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