Optimizing TDA7294 Output

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Guesstimation table for new production chips:
V7 TDA7293 = Real higher cost TDA7293
V6 TDA7293 = Real low cost TDA7294H (max 25+25vac transformer)
TDA7294 = TDA7295 or dirt cheap cull (terrible audio quality)
TDA7295 = TDA7295 or dirt cheap cull (terrible audio quality)
TDA7296 = Real TDA7296 OR a cheap substitute

This also affects authorized vendors. The ST chips that I have purchased from Mouser and Digikey have been identical to those in the eBay kits.

You really can buy an old-production quality TDA7294; but, it won't have "TDA7294" printed on the casing, AND all 15 pins will work.

Attempting to purchase TDA7296's sometimes works, and I guess that is because old stock of the less popular chip hasn't run out yet.

The use of the HCA topology by an integrated (monolithic) amplifier like an outline of a TDA7293V must actually be a good idea.
ckeck out this threads:
http://www.diyaudio.com/forums/chip...ed-topologies-yamaha-mx-10000-quad-405-a.html
http://www.diyaudio.com/forums/soli...lic-conversion-amplification-hca-circuit.html
http://www.diyaudio.com/forums/pass...s-ab-topology-about-hca-why-not-realized.html
 
Some curiosities.

First of all, the TDA7294S Datasheet (all 15 pins work, like TDA7293): http://www.startfetch.com/fidelityforce/datasheet/tda7294s.pdf

And next, in the the attachment, that TDA729X "Hi Fi" schematic that has been floating around the internet for some years. That thing was prone to explode so I have edited it to replace the BY diodes with more efficient MBR diodes to decrease explosions.
I didn't design this one!
It looks somewhat impractical and may produce speaker burning offset. Also 50x is beyond the 43x maximum, so the gain divider is serving up a catch function rather than helping resolution. That doesn't look appropriate, but it may produce interesting tone effects.
Now the good part:
The power circuit is worth a closer look . . . in the attachment. This MBR schottky diode bypass can allow playing with separate rails, without cracking the chip's substrate.
 

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any trick to spot fake 93 and 94 ?
here are some cheap ones on reichelt.de

Yes.

For the 93, just make sure all the pins work. It includes a TDA7293-specific bootstrap cap hookup, a built in clipping indicator pin, and it has a preamp output pin (the output of the built in small signal voltage amp). Version 7 is the higher tolerances power chip with a higher price. Version 6 has the voltage tolerances of TDA7294, a very low price, and functions nicely. A real version 6 routinely sells for $2 and is Exactly the same part, with the same markings, with the same audio performance as the $7 part at Mouser.
With V6, you can use a 24+24vac or 25+25vac transformer.
With V7, you can use a 28+28vac or 30+30vac transformer.


For the 94, here's how to tell:
If it sounds nice like a TDA7293, that's Old production TDA7294.
If it sounds dreadful but doesn't explode, it is a New TDA7294.
If it explodes, it is a lower voltage amp in a big TDA7294 casing.

*Some of the new plonkers and fakes can be redeployed as a decent ~12W amplifier by setting lower gain and using a 14+14vac transformer (max).

**The physical location of the terrible performance on New production TDA7294 is observable due to the bootstrap arrangement; therefore, I would like to guess that Post#54 will let you turn a bad 70W amp into a good 40W amp. Basically, the voltage amp power needs connected to the output buffer power via schottky diodes, so that the voltage amp's power supply voltage cannot go smaller (there is a fragile parasitic diode internal to the chip's negative rail, in the core, and we bypass this with a more efficient and more sturdy schottky diode to prevent exploded chip). And, then you run the voltage amp about 8v higher so you can remove the bootstrap cap. This is also your opportunity for "Front End On Regs" quality enhancement. With the output buffer run at 8v lower, you get 40W@8R, 60W@6R, 80W@4R, Music Power which is the point of clipping; HOWEVER, the built in limiter disallows bass beyond 45W RMS power. Therefore I think that ilimzn's (post #54) dual rails, no bootstrap, plan for a higher quality 40W amp doesn't reduce usable output power except for 5W worth of noise is gone and you won't miss that. ;) Remember to install the schottky before you play with quad rails. You can then figure out how to waste 8v on the way to the output buffer OR boost 8v on the way to the internal small signal voltage amp. That makes the bootstrap cap removable and thereby removes the audio problem of the new production TDA7294.
 
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Yes.

For the 93, just make sure all the pins work. It includes a TDA7293-specific bootstrap cap hookup, a built in clipping indicator pin, and it has a preamp output pin (the output of the built in small signal voltage amp). Version 7 is the higher tolerances power chip with a higher price. Version 6 has the voltage tolerances of TDA7294, a very low price, and functions nicely. A real version 6 routinely sells for $2 and is Exactly the same part, with the same markings, with the same audio performance as the $7 part at Mouser.
With V6, you can use a 24+24vac or 25+25vac transformer.
With V7, you can use a 28+28vac or 30+30vac transformer.


For the 94, here's how to tell:
If it sounds nice like a TDA7293, that's Old production TDA7294.
If it sounds dreadful but doesn't explode, it is a New TDA7294.
If it explodes, it is a lower voltage amp in a big TDA7294 casing.

*Some of the new plonkers and fakes can be redeployed as a decent ~12W amplifier by setting lower gain and using a 14+14vac transformer (max).

**The physical location of the terrible performance on New production TDA7294 is observable due to the bootstrap arrangement; therefore, I would like to guess that Post#54 will let you turn a bad 70W amp into a good 40W amp. Basically, the voltage amp power needs connected to the output buffer power via schottky diodes, so that the voltage amp's power supply voltage cannot go smaller (there is a fragile parasitic diode internal to the chip's negative rail, in the core, and we bypass this with a more efficient and more sturdy schottky diode to prevent exploded chip). And, then you run the voltage amp about 8v higher so you can remove the bootstrap cap. This is also your opportunity for "Front End On Regs" quality enhancement. With the output buffer run at 8v lower, you get 40W@8R, 60W@6R, 80W@4R, Music Power which is the point of clipping; HOWEVER, the built in limiter disallows bass beyond 45W RMS power. Therefore I think that ilimzn's (post #54) dual rails, no bootstrap, plan for a higher quality 40W amp doesn't reduce usable output power except for 5W worth of noise is gone and you won't miss that. ;) Remember to install the schottky before you play with quad rails. You can then figure out how to waste 8v on the way to the output buffer OR boost 8v on the way to the internal small signal voltage amp. That makes the bootstrap cap removable and thereby removes the audio problem of the new production TDA7294.
interesting to know and completly new for me. Jeff Rowland, Linn and some other manufacturers use this ICs and often the LM4766 just because by BjT power devices are many fakes resp. counterfeits on the market - go to
http://www.diyaudio.com/forums/solid-state/33597-how-identify-fake-transistors.html
So use the avoid of transistors nothing more.
 
If it sounds dreadful but doesn't explode, it is a New TDA7294.

Hi Daniel,

Please help me understand the meaning of 'sounds dreadful'.

How bad it sounds,does it sound like as if improper bias, coarse, grainy or is it very subtle audible to discerning ears?

I have made 7294 amp 7 years ago to my friends and they still plays very well.
Seeing this I have purchased a few more lately. One of them I have used for a sub-woofer amp,I felt nothing to be concerned.

Regards
 
That was too wordy! Sorry. 2nd try:
The easy way to spot a Fake TDA7294 is when it says TDA7294 on it; AND, the easy way to spot a Fake TDA7293 is when less than 15 pins work.
Obviously, place your bets on the TDA7293 when buying new.
And, as with all of audio, price does not indicate performance.

Was that better?
More confused.:confused:
If not TDA7294 then what else ?

It will be kind of you if you can give us some pics comparitively
 
For the 94, here's how to tell:
If it sounds nice like a TDA7293, that's Old production TDA7294.
If it sounds dreadful but doesn't explode, it is a New TDA7294.
If it explodes, it is a lower voltage amp in a big TDA7294 casing.

So it is therefore your opinion that these chips that Stanton sells are fakes and dreadful sounding? Have you tried them?
 
So it is therefore your opinion that these chips that Stanton sells are fakes and dreadful sounding? Have you tried them?

They're not fakes--they're really ST, but I'm certain they're the dreadful new production, which is a "functional clone" of the original, except slightly cheaper. I cannot think of a use for either that chip or that board layout. I have an unfortunate tendency to hoard, but I don't keep something as awful as that. So, I threw it out.
 
So are you saying you have tried that board and/or chip or are you assuming? Thanks

Yes, I have tried that board and that chip.
The linearity is severely inferior to Old-TDA7294 and New TDA7293, so that the New-TDA7294 will not dial in for practical performance when run at its rated voltage. SO, basically, the New-TDA7294 is not practical as a fragile and badly non-linear, high power amplifier. But it could do something much better as a sturdy, highly linear, lower power and Very cool running miniature amplifier.
Therefore:
I suggest to use the TDA7296 datasheet for new production TDA7294's, AND then derate it even farther--scale it small enough for linear performance.

Here's the extrapolated feedback resistor chart for New-TDA7294:
Voltage vs Feedback-Resistor (when its partner is 2.7K feedback-shunt).
36+36vdc, 110k Feedback-Resistor (not applicable--use TDA7293)
32+32vdc, 100k Feedback-Resistor (not applicable--use TDA7293)
30+30vdc, 91k Feedback-Resistor (not applicable--use TDA7293)
28+28vdc, 82k Feedback-Resistor (probably not applicable)
26+26vdc, 75k Feedback-Resistor, 50W@4R (same as inbuilt limiter anyway)
24+24vdc, 68k Feedback-Resistor
22+22vdc, 62k Feedback-Resistor
20+20vdc, 56k Feedback-Resistor, great imaging--good for High Fidelity table radio & mini systems
In this example, the input cap is 1u, the nfb-shunt cap is 220u+a 10n or smaller polyester bypass, and the amp board power caps are 220u. As you can see, the closest match datasheet for the charted linearity behavior of New-TDA7294, is the TDA7296 datasheet.

This is all a bit fishy. But, the chip really is an ST.
However, there may be some question as to which ST chip is in there, since it doesn't match either its own datasheet nor its older production sisters. This is not fake, but it *could* be a re-mark or cull. I have bought several new TDA7294's from different vendors and they all near-matched with TDA7296 datasheet. I wouldn't want to make assumptions; however, it is not an assumption to say that the new production chip requires MUCH more derating than the old production chip. I'll leave it up to you to figure out the cause of it.
The matter is a bit puzzling. And as surprises go, it wasn't good.

Edit: Uses for New-Production TDA7294:
Television and Table Radio amplifiers and mini systems (under-volted severely), and guitar amplifiers.
 
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TDA7294 board and it's SQ.

I recently bought a TDA 7294 board from Ebay for $6/- + shipping. The TDA7294 chip was bought locally. All parts were bought locally. Even the difficult to find uPC1237 protection chip was bought locally. I was concerned about the relay as the local one had 8 terminals. I just had to snip out the extra pins and everything works well. Great sound ! It's not the best you can get but it IS good sound. HF is not silky smooth , but on good recordings it sounds surprisingly good. No lack at the extremes also. As always a good input cap plays a big role. If the preamp or DAC output already has a blocking cap then don't use any input cap.

1pc TDA7294 TDA7293 Amplifier Amp DIY Board PCB | eBay

Note that only the transformer is off board ! It's a great kit.
You might remember that you can improve the sound of the TDA7294 chip by filtering the supply to the input stage. It audibly improves the sound. However implementation has to be done very carefully or you'll end up blowing the chip . That's why it got a bad rap long ago. Ilimzin put up several wonderful posts on this long ago explaining exactly why the chip will blow if you don't heed the supply pin specs. With all protection devices in place it should not blow under any condition. Unlike the implementation shown you can avoid using a voltage doubler and opt for extra windings on the trafo. Here it is easy to get that done.

On a test done long ago in a non inverting mode the 7294 came out better than the 3886. Never got around to trying the inverting mode.
 

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