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 AC Amplifier on DC source...
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Tutanchamun
New Member

4 Posts

Posted - Mar 14 2014 :  11:36:24 AM  Show Profile  Reply with Quote
Hello to everyone, i am new here and i saw that here my problem might be solved by the wisdom of the community:-)

I have built an custom Sound Box (out of a not used drawer).

i had the speakers laying around but the essential part, the amplifier was bought from dealextreme:
http://dx.com/p/tda2030a-2-1-3-channel-finished-subwoofer-amplifier-board-compatible-with-lm1875-267656#.UyMbRJK6-YE

The problem with this thing is, that it consumes alternating current, but i planned to built in an Lithium ion battery pack, so i do not provide AC.

I have uploaded pictures to an image hoster (trixuum), so you can see my problem (i hope).
Link is:








It will not work when powered by the DC source, thats what i tried for some seconds (to not destruct the device).

But as far as i know, the amplifier itself uses diodes (visible at the input) to make an DC of the Input AC.

So my question is (finally^^): Can i some way bridge the ac input or desolder the diodes and parts which do the AC-DC Transformation?

How can i use this device with batteries (i have bought an 11.1 V Lithium pack, but when i measure voltage after the rectifier, my multimeter shows some 16-17 Volts...so maybe i use just one or two cells more?

Ok. Stop, now i hope you can give advice...i wanted to use this box for causal listening, i know its not high end...;-)just spare parts glued together...sorry for this.

Michael

audioguru
Nobel Prize Winner

Canada
4218 Posts

Posted - Mar 14 2014 :  1:30:01 PM  Show Profile  Reply with Quote
The translation on the ad for the amplifier is awful. It talks about a dual-polarity power supply (positive power and negative power) and the photo shows two large filter capacitors for it. So you need two batteries.

The power transformer probably uses the 3-terminals strip with its center-tap probably on the center terminal. Connect your two batteries in series and connect the junction of the two batteries to the center terminal. Connect the unconnected positive pin of one battery to one outside terminal and connect the negative pin to the remaining outside terminal. The rectifier diodes will sort the polarities so they are correct.

The datasheet for the TDA2030A amplifier IC says its supply can be plus and minus 16V (a total of 32V) for 18W into 4 ohms. But the small heatsink on the amplifier limits the output from each of its 3 amplifiers to only about 10W each then the supply can be plus and minus 12V for 4 ohm speakers or plus and minus 15V for 8 ohm speakers.

If you use two 11.1V batteries then the output power from each of the three amplifiers is about only 3W or 4W into 8 ohm speakers.

You MUST have a low voltage cutoff or the Lithium batteries will be destroyed. An 11.1V Lithium battery must be disconnected when its voltage has dropped to about 9.6V.

Do you have a charger made for charging a Lithium battery? The battery will explode or catch on fire if it is not charged correctly.
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Tutanchamun
New Member

4 Posts

Posted - Mar 14 2014 :  6:37:46 PM  Show Profile  Reply with Quote
First of all, thank you for the quick reply, its awesome, that someone really can help me, i was really considering to buy a 12 v dc to 230 v AC inverter and then use a common transformer back to 12 v AC...

I am using the amplifier right now with an 12V 500mA Transformer (which makes just 6 watts, i know, but its not even warm, when i am on 50% loudness, i dont know, how it works...it sounds okay)

I have bridged the two outer inputs and feed the middle and the right one with the transformer and this works just fine.

I know taht the description i strange, i was wondering what the meant with positive and negative of an alternating current source...

I will try your suggestion with the batteries in series, it sounds good:-)

i am using a small fan on the heatsink, so it stays completely cool, will this help to increase the max. (thermal possible) output? (i am sorry, i am not that familiar with amplifiers)

I am using the common 18650 Lithium cells (as they are in notebook batteries) with protection, the better ones (real trustfire, not the fake ones) an they are in a battery holder in a series of three.

I guess that its better to buy a holder with four cells in a row, so i get 14,4 volts to get more power from the amplifier, right?

The cells have some kind of protection against overcharge ans discharge (cut off at 3.25 volts as far as i know) but i know, a disccrete solution against over discharge would be the better way, and i am planning something like this with a manual voltage control (small led-voltmeter to check the voltage on the go an switch off if necessary). I dont know a better solution for this, as my knowledge in voltage regulation/control and such things are very basic

And to your last question, of course i am using an appropiate charger for the 18650 cells!:-)

I am changing all my household devices to accept the bigger and more reliable 18650 cells, because i hate primary cells and the old NiMH secondary cells also, they are not reliable and have so much less power...but thats another story....sorry

PS: my english might be a bit strange, its not my mother tongue, i apologize for that.

Michael
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audioguru
Nobel Prize Winner

Canada
4218 Posts

Posted - Mar 14 2014 :  8:18:21 PM  Show Profile  Reply with Quote
Since your 12VAC transformer does not have a center-tap and you bridged the outer terminals for it on the amplifier and used it and the center-tap then only two of the four rectifier diodes were used which might cause very low frequency hum at high levels.

Many things from China are fakes. In addition to fake Ultrafire batteries there is a Chinese company advertising a cheap fake protection circuit for a 18650 cell, it is identical because it is simply a photo of the real circuit.

Our hearing's sensitivity to loudness is logarithmic. Half the level is only a small reduction of loudness. 1/10th the level sounds half as loud. 10 times the level sounds twice as loud.

Here is a good video of a fake Ultrafire battery cell:
http://www.youtube.com/watch?feature=player_embedded&v=eOshOXcSkDA
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Tutanchamun
New Member

4 Posts

Posted - Mar 15 2014 :  11:00:49 AM  Show Profile  Reply with Quote
Thanks for the advice with the batteries, i know, that there are many fake one, tahts why i do not buy the cheapest "4000" mAh Monsters (which is even quite impossible for the 18650 form factor) but the good reviwed and tested ones (by other buyers) at dealextreme, i have the 2500 mAh ones and they last for hours in my torchlights (3 xm-lt6 bins, so i guess they are not that bad.

The thing with the logarithmic sound perception...i have heard of that, it might be the best explanation for this:-)

I will report when i have tried your solution

Regards, Michael
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Tutanchamun
New Member

4 Posts

Posted - Mar 18 2014 :  12:21:31 PM  Show Profile  Reply with Quote
Hi again, i tried your solution and it worked like a charm, so i need now just another battery holder for three 18650 cells...

Its so great that i can now use it remote from wall power supply. I just need to build something to prevent deep discharge as you recommended, don't know if i will find a cheap diy solution

Thank you for the hints and the solution!

Michael
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