AA 8 pack vs. 9v battery

[deadspeaker]

I have a couple questions:
1) I've read in a couple places that AA batteries will last longer than a 9v. About how long will each method last and why?

2) I built a bridged LM386 amp for my car(.....the car is just so bad...) and I'd like a hair more volume. Will using 12v provided by an 8 pack of AA give me more volume or headroom? The circuit is a little gem mkII without the gain caps.

I tried searching but could not get a straight answer. At least this thread can serve as an easily searchable answer to a common question. Thanks a lot.

[Mark Hammer]

Since the current that can be supplied is a function of a chemical process, the more chemical content, the more current can be provided, right?  Hence, 6 AAAs will provide more current over time (i.e., ampere-hours) than a 9v battery (which is just 6 small 1.5v slugs inside the case), 6 AAs will deliver more than a sextet of AAAs, and 6 D cells will kick their collective asses around the block.  You can expect C and D cells to last MANY times longer than AAs and certainly 9v batteries.

When it comes to small battery-powered power-amp chips, providing more current, and more voltage, can deliver more output power for longer time, as well as cleaner output.  Just keep in mind that all power-amp chips are rated with respect to how much current they can pass.  As the supply voltage goes up, and the speaker impedance goes down, the chip will be more timid about passing large amounts of current.  So, while a 386 chip can handle 8 D-cells, do NOT expect it to live very long if you feed it to a 2-ohm or possibly even 4 ohm speaker load. 

get yourself the datasheet for the LM386 (or JRC, depending on whatyou have) and look at the chart that shows power dissipation by supply voltage and output load.  It'll give you some advice on how to use 12v safely.

[deadspeaker]

Thanks for clearing that up Mark. Clearer sound is very important to me. I'll go with 8 AAs for now. I found out that the LM386 is rated up to 12volts with an absolute maximum rating of 15v. I'm pretty sure that means that bad things happen after 12 but it's ok if my pack measures 13v. I'll have to test my 8 AAs once they are in the holder.

As for the load, the speakers distorted at 4 ohms. I don't know why, but I can only fit one 12" PA cab on my back seat (and still be able to fit more people in my minivan), so 8 ohm is fine.

[Mark Hammer]

You'd be surprised how much punch 12V of AA power into a 386 can pack.

[deadspeaker]

Well it measured about 13volts. It did make the cab a little louder. Maybe I'll get a new car before the AAs finally run out of power. Thanks again.

[deadspeaker]

Well my car stereo system has grown to a PA cab and a homemade 2x3' box on each channel. That's 4 lm386s! The 8 AAs didn't stand a chance. I think it sounds best at 13volts. 15volts is the absolute max rating.

My question: Would it make sense to regulate a battery? I'm thinking like 12 D size regulated to run at 13volts.

[deadspeaker]

Regulated batteries? Anyone?

[Mark Hammer]

Regulation is what you use to eliminate ripple from a rectified AC supply and make it behave as if it were DC.  So, in one sense, if you already have batteries, you don't need further regulation.

On the other hand, there are plenty of commercial pedals that take a battery supply and use regulation to drop it down to some other voltage that is either needed by the circuit, or else will allow more stable operation over time if it depends on the lower voltage rather than the battery per se (the battery will lose voltage over time/use, but the lower regulated voltage will still be the same for a longer period of use).

It seems to me that you don't really need regulation in your case.  I mean you could, but there isn't any real need that I can see from your description.