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.

cheap 150 w boost reg

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    \$\begingroup\$ There are lots of class D amps out there suitable for use with a Li-ion battery. Why not use one of those instead? \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 26 '16 at 15:38
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    \$\begingroup\$ I would look at the the boost regulation option, but be sure to add under voltage lockout to endure that you don't deep discharge the batteries (current draw from the battery will increace as their voltage drops). \$\endgroup\$ – user1582568 Jan 26 '16 at 15:45
  • \$\begingroup\$ Why do you assume that "the sound will begin to suffer" when the supply voltage for the power amp. becomes lower ? I would say: the maximum power before clipping would become a little bit lower but your ears work in a logarithmic way. The difference between 10 Watt and 5 Watt is very small. As long as no clipping occurs, the Amp's feedback will make the sound the same for low and high supply voltages. \$\endgroup\$ – Bimpelrekkie Jan 26 '16 at 15:56
  • \$\begingroup\$ @user1582568 Yes and thanks... I already have made provision for locking out the cells beyond a certain discharge point. I'd like to look into the boost regulator option, but am hoping to gain some insight here as to how well they work with such transient loading as a music source would have to work with. As far as the sound beginning to suffer, it is not an assumption, but is based on experience. But it is difficult to know for sure whether the quality is purely about clipping, or is partially the fault to the feedback circuits failing below the spec. voltage. \$\endgroup\$ – Randy Jan 26 '16 at 16:05
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    \$\begingroup\$ I would use a boost-only topology. It should be OK for the input voltage to be the same as the output voltage for most boost regulators. You can set the output voltage at 13 V if you want to make sure Vout > Vin. Most regulators have pretty good transient response. Audio load changes are slow compared to the response time of a regulator, so this is actually not a demanding application. Read the datasheet for the regulator carefully. \$\endgroup\$ – mkeith Jan 26 '16 at 18:09

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.

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).

So with this choice, the sound will begin to suffer long before the pack has exhausted its useful charge.

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.

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  • \$\begingroup\$ 10.5 is 3.5 V/cell. It is true. I have seen systems that cut off at 3.5V. The voltage is already falling steeply. Extending to 3.4 or 3.3 adds very little extra run time. It might be desirable to go down to 3.4 / cell, but the added cost and complexity is probably not worth it. \$\endgroup\$ – mkeith Jan 27 '16 at 6:16
  • \$\begingroup\$ It is possible that some of these lower priced automotive class-D's I've been working with are even less tolerant of lower voltage then they should be. After all, a car battery should seldom be below 11 volts, and if anything you'd expect higher then 12. But bottom line, I can hear unwanted distortion too early in the discharge cycle. So maybe a boost regulator module, possibly like this one, might work for me. gearbest.com/multi-rotor-parts/pp_294545.html \$\endgroup\$ – Randy Jan 27 '16 at 12:38
  • \$\begingroup\$ Which units do you have that distort below 12V? \$\endgroup\$ – Bruce Abbott Jan 27 '16 at 18:29
  • \$\begingroup\$ Well here's one I really like, because it has some built in auto-limiting to prevent the God awful sound of Class-D overdrive. Since I'm building practice guitar amps, something like that is important. But it seems a lot of feature start breaking down below 11V. dx.com/p/y148-audio-amplifier-module-93121#.VqophporJiw \$\endgroup\$ – Randy Jan 28 '16 at 14:48
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    \$\begingroup\$ That module is rated for 9-15V, and the YDA148 IC it uses is rated down to 8.0V. Therefore it should work fine down 10.5V. Some users have reported better sound quality with a larger power supply filter cap (eg. 4,700uF). \$\endgroup\$ – Bruce Abbott Jan 28 '16 at 18:18

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