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Old 28th November 2016, 08:46 PM   #21
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They are all op-amps somehow, since they have a non-inverting and an inverting input. Yet some of them do not integrate input protection diodes. As i wrote, this lack of input protection may be good for a comparator, for which the circuit designer will tailor input protection, but not for a linear amplifier.
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Old 28th November 2016, 08:48 PM   #22
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not just somehow, they are high power opamps basicly.
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Old 29th November 2016, 12:03 AM   #23
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Protection diodes are a good idea. I always include them in all my circuits.

They work like op amps of course but have application specific features. Their phase/bandwidth performance is poor for an op amp but they work great as power amplifiers.

In spite of their price, I don't think engineers use them in servo circuits. I think you could use them in some servo circuits, though.
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Old 29th November 2016, 07:35 AM   #24
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Audio chip amps are assumed to be operated closed loop, and when the loop opens during clipping it is assumed the signal is low (2...3Vpeak or so) and that there is a source resistance in the low kOhm range at both input pins. Also, common mode operation point is assumed to be very close to 0.5* total supply.
An input protection and a large input common-mode range is therefore not deemed necessary for these specialized opamps.
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Old 29th November 2016, 01:56 PM   #25
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Diode protection on the input ensures that the input coupling capacitor never causes the unit to latch up on power down. This is especially important when a single ended supply is used.

I use diode clamps on line level circuitry too. The diodes do nothing at all unless they're needed. They cost around 2 cents apiece if bought in bulk so the biggest cost is board space.

A couple of diodes and a resistor can provide virtually unimpeachable input circuitry protection. I don't see a downside to that.
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Old 29th November 2016, 05:14 PM   #26
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Here is a circuit, which likely breaks on power-up or -down, if there were no input protection diodes:
Click the image to open in full size.

In a single-chip circuit, in which i accidently shortcutted the resistor of the 2R2/470nF network, another chip broke. In both cases only the differential-input stage broke, as fuses did not.
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Last edited by Grasso789; 29th November 2016 at 05:17 PM.
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Old 30th November 2016, 03:56 AM   #27
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There's also a slow start circuit, as well as at least one error in the schematic.

A reverse biased diode from R6-R7 junction to V+ would provide positive latch up protection. If that diode forward biases on power down, then it's needed.
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Old 30th November 2016, 09:37 AM   #28
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The error was the missing 3u3/50V input series capacitance. A reverse diode over R8 was also cool but not mandatory in this circuit. Lastly, this 20V= generator is one for all amplifiers and so located before the fuses.
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Old 1st December 2016, 07:37 AM   #29
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Ever so astonishing!
However, it is also astonishing that it is possible to get the TDA7293 to work above the claimed specs so long as one is not entirely reliant on the datasheet insistent specs.
So far as today, that is the comment that I wish to make.
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Old 1st December 2016, 01:41 PM   #30
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Thank thee, Daniel! I have to use the single rail supply, which i already have. I was worried, the nine chips could use too much idle power, heating up the cooler (9*50mA*46V=27W), but that is not the case at all; even if i cover the cooler for a while, it gets barely luke-warm. UTC state 30mA idle current but have prolly futher reduced it and shrunk the die. So it makes sense to build the circuit as small as possible, with feedback network and 100nF capacitors right at chip legs at board bottom. And use a single rail supply indeed, so the chips can be easily cooled without insulation, what also results in a low-resistance ground path, so i screwed loudspeakers ground to the cooler, too.
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Last edited by Grasso789; 1st December 2016 at 01:58 PM.
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