Protecting Balanced audio amplifier inputs from overvoltage

[pleslie]

I am looking to protect the inputs of a professional line receiver SSM2143 from large overvoltage signals in a situation where a user plugs speaker outputs into my line input. The line input must also be protected from ESD and EFT (lightning strikes) events.

I have selected a TVS diode (Littlefuse SP4021) to protect the input amplifier at the XLR input jack from fast transients. I am concerned with overvoltage protection for a continuous signal above my ±5V rails. I would generally use Schottky diodes to protect the inputs, however Schottky diodes would turn on before the TVS diodes and most likely would be damaged if a large transient voltage event occurred. Does anyone have any recommendations as to what I should do?

Cheers

[Messtechniker]

I would start with thinking about an input transformer
as one part of the equation.

Yours - Messtechniker

[bson]

Back-to-back zeners between the inputs or inputs and ground, and inline pptc fuses to save the zeners?

The main problem is it's hard to design a HV high power proof input that doesn't degrade the signal.  Big HV capable power parts will by necessity have reactive effects.  For this reason most quality small-signal designs will either simply break if abused, or aren't expected to perform past PA grade.

[Cerebus]

I would start with thinking about an input transformer
as one part of the equation.

Yours - Messtechniker

Exactly my first thoughts too. Transformers can be hard to accept in terms of distortion, frequency response and cost but they're a really big hand up in input protection. Back in my professional audio days (many years ago now) I can't remember a single instance of an input that was transformer based getting killed whereas I can't say the same about 'direct' inputs. Beyond that it's just the usual things - back to back zeners across the line, reverse biased diodes to the rails from the inputs, some impedance between that point and the op amp inputs.  I seem to remember that "That Corporation" have some very useful application notes on input stage design and protection - worth a search/look at their website.

[DaJMasta]

The little I've seen of boards has been the back to back zener variety, but speaking on transformers, there are actually some very low distortion transformers that work in the audio range - they tend to be pricey, but for under $100 US you can find one that's pretty transparent and gives full isolation.  If you're on a tighter budget than that, you can look at cheaper transformers or what's called a direct box (though some use the nice transformers and cost much more).  Transformer audio isolation also eliminates potential ground loops, which could definitely be a problem if you're looking at a long input line.

I'm not certain if the commercial solutions will be rated for lightning protection, because I don't know of many applications where it would be relevant, but I think a solution could be devised.  There are some designed for use with outdoor speakers, but I'm not sure how their signal distortion specs look.

[JS]

  For line level audio is OK to have a ~10k resistor in series at the input, then a back to back zenner from each input to ground would do the trick. Using 15V 0.5V zenners would take overload way bigger than you may expect in the real world, only limited by the resistors power handling. Using 1W resistors (or two 0.5W 1%MF) you would be able to handle up to ±100V on each input, RMS and long enough to realize, it might degrade the resistance value taken to that limit which would have an impact on CMRR for both, temporary (not important) and permanent if the over voltage is presented for a long enough time.

  The problem with that approach is, of corse, you already have the nicely laser trimmed resistors inside your IC and you can't access the needed end of them, CMRR could be trimmed to a decent level, provided a proper and stable trimming solution. Using this ICs solutions takes away the flexibility to handle many things, as protection in this case. Still it can handle over 100W 4? power amp connected directly. Personally I'm not a lover of this ICs line driver/receivers but they do provide a solutions for reasonable cases. Usually really good for DIY line management or specific products as studio tools and such.

  When you need to go out into the wild you need serious protection so the reliability isn't compromised, big live desks isn't a place for this to go to die for example, at least not as in the data-sheet examples. Still there are work arounds, for 12? miss match in the input resistors you are looking 64dB CMRR (I'm assuming 0.5 gain config) which is something you should totally be able to accept, as the source might have an even bigger miss match, even 100? is not rare. Then you can have two polyswitches in there ±12? between each one and a diode configuration that can handle the expected input transient depending on the selected poly. With that approach you may be able to handle hundreds of watts while keeping a decent CMRR. Distortion shouldn't be a problem I guess, the polys should be stable enough under normal signal levels.

  If you don't care about the actual gain the amp has you could use 10k 0.1% input resistors and the diodes or something like that. 18k in series with the 6k would give the same 0.5 gain, you could replace the 0.1% resistors with matched 1%, your choice, would depend on the scale of the project.

  Even the transformer approach isn't perfect, it has many advantages but they probably can't handle a high power amp because two things, burning the primary and depending on the particular circuit they might also feed a harmful over voltage to the next stage.

JS

[Cerebus]

I'm not certain if the commercial solutions will be rated for lightning protection, because I don't know of many applications where it would be relevant, but I think a solution could be devised.

Situation? Outside broadcasts often have mike lines snaking in the open, over fences, past generator trucks, up trees, through rivers, past animal enclosures, just about anywhere and near all sorts of hazards that could dump a few kV into them if something went wrong. I would not be surprised if there was an old BBC monograph somewhere on exactly how to lightning protect a mic input.

[pleslie]

These are some really great suggestions, thanks for your input guys. I think I'll try and implement a series resistance followed by back to back Zeners to protect the amplifier inputs, I'll just have to tweak the gain settings to get back to 0.5. Fortunately signal quality isn't paramount in this design as the Audio is being listened to just as an input to modulate an audio reactive  lighting product. Thanks again so much for your suggestions, helped tremendously.

Just to clarify back to back zeners means two zeners in series in opposite polarity on the input line to GND such that it clamps the input line to Vz + 0.7?

[JS]

These are some really great suggestions, thanks for your input guys. I think I'll try and implement a series resistance followed by back to back Zeners to protect the amplifier inputs, I'll just have to tweak the gain settings to get back to 0.5. Fortunately signal quality isn't paramount in this design as the Audio is being listened to just as an input to modulate an audio reactive  lighting product. Thanks again so much for your suggestions, helped tremendously.

  Shall started there. As most things aren't important in this case, (CMRR, precise gain, THD, noise, etc) I'd probably go for a PTC or a polyswitch in series with the input, so the protection network isn't in risk and can stand the faulty condition for long periods of time. Using a proper configuration it's not hard to protect agains 1kV indefinitely.

  You could also want it to be able to work under that conditions, for example, if you feed the signal from a power amp instead of a line level, as it could work with no problems. You just need to take that into account.

Just to clarify back to back zeners means two zeners in series in opposite polarity on the input line to GND such that it clamps the input line to Vz + 0.7?

Yes

JS

[grifftech]

just use many Schottky diodes in paralel