6AA or 1 9v? to power lm386

[dmartn149]

Hi, I'm building a battery powered amp. Should I use one 9v or 6AA? Space isn't an issue, I just want to get the most battery life.
Thanks, Dan

[Ripthorn]

6 AA batteries will have a better life, just like 6 C size or D size will have longer life than the AA's.  This has to do with how much of the chemicals are contained in the battery.  If you crack open a 9V (not really recommended) you will find six tiny 1.5V cells (at least, that is how most of them are I believe).  So if you really want long life, go with the largest size battery you can.

[dmartn149]

Thanks Rip. That's what I thought, but I wasn't sure. I don't know much about electronics. kind of learning as I go. I didn't realise that there were smaller cells inside a 9v. There must be something about the technology that makes them come out to 1.5v ea.
Anyway, Thanks!

[Mark Hammer]

It's not the technology, but the chemistry.  1.5V, or thereabouts is what you get as the potential difference between + and - with those chemical cells.  Cheaper carbon-zinc 9v batteries will have 6 cells inside, but they look more like black slugs.  Higher-quality alkaline batteries will have 6 little sub-AAA cells inside, with a metal strip spotwelded to them to provide contact.  9v batteries are not a different technology than 1.5v batteries.  Rather, they are simply a packaged convenience.  There are also 6v and 12v batteries that you can find commercially, which will undoubtedly have 4 and 8 cells inside to suit their particular application, but "our" 9v is suited to our needs.

The difference between chips like the LM386 (or any other power chip) and more "normal" op-amps, is that power amp chips need to be able to pass a lot of current through a low-impedance load.  The voice coil of the speaker/s you will feed with the chip need lots of current to be able to produce movement of the cone.  In general, the amount of current available from any battery will be a function of the size of the battery, with D-cells providing more than C-cells, which provide more than AA-cells, which provide more than AAA-cells, and so on.  A 9v battery will be able to provide that current for a short while, but will soon fall under the minimum needed to do the job, unless you are using it to power earphones or something else where the voice coil is not expected to move very far.

The upshot s that, if you can have the space for it, a 6-pack or 8-pack of either AAs or AAAs will likely give you better performance for a longer period of time.

[dmartn149]

Thanks for the short lesson, Mark. Now I'm thinking I'll use C or D cells. Space is no issue, I am building it inside an Army night vision goggles case.

[petemoore]

12v affords more headroom.

[Mark Hammer]

12v affords more headroom.

Yes, but observe the output load guidelines provided by the datasheet.  Some 386 chips are not crazy about feeding a 4ohm load using a 12v supply.  They may behave well feeding 8ohms with a 12v supply, or 4ohms with a 6v supply, but higher supply voltage and lower speaker impedance may let the magic bue smoke out.