There are lots of Class D audio amplifier modules for 12V use that are both readily available and inexpensive, intended for automotive use (hence the 12V rating). I would like to use one of these, rated at about 20 Watts max into a 4 ohm speaker load, as part of a portable "practice" guitar amp, and I'd like to power it with a Lithium-ION or LI-Polymer cell pack. The problem is, all the modules I've looked at have a maximum supply limit of about 14.5V and worse, their sound quality starts to diminish at about 10.5V. So the problem, of course, is that a 3 cell Lithium pack is going to output between about 9.6V (3.2 x 3) and 12.6V (4.2 x 6) over the bulk of its charge range. So with this choice, the sound will begin to suffer long before the pack has exhausted its useful charge. I don't need precision voltage regulation, but I DO need to at least do a little better then a raw 3 cell solution.
I'm not sure what options I can consider here and could use some advise. I suppose I could add another cell and look into a buck regulator circuit, with the disadvantage of adding more bulk to the battery pack, and losing some efficiency to the regulator. Maybe there is a boost regulator I could look into for the original 3 cell approach, but it would have be nearly bypassed at full charge, and of course be able to efficiently supply about 2.2 amps (assuming 100% efficiency of the class D amp). Does anyone have any thoughts, or maybe could recommend other options I've not considered here?
Addendum: Here's a link to one possible inexpensive solution. Based on answers indicating a boost regulator might be my best choice, I started poking around for a ready made module, and I'm finding this one all over e-bay. So I'm just passing this info along for anyone else in the same boat. It seems that with a boost regulator, excess voltage is passed through, so if adjusted to 12V and the input is 12.6, then the output will be 12.6. But thats not a problem for me, and if it was I could simply adjust to 12.6, or even 13V. At 150 watts, this one might be overkill, but they are not as huge as they look in the picture (65mm long). Of course this particular seller may go away. But there seem to be many suppliers, and for around $3 USD, its hard not to get a few just to experiment with. Here's a screen shot in case the link stops working. They seem to be pretty easy to find at this point.
Best Answer
A good 3S Lipo has a usable range of 10.5-12.6V when discharged at a reasonable rate (1C or lower), so it should suit your application perfectly. 9.6V is into the over-discharge zone, where the battery has no capacity left. You should never let the voltage drop below 10.5V under load (11V resting).
If the amp sounds good above 10.5V then by the time you notice the sound quality degrading the battery is already over-discharged and will die very soon. You should use an under-voltage alarm and/or cutoff device to prevent the battery voltage from going below 10.5V.