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Old 24th September 2003, 12:40 PM   #1
tab30 is offline tab30  Germany
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Question More Power out an TDA7294/3

What do You think of this arangement ?

Will it work proper ?

I want use TDA7293 and insed of BD 249 / 250
Sanken Like SA1494 / SC3858

The value of Resisor at the Output of the IC is 0,18 Ohm.

The Picture is from:

http://www.freak-audio.de/amp01_anleit.html
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Old 24th September 2003, 12:57 PM   #2
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a similar circuitry was discussed somewhere here and at least one forum reader tried it without much success.

It is a classic example of expanding opamp's current output capabilities and you may find it in textbooks. So it works first of all.

The bad news, per my simulation, is that it doesn't work well in that THD is very high. I used a 541 driving mje15030/31 and at 4ohm, THD is in single digital. Not something to write home about.

Also, 0.18 may be a little too small as you need 3+amps from the IC for the transistors to start to conduct and contribute.

In a nutshell, if you need more current output capabilities (to drive low impedance speakers) and don't mind lower sound quality, it will do for you. Otherwise, you may want to take a different route.

Also, the circuit is unstable with mosfets (I tried 540/9540), in case you are wondering.
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Old 26th September 2003, 08:56 AM   #3
sangram is offline sangram  India
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The 7294 datasheet has a nice circuit using supply voltage boosters to increase power output beyond nominal output rating. Very interesting stuff.

Also you can consider running two or more amps in bridge/parallel configuration...

Running in the way you have mentioned introduces a lot of crossover distortion. Not recommended.
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Old 23rd February 2005, 12:10 PM   #4
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The best you can do is what Linn apparently does on the Chakra twin:

-use current sensing resistors (O,2 Ohms?)in the output stage power supply pins ONLY (and not on the "signal" power supply)

-drive from the voltage drop transistors to boost the output current.

with this configuration, the TDA provides the current up to 3 amps, then the bipolars will provide the extra current.
Distortion will be that of the TDA up to 3 amp outputs, and then probably be slightly better than that of the TDA at higher current due to the reduced output resistance


From the chakra twin specs and reviews, it seems very effective, and I am currently implementing it
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Old 23rd February 2005, 02:13 PM   #5
danyel is offline danyel  Romania
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Why don't u use 2 ic's?U can get a clear 150Watts?
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Old 23rd February 2005, 02:13 PM   #6
danyel is offline danyel  Romania
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I've tried it and it works smoth.Becouse of the lack of big speakers i made an stereo amplifier with only one 7294 per channel and it works just well.I have a little problem with the tone controller(tda1524):it's a little bit noisy,probably not grounded well(no hum but a little fuss).Still working on it.
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Old 23rd February 2005, 05:09 PM   #7
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Why has Linn moved from 3 ICs to one?

Just for marketing? Or because there is an impact on the sound (probably on THD at high voltage swings)?
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Old 24th February 2005, 02:01 PM   #8
driver is offline driver  Latvia
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I used a driving KT819/818

http://u-sound.narod.ru/files/tda7294subamp.pdf
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Old 24th February 2005, 06:54 PM   #9
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The booster of the "emitter follower" type will generate crossover distortion and you will loose the benefit of the monolithic output stage.

The CFP configuration with current sensing resistor inthe power supply section gives much better results. It will allow:

-an ouput current coming from the TDA7293 only , for current up to 3 amps (hence same crossover distortion as TDA7293)
- an additional output current coming from the bipolar output stage for currents greater than 3A, the TDA7293 will provide abit more than 3 A, and the bipolar will provide the rest.

( And because Vs- < PVs- and Vs+ > PVs+, the IC won't blow )
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