Forums

"Efficient" audio amp selection

Started by Don Y January 8, 2012
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 (<grin>) 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
On Jan 9, 1:04=A0am, Don Y <t...@isnotme.com> wrote:

<snip>

> So far, my searches keep bringing me to TI's class D offerings -- though > none seems to be the perfect cherry. =A0And, 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?
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
"Don Y" <this@isnotme.com> wrote in message 
news:jedav5$sqt$1@speranza.aioe.org...
> 3) Everything is PoE powered (though I will support PoE+ as well) > so the power budget is extremely critical.
I'm not familiar with PoE, what V, I, P is available?
> 4) Size is important. E.g., a few cu in for the whole device. > (note the implications on heat dissipation!)
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.
> 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)
That shouldn't be a problem. Even a simple design like this, http://myweb.msoe.edu/williamstm/ClassD2_Schematic.pdf 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. http://myweb.msoe.edu/williamstm/Images/ClassD_4.jpg http://myweb.msoe.edu/williamstm/Images/ClassD_5.jpg 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..).
> 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...
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
"Tim Williams"

> on a class A amplifier (~20% efficient, assuming continuous full volume of > course)
** 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
Tim Williams wrote:
> "Don Y" <this@isnotme.com> wrote in message > news:jedav5$sqt$1@speranza.aioe.org... >> 3) Everything is PoE powered (though I will support PoE+ as well) >> so the power budget is extremely critical. > > I'm not familiar with PoE, what V, I, P is available? > >> 4) Size is important. E.g., a few cu in for the whole device. >> (note the implications on heat dissipation!) > > 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. > >> 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) > > That shouldn't be a problem. Even a simple design like this, > http://myweb.msoe.edu/williamstm/ClassD2_Schematic.pdf > 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. > http://myweb.msoe.edu/williamstm/Images/ClassD_4.jpg > http://myweb.msoe.edu/williamstm/Images/ClassD_5.jpg > 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..). > >> 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... > > 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 >
Try an 8 inch high quality speaker in an infinite boffle; you will be amazed at the result.
On Sun, 08 Jan 2012 21:09:37 -0800, Robert Baer <robertbaer@localnet.com> wrote:

>Tim Williams wrote: >> "Don Y" <this@isnotme.com> wrote in message >> news:jedav5$sqt$1@speranza.aioe.org... >>> 3) Everything is PoE powered (though I will support PoE+ as well) >>> so the power budget is extremely critical. >> >> I'm not familiar with PoE, what V, I, P is available? >> >>> 4) Size is important. E.g., a few cu in for the whole device. >>> (note the implications on heat dissipation!) >> >> 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. >> >>> 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) >> >> That shouldn't be a problem. Even a simple design like this, >> http://myweb.msoe.edu/williamstm/ClassD2_Schematic.pdf >> 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. >> http://myweb.msoe.edu/williamstm/Images/ClassD_4.jpg >> http://myweb.msoe.edu/williamstm/Images/ClassD_5.jpg >> 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..). >> >>> 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... >> >> 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 >> > Try an 8 inch high quality speaker in an infinite boffle; you will be >amazed at the result.
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.
omg@grrr.id.au>

> Infinite baffle, aka acoustic suspension, nice.
** 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.
> I don't like the boomy vented enclosures common these days.
** Vented ( ie tuned port) boxes are not boomy. Not if Messer's Thiele and Small have anything to do with it. .... Phil
"Phil Allison" <phil_a@tpg.com.au> wrote in message 
news:9mv67bFvbaU1@mid.individual.net...
> ** 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.
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
"Tim Williams = Wanker "

> "Phil Allison" > >> ** 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. > > 25% being the case for single ended, resistive loaded stages,
** Totally irrelevant to an amplifier driving a speaker. You pathetic, context shifting, over snipping bullshitter. ... Phil
Hi Bill,

On 1/8/2012 5:39 PM, Bill Sloman wrote:
> On Jan 9, 1:04 am, Don Y<t...@isnotme.com> wrote: > >> 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? > > Put some high-frequency filtering at the output of the amplifier.
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.
> 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 > >