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Don Y
Guest
Mon Jan 09, 2012 1:04 am
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
Bill Sloman
Guest
Mon Jan 09, 2012 2:39 am
On Jan 9, 1:04 am, Don Y <t...@isnotme.com> wrote:
<snip>
Quote:
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.
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
Tim Williams
Guest
Mon Jan 09, 2012 4:29 am
"Don Y" <this_at_isnotme.com> wrote in message
news:jedav5$sqt$1_at_speranza.aioe.org...
Quote:
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?
Quote:
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.
Quote:
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..).
Quote:
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
Phil Allison
Guest
Mon Jan 09, 2012 4:45 am
"Tim Williams"
Quote:
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
Robert Baer
Guest
Mon Jan 09, 2012 6:09 am
Tim Williams wrote:
Quote:
"Don Y" <this_at_isnotme.com> wrote in message
news:jedav5$sqt$1_at_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.
Guest
Mon Jan 09, 2012 12:55 pm
On Sun, 08 Jan 2012 21:09:37 -0800, Robert Baer <robertbaer_at_localnet.com> wrote:
Quote:
Tim Williams wrote:
"Don Y" <this_at_isnotme.com> wrote in message
news:jedav5$sqt$1_at_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.
Phil Allison
Guest
Mon Jan 09, 2012 2:05 pm
omg_at_grrr.id.au>
Quote:
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.
Quote:
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
Tim Williams
Guest
Tue Jan 10, 2012 2:05 am
"Phil Allison" <phil_a_at_tpg.com.au> wrote in message
news:9mv67bFvbaU1_at_mid.individual.net...
Quote:
** 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
Phil Allison
Guest
Tue Jan 10, 2012 2:11 am
"Tim Williams = Wanker "
Quote:
"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
Don Y
Guest
Tue Jan 10, 2012 7:30 pm
Hi Bill,
On 1/8/2012 5:39 PM, Bill Sloman wrote:
Quote:
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.
Quote:
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
Guest
Tue Jan 10, 2012 7:48 pm
Hi Tim,
On 1/8/2012 8:29 PM, Tim Williams wrote:
Quote:
"Don Y"<this_at_isnotme.com> wrote in message
news:jedav5$sqt$1_at_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?
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.
Quote:
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.
Exactly.
Quote:
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.
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... <shrug> 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...
Quote:
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.
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)
Quote:
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..).
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.
Quote:
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.
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.
Vladimir Vassilevsky
Guest
Wed Jan 11, 2012 3:19 am
Don Y wrote:
Quote:
Hi,
I've been designing a "network audio client" (aka "network loudspeaker")
and now have to select a suitable amplifier to use in it.
TPA2018
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Tim Williams
Guest
Wed Jan 11, 2012 3:48 am
"Don Y" <this_at_isnotme.com> wrote in message
news:jei14t$96h$1_at_speranza.aioe.org...
Quote:
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... <shrug> 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...
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.
Quote:
http://myweb.msoe.edu/williamstm/Images/ClassD_4.jpg
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)
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.
Quote:
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.
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.
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!
Quote:
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.
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
josephkk
Guest
Sat Jan 14, 2012 6:58 am
On Tue, 10 Jan 2012 11:48:04 -0700, Don Y <this_at_isnotme.com> wrote:
Quote:
Hi Tim,
On 1/8/2012 8:29 PM, Tim Williams wrote:
"Don Y"<this_at_isnotme.com> wrote in message
news:jedav5$sqt$1_at_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?
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).
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)
Quote:
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.
4) Size is important. E.g., a few cu in for the whole device.
(note the implications on heat dissipation!)
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.
Quote:
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.
Exactly.
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.
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... <shrug> 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...
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.
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)
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..).
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'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.
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.
josephkk
Guest
Sat Jan 14, 2012 7:04 am
On Tue, 10 Jan 2012 11:30:02 -0700, Don Y <this_at_isnotme.com> wrote:
Quote:
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.
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.
Quote:
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
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