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Old 14th July 2007, 10:36 PM   #1
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Default Please help about the minimum VA of transformer to drive TDA7293

If going to make one TDA7293 working at AC25 x 2, driving 4 ohms bookshelf speakeers, what will be the minimum VA of the transformer will be needed? Since I wanna built something compact and slim, I try rread through the TDA7293 odf file but still can't make sense, could anyone please help? Thanks!
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Old 14th July 2007, 11:45 PM   #2
v-bro is offline v-bro  Netherlands
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It's in German, but...

Click the image to open in full size.
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Max. cone displacement can be several foot on any speaker!Too bad it can be done only once......
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Old 14th July 2007, 11:47 PM   #3
v-bro is offline v-bro  Netherlands
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So 135 VA...
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Max. cone displacement can be several foot on any speaker!Too bad it can be done only once......
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Old 15th July 2007, 10:30 AM   #4
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Thank you very much! So I will need at least a 270VA transformer for 2 channals, right?

By the way, a stupid though, if I finally force to use a transformer under 270VA, will using bigger filter cap could help?
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Old 15th July 2007, 11:29 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
2*25Vac is a bit high for 4ohm speakers.
This is the more usual voltage for 8ohm speakers.
18Vac to 22Vac is the usual range for 4ohm, but I would recommend towards the lower end.

Yes, you can compensate for slightly low VA rating by increasing the smoothing capacitance.
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Old 15th July 2007, 02:53 PM   #6
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Thanks again.

Since I have two slim (43mm overall height) transformers rate at 150VA 240V / 25V x 2 don't wanna waste, since here the mains supply is at 220V, therefore the output should not exactly right at 25V x 2.

My speakers are rated at 4 ohms, therefore I am wondering I use just one transformer or use both but parallel them as one 300VA transformer, maybe I try using one transformer first, with 10 x 5600uF smoothing caps, and fit everything in a 180mm x 300mm x 50mm slim case, if no good, then maybe take out the transformer put in another case as external PSU, but will then using both but parallel as one 300VA transformer.

I don't expect much from a chip amp although I know they could be real great, but since I wanna save cost, and I never really play loud, plus I am going to use CPU fan to help cooling, I think as long as the dynamic impedance of the speakers stay stable, there should have no problems?
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Old 16th July 2007, 08:59 AM   #7
Nordic is offline Nordic  South Africa
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With lover voltages, long as you keep the gain low enough not to exceed the maximum voltage available, it should not be a problem...

Remember a 25VAC transformer will give 25 x sqrt(2) - diode losses from rectifier... so you are looking at about 35V rails....

When you divide 35 by your load (4 ohms), you will see that the chip can not handle enough current to deal with that voltage...regardless of transformer... and this hold true for most chips currently available....

So your solutions to are, get 8ohm speakers, or lower voltage tansformers...

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Old 16th July 2007, 12:23 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
When you divide 35 by your load (4 ohms),
it's much much worse than that.

The reactive speaker load on continuous soundwaves can have an impedance around 60% to 70% of the nominal impedance for some narrow bands of frequency.
That alone increases the current demand by about 50%.
But on discontinuous sound, particularly fast starting or fast stopping signals, the effective impedance for the fast transient can be as low as 35% of the nominal impedance.
Your amp should be designed to meet this requirement.
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Old 16th July 2007, 03:58 PM   #9
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I'm a beginner. Please hunt me!
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Old 16th July 2007, 04:13 PM   #10
AndrewT is offline AndrewT  Scotland
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hunt=? or maybe =help?
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