4 ohm load with tda7294 in bridge mode.!! - diyAudio
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Old 21st June 2010, 10:42 PM   #1
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Question 4 ohm load with tda7294 in bridge mode.!!

DIY Audio 300 Watt IC TDA7294 Chip Power Amplifier Circuit for Home Stereo or Subwoofer
TAKE A LOOK THEY USE 4 OHM SPEAKER WITH TDA7294 IN BRIDGE MODE THEY SAY IT`S 300WATT..!!!!!!
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Old 22nd June 2010, 02:25 AM   #2
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Depends on the rail voltage. But no matter what the rails it will not be for very long.

George
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Old 22nd June 2010, 03:37 AM   #3
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Funny, it says 2x TDA7294 100W ICs... I wonder how 2 x 100 = 300?
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Old 23rd June 2010, 09:26 AM   #4
space is offline space  Norway
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It also says 75 W in the serch terms. 2x75=150W RMS = 300W peak
With the 50V CT transformer it might do it for a few milliseconds...

space
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Old 23rd June 2010, 06:12 PM   #5
asbjbo is offline asbjbo  Norway
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The datasheet says 70 W RMS at 0,5 % distortion into 8, 6 and 4 ohms. (100 W is "music power" at unlimited distortion for a limited time.)

A bridged amp doubles the voltage swing, which may quadruple the power, if the amp and power supply can deliver the current. In other words, the TDA7294 could conceivably produce 4x70 W = 280 W RMS when bridged. But it does not have the current capacity, and it is not able to get rid of the resulting heat. Notice that the power is the same into 8, 6, and 4 ohms load. The data sheet only promises 150 W if bridged across 8 ohm load, and the distortion will start rising sharply from 100 W.

Bridging across a 4 ohm load effectively creates a 2 ohm load for each amp. The TDA7294 cannot drive lower impedances than 4 ohms, according to the data sheet, so there is no way you will get good results, much less 300 W power, by bridging these amps across a 4 ohm load. It will distort like mad.

As to the link that started the thread: No way. Meaningless numbers.
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Old 23rd June 2010, 10:10 PM   #6
space is offline space  Norway
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What the data sheet is one thing.
What it does in real life another.
What marketing says is a totally different third.

Pick among the above three and you can say just about anything.


space
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Old 24th June 2010, 12:11 AM   #7
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Quote:
Originally Posted by space View Post
What the data sheet is one thing.
What it does in real life another.
What marketing says is a totally different third.

Pick among the above three and you can say just about anything.
Very true.

I know for a fact that bridged 7294s can run a 6 ohm load no problems (the Logitech Z5500 has a 6 ohm woofer), but if you factor in power compression you realize that the impedance rises anyway when the system is played loud for extended periods of time, so it'll still be driving a 8 ohm+ nominal load thus no trouble.
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Old 24th June 2010, 08:13 AM   #8
asbjbo is offline asbjbo  Norway
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Sure. The question is really how much power you can expect at a reasonable quality. If bridged gives 150 W into 8 ohms at decent quality and we assume that the chip is current/heat limited, dropping the impedance to 6 ohms will limit you to somewhere around 110 W with the same current, and dropping further to 4 ohms will leave you at 75 W. That is not much benefit over a single 70 W chip.

Also, heating the voice coil by playing loud for some time will reduce the efficiency of the speaker, so you will need some more power to keep the same sound pressure level. I would not bridge this chip for a 4 ohm load, and if I did, I would certainly not claim 300 W.
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Old 24th June 2010, 10:23 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
bridging produces double the power into double the impedance.
This rule always applies. I do not know of any exception.

eg. a pair of 75W into 4ohm amplifiers when bridged will give a maximum of 150W into 8ohms.
But that 100W is identical to the sum of the maximum outputs of the two single amplifiers. i.e. 75W + 75W to drive two 4ohm loads is exactly the same as 150W to drive a single 8ohm load consisting to the original two 4ohm drivers connected in series.
Overall nothing powerwise is gained by bridging.
Personally I would use two drivers each with their own dedicated amplifier to give me the 150W of speaker input power that I desire.
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Old 24th June 2010, 03:02 PM   #10
asbjbo is offline asbjbo  Norway
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IF (and that is a capital if) the amp can double its power when the impedance is cut in half, that also means that the amp could give four times the power into the same load when bridged. For example, a large power amp with a hefty power supply might be rated at 150 W into 8 ohms, 300 W into 4 ohms, and 600 W into 2 ohms. Bridged, the voltage swing across the load would be doubled and each amp would see one half the load (e.g., 4 ohms instead of 8). The result is that the load gets twice the voltage and twice the current, for four times the power.

Of course, this is all in theory only, since the 7294 gives the same 70 W into 4 ohms, 6 ohms, and 8 ohms. It just cannot cope with the needed current, and I agree with your game plan: Two separate amps driving two separate drivers, 4 ohm or 8 ohm. The added driver area would also give a useful contribution. Or dig up a single 16 ohm, high efficiency driver, bridge the amps, and get 170 W or so (again according to the datasheet). If the driver can give around 98 dB @ 1 W, that will also be plenty loud.
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