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Building small stereo speaker w/ dual LM386 amp

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jivix

New Member
Hello everyone. I've been working on a small project using 2 LM386's I purchased from Radioshack, because to the best of my knowledge an LM386 can only amplify one channel of audio. I want to build it in a small wooden enclose, and for it to operate off of battery power.

This is the circuit I used:
Chapter 10: Computers and Electronics

I have built 2 amplifier circuits already, but I still need to build the enclosure and mount them, etc.

For the battery power, I would like to operate off of a supercapacitor or the like. I don't want to shell out for dozens of 9V batteries, and I like the fast charging qualities of capacitors. I read an article about building your own 9v battery replacement, but the link was missing:
SuperCap 9V battery - Hack a Day

I have found a 1000uF 11V capacitor, and was wondering how I would go about stepping down the voltage. Would an inductor work for something like this, or should I go with an integrated solution? I would like it to be as efficient as possible without going over a $20 budget.

My estimate to the current requirements of the current circuits are about 250mA to 350mA, but I have not measured it yet. If it is required, I can do it.
 

Nigel Goodwin

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Most Helpful Member
For the battery power, I would like to operate off of a supercapacitor or the like. I don't want to shell out for dozens of 9V batteries, and I like the fast charging qualities of capacitors. I read an article about building your own 9v battery replacement, but the link was missing:
SuperCap 9V battery - Hack a Day
Use batteries, NiMh so you can recharge them - Supercaps are really pretty useless for most purposes, and don't last very long.
I have found a 1000uF 11V capacitor, and was wondering how I would go about stepping down the voltage.
A 1000uF might last a second or two, it's a LONG way short of been a Supercap.
 

audioguru

Well-Known Member
Most Helpful Member
The link to the project does not work.
A reader commented that a little AAA cell has 1000 times the capacity of a 11F super-cap. A 9V battery has six AAAA cells in series so it has the capacity of 500 super-caps.

A 9V alkaline battery has a capacity of 625mAh at a current of 25mA. A 9V Ni-MH rechargeable battery has a capacity of only 175mAh.
 

bountyhunter

Well-Known Member
The link to the project does not work.
A reader commented that a little AAA cell has 1000 times the capacity of a 11F super-cap. A 9V battery has six AAAA cells in series so it has the capacity of 500 super-caps.

A 9V alkaline battery has a capacity of 625mAh at a current of 25mA. A 9V Ni-MH rechargeable battery has a capacity of only 175mAh.
And you can get AA "camera" type NIMH batteries all over the place that are 2000 mA-hr. Stack six and get a ton of power.

Even generic AA NI-MH are in the 1200 - 1400 mA-hr ballpark and you can find them cheap a lot of places. I see them at places like Harbor Freight tools for about $6 for a four pack.
 

jivix

New Member
My bad, I overestimated the power of a Farad. I just graduated high school and I am going to start out in college. I took 3 years of engineering classes but we've only gone so far as watts/horsepower and not so much into joules and farads.

Is there any way for me to set up a super-durable (long lasting) rechargeable setup for this? From the looks of it so far there isn't any way to do it, save solar panels.

What about D batteries? How would I go about stepping up the voltage? Should I use one of those big dual-coil wraps that step up voltage, or should I do an electronic voltage stepper?
 

audioguru

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If you stepup the voltage of a single battery cell then its current increases the same amount so that its power output is what you want. So it won't last very long. Then you shoulld use six AAA or AA Ni-MH cells in series to make 9V instead.

Watch out for huge D cells. The one from Energizer is a big package with a little AA Ni-MH cell inside. Their C cell is the same.
 

jivix

New Member
Ah but then why do C cells and D cells have different amounts of mA? Or was this change with Duracells very recent? According to This, "D" batteries have about 50% more capacity than "C" batteries. I have seen that they can also provide a significant amount of current at once, and they create some very potent electromagnets when tied up to a coil w/ steel rod. However, if it would be easier for me to stick with the 9V's, I suppose I will stick to them. I'd just rather not have to buy 2 9V batteries every time they die on me (as I would have one for each channel of audio, or have them parallel connected to both.) I will also be adding a power adapter to it in case I would like it plugged in. Will I have to add diodes to the 9V's to keep the adapter power from going through the batteries?
 

audioguru

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Most Helpful Member
Duracell is the name of a battery manufacturer, not the name of a certain type of battery.

Like every other battery maufacturer, Duracell makes throw-away alkaline batteries and Ni-MH rechargeable batteries. The datasheets on their website show that they are the same as every other battery manufacturer (except cheap Chinese ones).

Most products that are powered from a battery and an AC adapter use a jack with a built-in switch to disconnect the battery when the adapter is plugged in.
 

jivix

New Member
I understand that Duracell is a company, I am talking about "D" cell batteries... the largest consumer cylinder cell they produce (for small electronics).

Yes, I know they have switches to keep the batteries safe when running on the DC adapter. What I am asking is, what method should I use to protect the batteries? The first thing that came to mind were diodes.
 

audioguru

Well-Known Member
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Some D-size Ni-MH rechargeable battery cells are cheap and lightweight. Because they have a little AA cell inside. A good D-cell is expensive and heavy because it is full of battery stuff.

You do not need a switch. You also don't need a diode.
The jack from the AC adapter has a built-in switch that disconnects the battery when the adapter is plugged in.
 

bountyhunter

Well-Known Member
Some D-size Ni-MH rechargeable battery cells are cheap and lightweight. Because they have a little AA cell inside. A good D-cell is expensive and heavy because it is full of battery stuff.

I believe you will find the little cell inside of the fake D and C cells is actually called a sub-C cell. It's a bit smaller than a standard C cell. I have cut them open before, that's what was in there. Sub C cells are very cheap because hey are used in the hundreds of millions to power tools and consumer junk.
 

audioguru

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Energizer sells Ni-MH AA and AAA cells that are made in Japan.
Their Ni-MH 9V battery is made in Germany.
Their cheap Ni-MH C and D cells are made in (guess where?). China.

They still make disposeable alkaline cells in the United States.
I don't know where their lithium cells are made.
 
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PatM

Member
I used to use NiMh in my camera's but found that if not used soon, they loose charge within several weeks.
I have replaced all of my NiMh cells with Sanyo Eneloop cells.
These cells can sit for a year and still maintain about 85% of their charge.

If you are constantly using and recharging, the NiMh are fine, but I prefer the Sanyo eneloop cells for my usage.

PatM
 

audioguru

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Most Helpful Member
Energizer's new AA Ni-MH cells also keep their charge for a long time. Maybe they are the Sanyo ones.
 

bountyhunter

Well-Known Member
I used to use NiMh in my camera's but found that if not used soon, they loose charge within several weeks.
I have replaced all of my NiMh cells with Sanyo Eneloop cells.
These cells can sit for a year and still maintain about 85% of their charge.
When rechargeable batteries lose charge fast (like a few weeks) it means the internal insulating layers are degraded and allowing leakage current which discharges the batteries. Very common problem with NI-CD or NI-MH cells.
 

audioguru

Well-Known Member
Most Helpful Member
Old Ni-cad and Ni-MH cells were discharged when purchased. They self-discharged very quickly. New ones are pre-charged because they are made differently with a much longer shelf life for each charge. Their capacity is a little less than the old ones.
 

bountyhunter

Well-Known Member
Old Ni-cad and Ni-MH cells were discharged when purchased. They self-discharged very quickly. New ones are pre-charged because they are made differently with a much longer shelf life for each charge. Their capacity is a little less than the old ones.
Must have been before my time, and I am a dinosaur. I heard tales of REALLY old NI-CADs shipped with shorting bars across them to keep them fully discharged. With no potential difference between the cathode and anode, they can't grow those internal crystal structures some of them do when the separators are not sealed well enough. Every NI-CD I have seen in the last 40 years shipped with a partial charge... and when started self dying quickly, it meant the separator was hosed. And they had to be ZAPPED with a capacitor to blow out the internal short to get it to take a charge again.
 
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jivix

New Member
Audioguru, how does the AC adapter magically route power away from the batteries? If I wire the adapter parallel to the batteries, won't some current be routed through the batteries, dangerously? I will remind you that I have not constructed this yet. It is not a prebuilt stereo that automatically switches between batteries and adapter. I need help making a circuit that will switch between batteries and a DC adapter. Or, I suppose, I could just make a manual switch so I can use either. But automatic would be much neater.

Also, I have no idea what you guys are talking about fake "D" cells for... I buy Duracell D's and they are extremely heavy for their size. They are no doubt different from C cells. I have measured the cells with a multimeter and they do output at different current levels. I do not know about other brands, however. Can anyone help me make this thing?
 

bountyhunter

Well-Known Member
Audioguru, how does the AC adapter magically route power away from the batteries? If I wire the adapter parallel to the batteries, won't some current be routed through the batteries, dangerously? I will remind you that I have not constructed this yet. It is not a prebuilt stereo that automatically switches between batteries and adapter. I need help making a circuit that will switch between batteries and a DC adapter. Or, I suppose, I could just make a manual switch so I can use either. But automatic would be much neater.
It will be automatic if you use a standard 1/8" jack connector and matching plug. The jack has a normally closed set of contacts you route the battery power through (these kind of jacks are often used for the speaker of a portable device, which is disconnected when the headphone plug is inserted). In your case, the power from the AC adapter goes to the plug. When the power plug is inserted into the jack, it breaks the line to the batteries and provides power directly to the radio from the AC adapter. Very easy to wire up.
 
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