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Old 14th August 2006, 09:37 PM   #1
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Default tda7294 current, power transformer selection

I want to use the tda7294 in a bridge configuration, as seen in the data sheet.

tda7294 datasheet, bridge circuit, p.13, figure 25

I am clueless, however, as to how much current this circuit will draw. I need to know this, of course, because I don't want to buy a power supply that doesn't have a high enough current rating. BTW, I plan to use about +-40V to the supply rails.

Anyone have any experience with this circuit? Or, can anyone give me an estimate, or tell me how to make my own estimate?

Thanks in advance,
Brian
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Old 15th August 2006, 07:39 AM   #2
space is offline space  Norway
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Location: Oslo - Norway
The current draw only depends on what kind of impedance the speakers you plan to drive have. Bridging a pair of 7294 is only good for 8 ohms of load. (4ohm loads paralleling is better) Using +/-40V supply rail are a bit too high for bridge configuration even at 8-ohm loads. I would not go higher than +/- 35. +/-30 would be even better. (When I say "are only good for", I mean output power will be very limited because of power dissipation in the IC.)

Current draw you calculate from supply voltage divided by impedance. U/Z=I For bridge configurations you multiply the supply voltage by 2. This will give you peak current draw. Maximum RMS current draw will probably be less than half of this.

If we say +/-35V supplies on a bridged config. on 8ohm speakers:35*2=70V maximum (actually 2-4 V lower due to voltage drop in the output mosfets) over 8 ohm load gives 70/8=8,75 Amp peak. The spreadsheet for the 7294 tell us it has a 10 Amp peak output limit so 8,75 is pushing it. This because of speaker impedance varying to frequency.

Peak output power at 70V over 8 ohm are 70*70/8>600W which is much more than what the 7294 can sustain due to power dissipation, so a lower voltage should be chosen. If one read the spreadsheet carefully, one will find that +/- 29V is the voltage to get maximum (music) power for an 8ohm bridged load. +/- 27 for maximum continues RMS power.

From your questions rbruss82, I believe you should study this amp building some more and get the basic understanding of amp operation. An excellent place to start: http://sound.westhost.com/articles.htm

Best of luck!

space
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