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Re: ESD protection for HiZ inputs?
开云体育
On 06/10/2024 12:10, Richard Lee via
groups.io wrote:
OK, so I accept it is impracticable on a condenser mic - the broader question for me is? - What is the upper limit of input impedance where a diode ESD protection scheme can work effectively with low noise? And which diode is best suited to that?
do you mean this one? From a brief look it seems it explains the protection circuit,
but I couldn't see anything about why they chose 1N4004GP
specifically. Unless it is implicit in some way I didn't
understand. They also seem to be mostly concerned about the sorts of faults caused by plugging in live phantom powered cables to a low Z mic input, which is quite a different situation from ESD on an exposed HiZ input. A mic preamp input is concerned with noise because it has high gain following it, wheras my buffer circuit is unity gain or less and concerned with noise in quite a different way.? I'm at least as concerned about diode leakage throwing off the
fet bias as I am about noise. Though the solution may be the same.
yes, as above, I'm wondering where the practical upper impedance
limit is.
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Re: ESD protection for HiZ inputs?
BAV199 are suitable for mike preamps like THAT 1512 but are too noisy for condensor mike stuff.? You want something like Ir 0.2pA.? There's nothing spec'd to that level as its very temp. dependent.? You just have to guess that the paper spec. is pessimistic and by experience.
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BTW, the THAT datasheets recommend 1n4004GP.? There's some AES papers on The Phantom Menace by THAT & Wayne Kirkwood that explains the choice.
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I don't think diodes with leakage low enough not to introduce noise to a 1G input will protect anything.? They? will be too fragile and too low Vr Reverse Voltage.? They are suitable to run FETs at Idss for something like simpleP48.
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That's not to say they don't exist cos some must be integrated in OPAs |
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Re: ESD protection for HiZ inputs?
How would you know when it's done its job? just the noise levels?
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If I have a build where the protection diodes are not replaceable separately from the fet they are protecting, what specifications of the diodes should I consider for robustness? On 06/10/2024 09:21, Jerry Lee Marcel wrote:
All P-N junctions become noisy under circumstances, particularly when they're driven in their Zener region. |
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Re: ESD protection for HiZ inputs?
Thanks Richard this is setting me on the right track, but I have more questions...
On 05/10/2024 22:04, Richard Lee via groups.io wrote: Dunno about piezo amps @ about 10M but for a LN condensor mike over 1G, the diodes must have VERY low leakage.yes, I think it is degrading my noise levels even at the <5M input impedance of the circuit I linked to. Guru Scott Wurcer liked a smoke alarm FET used as a diode. Another 'diode' for this is the BE junction of an old fashioned LN BJT like BC214 or 2n5087.no space for this in my piezo circuit but I'll keep it in mind for future higher impedance pluggable setups. Schottkys have very poor leakage which gets worse with time. The latest THAT 1510/12 datasheets for microphone preamps recommend cheapo 1n4003GP (Glass Passivated).Why 4003 rather than say 4007? and what is the advantage of the glass passivated? does it reduce the leakage? I've seen BAV199 (dual low leakage surface mount) mentioned in some searching for fet input protection. The small size, though hard to solder, would actually suit me for my current application. any comments on BAV199? is overall beefiness a consideration? ie max forward current, max reverse voltage? BAV199 are small diodes with low voltage and current specs. |
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Re: ESD protection for HiZ inputs?
All P-N junctions become noisy under circumstances, particularly when they're driven in their Zener region.
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I would say that a diode that has done its job of protecting a low-noise circuit should be replaced one day or another. Le 05/10/2024 à 23:28, Ralf R Radermacher via groups.io a écrit?:
Am 05.10.24 um 23:04 schrieb Richard Lee via groups.io:I've personal experience of Schottky protectionNever noticed anything like that with my traveler MK3. Then again, I use |
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Re: ESD protection for HiZ inputs?
How many months have you had your Traveler Mk3?
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Can you open it up and tell us what diodes it uses on the mike amps?? They are large and pretty easy to spot.? Should be 4 on each input near the IC.
BTW, its a very bad idea to have a 1G or greater input plugable.? Stuff like replaceable capsules on condensor mikes take pains to shield the HiZ pin and ensure a shield connection is made before anything touches this. ?
RCA plugs & cables are about the most evil possible.
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Re: ESD protection for HiZ inputs?
开云体育I’m using the diodes ?am as this is text book protection for opamps ? Can’t speak to FET’s or a >10M input impedance?Best Regards, Jules Ryckebusch? On Oct 5, 2024, at 16:04, Richard Lee via groups.io <ricardo_lee@...> wrote:
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Re: ESD protection for HiZ inputs?
Am 05.10.24 um 23:04 schrieb Richard Lee via groups.io:
I've personal experience of Schottky protectionNever noticed anything like that with my traveler MK3. Then again, I use it mostly for recording... errr... noise, i.e. street scenes and industrial ambiences. Are these diodes in the mic amps? Ralf -- Ralf R. Radermacher - K?ln/Cologne, Germany Blog : Audio : Fotos : |
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Re: ESD protection for HiZ inputs?
Dunno about piezo amps @ about 10M but for a LN condensor mike over 1G, the diodes must have VERY low leakage.
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1n4148 far too leaky and noisy.? Guru Scott Wurcer liked a smoke alarm FET used as a diode.? Another 'diode' for this is the BE junction of an old fashioned LN BJT like BC214 or 2n5087.
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Schottkys have very poor leakage which gets worse with time.? The latest THAT 1510/12 datasheets for microphone preamps recommend cheapo 1n4003GP (Glass Passivated).? I've personal experience of Schottky protection diodes going noisy in several MOTU Travelers withing 12 mths and indeed all da MOTU designs cured by replacing with 1n4003
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Dunno if 1n4003 will protect Jule's piezo amps. |
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Re: ESD protection for HiZ inputs?
开云体育thanks. So it works with a 1M input impedance. How high input impedance can you do this before diode leakage starts messing with the fet bias or lowering the input impedance? BTW I have built the circuit I linked to and it works ok, but I'm
not sure if it sounds quite as good as without the diodes, nor how
much ESD protection I'm actually getting. Which schottky diodes did you use? what current capability and reverse voltage? there's no point in protecting the fet only to blow the diode
which I fear could happen with the tiny 1N4148.
On 05/10/2024 21:01, Jules Ryckebusch
wrote:
Yes!!! I just did this with my piezo buffer for my hydrophones. You need Shottkey diodes. See this? |
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Re: ESD protection for HiZ inputs?
开云体育Yes!!! I just did this with my piezo buffer for my hydrophones. You need Shottkey diodes. See this?Best Regards, Jules Ryckebusch? On Oct 5, 2024, at 14:55, thet via groups.io <parenthetical@...> wrote:
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ESD protection for HiZ inputs?
I hope this is OK to ask here as it isn't exclusively a mic question but a general electronic question that might be applicable to some mics.
Consider a fet input with a high impedance, that is connected to a exposed bare wire or RCA type plug, because it needs to be unplugged from the source. This could be for a piezo pickup with an input impedance of 5-10M or it could be for a condenser mic with an impedance of 1G Do I need to protect that bare fet gate from ESD it might encounter in the world? here's a schem to illustrate: I have naively added diodes but I'm not sure if they will actually lower the input impedance which obv I don't want. The real-life behaviour of diodes at high impedances is beyond my current level of understanding. I'm also not sure if 1N4148 are big enough to offer much protection. Nor am I sure if R1 offers any protection. Could any of the experts here talk me through what would be wrong with the attempts at ESD protection in this schematic? |
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Re: choosing SimpleP48 resistors
Jerry (and pmfalcman) is (are) of course right.
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By all means measure your P48V and 9V before doing the arithmetic.? Da P48V is specifically important as many so called Prosumer 'P48' devices may be much less.? I ask people to do this in simpleP48.pdf to give tested values for various capsules.
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Please keep sending me these test results for simpleP48.pdf for those (shock horror !!) who are MicBuilding without a multimeter.
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But we are shooting at a broad target; 5 - 10V.? Drawing Fig 6 in FETbias.doc has highlighted the consequences of just using E24 resistors.? It has also prompted me to personally go again for 5V instead of 10V .. something I keep vacillating between. :)
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I have to resist making the circuit more complex to 'cure' its shortcomings.? But then my failing eyesight and deteriorating soldering skills won't let me fit even one extra bit inside a male XLR.
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There's also an even simpler circuit but IMHO, this sacrifices stuff like simpleP48's excellent RFI/EMI immunity etc .. lesser known features that distinguish good mikes from great mikes. |
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Re: choosing SimpleP48 resistors
开云体育A more elaborate answer would take several pages of discussion,
so "yes". Le 29/09/2024 à 15:17, pmfalcman a
écrit?:
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