eBay TDA7294 kit + Parts Express Buyout 32V CT 200VA Toroidal Power Transformer? - diyAudio
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Old 17th February 2009, 08:32 PM   #1
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Default eBay TDA7294 kit + Parts Express Buyout 32V CT 200VA Toroidal Power Transformer?

I've recently been playing around with the idea of building an amp to power the mains in my home theater. I was originally thinking ICE ASC200/AC200 modules but the cost seemed a bit high for my first try, even though a bit more fool proof.

I came across this kit on eBay.

Would I be able to use this toroid from Parts Express as the transformer?

I'm going cheap to experiment with DIY amplification. I've built speakers and subwoofers in the past, but that's just carpentry and basic soldering. The price of the PE Buyout Transformer is its main selling point... will it work with this kit (cryptic description calls for what I assume is 35+35 but doesn't say what VA)?

Any help for a new guy is appreciated. Thanks in advance.
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Old 18th February 2009, 12:24 AM   #2
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Read the datasheet on the TDA7294. Figure out what speakers you want to drive, then look up what voltage you can/should run at the impedance of your speakers. Other than that, a 200VA transformer is a bit small for a 200W amp. You want more than 300VA if you plan on taking full advantage of the power.
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Old 18th February 2009, 12:32 AM   #3
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Upon looking at the amp, I notice it's Bridge Tied Load (BTL) aka bridged. That means with 8 ohm speakers, each chip will be driving the equivalent of a 4 ohm load.

In other words, 1) you can't use this amp with 4 ohm speakers, and 2) you are limited to +-30VDC power supply. Since AC * 1.4 = DC, you're looking for a 20VAC or less transformer.
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Old 18th February 2009, 01:48 AM   #4
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Hi!

To be honest, I wouldn't do that. I don't know exactly if this would work, as the description on the ebay offer is bit incomplete. But in general, this would be nothing else than "just another amp".

For the amp itself:
I build an amp with the TDA7294 and this chip likes to be used exclusivley, which means paralleling them will give you more output power, but less convincing sound. Additionally, there is the younger brother of this chip, the TDA7293, which performs even better. I build an amp with these chips, too, and the result was quite nice! But these chips are very sensitive concerning the other parts like caps and resitors, and, like the 7294, they play better in a simple one-chip-per-channel application.

The transformer:
This trans is only 200VA rated, this might be not sufficient to power those 4 TDAs. Then, it provides mutiple secondary windings, which you don't need at all for this amp. Another trans might be a bit more expensive, but would fit in way better here.

Instead, I would go a more serious version of this amp, like this: http://www.diy-selbstbau.de/produkte/a80-en.htm
(This is the one I build)

or, if more power is required, this one:
http://www.diy-selbstbau.de/produkte/a200-en.htm

Kind regards - Martin
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Old 18th February 2009, 02:56 AM   #5
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Oops, my bad. Upon looking closer at the board, you can use up to 40VAC, since it splits that for you. So the transformer you linked would work.
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Old 18th February 2009, 11:04 PM   #6
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Would I be able to use the 16v secondary of the PE transformer to power this this speaker protection circuit from the same seller (which calls for 12-18v)? My speakers 8ohm nominal and not complex loads (lowest dip is 7ohm around 200hz)... if this amp just fails to impress it will can easily be regulated to infrequent 2ch use in another room with pretty efficient, though not great, speakers. Any guess on how much power I could expect to get out of this design?

Thanks in advance.
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Old 19th February 2009, 12:51 AM   #7
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Warning, the 32V output is NOT center tapped. Read the 4th review, and the 2nd for the wire labeling.
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Old 19th February 2009, 09:44 AM   #8
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Quote:
Originally posted by IsleOfMan
Any guess on how much power I could expect to get out of this design?
With 8 Ohm speakers and a 32 V center-tapped transformer expect about 16-17 W from a single IC, four times that figure, if they are bridged.

Buy a 60 V center-tapped transformer and you get the same amount of power from a single TDA that you get with 32 V from two bridged ICs.
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