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I work with audio electronics in the touring concert realm and I've been seeing a gross failure of a certain balanced line driver IC, the THAT 1646. Main symptom is no output and most common failure mode is the OUTPUT pins latching to the supply rails (-15V or +15V). I've seen both outputs at +, both at -, or one at each. Usually all other pins look ok but at times I have seen full rail voltage on the SENSE pins or a <10V on the INPUT pin. Most of the time the chip looks ok and shows no sign of heat but I have had a few that were giving off lots of heat.

It's not my business to know what's causing these failures but I am left wondering; whether it be inherent to the chip itself or the circuit surrounding it. Often the equipment is simply the THAT 1646 feeding a passive filter and suppression diode into XLR jack. Are these chips prone to fail on their own? Do they need extra "outside" suppresion and protection from the abuse of plugging an XLR plug in? In addition to the clamping diodes already present.

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    \$\begingroup\$ thatcorp.com/datashts/THAT_1606-1646_Datasheet.pdf Check the paragraph around fig. 8. \$\endgroup\$ – jippie Jan 7 '16 at 19:45
  • \$\begingroup\$ Without seeing the circuit, I would guess what is happening is surges cause the output stage to blow up as described on page 9 of the datasheet. It recommends clamping each output to the respective supply rail. \$\endgroup\$ – ACD Jan 7 '16 at 19:47
  • \$\begingroup\$ @ACD this gear actually does just that. That's what I meant by "suppression diode" \$\endgroup\$ – disorder Jan 7 '16 at 20:49
  • \$\begingroup\$ Can you post the schematic of an exemplar output stage, or are they using the Figure 8 reference circuit? Are the diodes blowing in a vain attempt to protect the line driver IC? Finally, what is the output plugged into when it fails? (XLR makes me think they're plugging a line-out into a mic-in, which works fine, until you do it with the phantom power turned on...) \$\endgroup\$ – ThreePhaseEel Jan 8 '16 at 1:27
  • \$\begingroup\$ @ThreePhaseEel Can't post schematic right now but it involves filtering and protection. The diodes it uses are part of a TVS array package (SMDB12C). Most of the failures we have had in the past the line driver equiped unit was being plugged into a powered speaker or a line-level signal analyzer... never plugged into mic-in never coming close to seeing phantom). \$\endgroup\$ – disorder Jan 8 '16 at 21:17
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I'm not sure this question is well suited to the SE format, but anyway here is one way that the chip could fail due to output abuse.

If the chip (with the diodes) is plugged into a 48V microphone input that has relatively large capacitors it can instantly drive the power supply beyond abs max limits.

Usually regulators cannot sink current, they can only source it, so connecting a much larger capacitor to the output will cause the voltage to divide depending on the capacitor ratio. After that it would be limited by the phantom power current.

This could probably be solved by putting a TVS across the supply, rated to take the highest plausible 48V current. That might require a large package.

There are other possibilities involving putting something in series with the output in addition to a TVS.

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