TDA7293 datasheet - Page 2 - diyAudio
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Old 15th January 2013, 07:34 PM   #11
Kitax is offline Kitax  Latvia
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w8... if i use 24x2 (2.08 amp) transformer, and the actual supply voltage is +/- 24v, not 24v, doesnt it mean, that the full voltage is 48v, and if i put a 100w amp it uses 100/48 wich is about 1.04A, so if i put 2 of the 100W amps the total amperage is 2.08A, and it shouldent burn...
am i right?
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Old 15th January 2013, 07:58 PM   #12
Mooly is offline Mooly  United Kingdom
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OK, so 100 watts RMS into 4 ohm means you need to develop 20 volts RMS at the amp output. Thats worked out as V = square root (100*4).

20 volts RMS is 28.28 volts peak and 56.56 volts peak to peak (that's Vrms * root 2)

So we need at least 28.28 volts DC as a rail PLUS losses which are losses in the chip and general resistive losses. So around 35 volts DC would seem suitable. Thats 35-0-35 volts. If you built a single rail amp then that would be a single 70 volt rail. Yes

So the transformer needs to be 35 divided by root 2 which is 24.75 volts AC. So a 25-0-25 volt transformer should cover it.

The VA rating of the transformer needs to be determined.

Ipeak = Vpeak/R so that is 28.28/4 which is 7 amps

Iaverage is Ipeak/pi which is 7/3.142 which is 2.23 amps.

That's for one channel, so you are looking at a 160va transformer for mono and over 300va for stereo. Build in a good margin and that means using a 500va transformer (as a standard off the shelf part)

Next question.... do you really (really really) need 100 watts RMS ?

Have a read at this
A Test. How much Voltage (power) do your speakers need?
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Old 16th January 2013, 08:19 AM   #13
Kitax is offline Kitax  Latvia
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well... actually the 100w arnt the important ones... i just picked the chip, and one of the reasons was the flexability it provides...
i have a 24x2 toridial transformet, wich i could use it would provide 24*sqrroot of 2... wich is around 33,6 DC volts to each of the windings. the transformer has a 2.08A limit, so if we squeeze all that we can, then 33,6*2*2,08=140W... am i right?
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Old 16th January 2013, 08:29 AM   #14
Mooly is offline Mooly  United Kingdom
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It doesn't quite work like that in practice. Transformers are rated for their max current into a resistive load. A bridge rectifier and reservoir caps draws "a lot of current for short time" on each cycle as the bridge only conducts at the tops of the sine wave to bring the caps back to full voltage. For most of the cycle no current flows.

So in practice... its a bit on the small side for two channels.
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Old 16th January 2013, 08:44 AM   #15
Kitax is offline Kitax  Latvia
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oh ok... i know i can always put a rezistor between input, and ground, so i cant actually use the 100W power of the amp, but on the other hand, maybe i should just choose a better transformer, or a different chip... :P
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Old 16th January 2013, 08:56 AM   #16
Kitax is offline Kitax  Latvia
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heh... was looking for a chip, and came across this one
http://www.datasheetcatalog.org/data...onics/1565.pdf
and i think this one could work :P
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Old 16th January 2013, 09:03 AM   #17
Kitax is offline Kitax  Latvia
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oh yeah...
http://lv.xn--sondae-tpb.com/_y/agif...aecli_/bo1.jpg
what is this called... i just dont know the term... and the main question was, can i use nail polish to draw the lines, or the acid would just burn througs...
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Old 16th January 2013, 12:01 PM   #18
Mooly is offline Mooly  United Kingdom
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The TDA7295 looks to have a lower max supply rating.

Making PCB's is something you have to learn by doing. Nail varnish is probably etch proof but the end result won't be pretty. You can get etch resist pens and transfers. The board in your picture looks a fibre glass roller tinned board.

Why don't you get a PCB ready made, I think they are advertised from the usual suspects
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Old 16th January 2013, 12:36 PM   #19
Kitax is offline Kitax  Latvia
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well thats not what i ment... well nevermind that
only now i realize, how bad i know english terms

actually the transformer i was talking about is 2x24V 2x2.08A and i found a datasheet with graphs Here
http://www.datasheetcatalog.org/data...08fj3sqq3y.pdf
it seems that with 8 ohm speakers, and +/- 34V power supply the max output power is somewhere around 80w, that means, that i can safely put 2 of the chips with the transformer, cant i? it doesnt have the 4ohm speaker supply vs power output graph though...
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Old 16th January 2013, 02:35 PM   #20
Mooly is offline Mooly  United Kingdom
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Yes, you can safely use two chips on that transformer but the transformer is the limiting factor. Fuse the transformer correctly and there is no problem. You can always replace with a larger transformer if you want later on, but for normal use the amp will be just fine.
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