don Y

There are fairly high-power AB power amps, used in the automotive industry. Cooling won't be easy and they may be hard to find, onesy-twosy, though.

Reply to
krw

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