JFET's BTW tend to be very robust to static discharge. This is mainly from experience there were no ESD devices on the hybrid FET amps (now obsolete) which used JFET's almost the same as the ones Linear Systems still makes. A large input coupling cap does not prevent ESD BTW.
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Yes, agreed....sort of. Except some carts, such as the one Bill posted an image of, connect cart ground to one coil pin at the cartridge; and common TT wiring, with braid to cart coil pin, then sets up the situation you describe unless wiring is explicitly not the obvious braid and centre used as balanced inputs. But, for balanced floating prototyping, this is fine because we all know how the wiring must be done, and the cart modified as necessary, to avoid this
As may be expected. In fact whatever protection provides the 1632 with a significantly inferior ESD rating to the 1642.
LD
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Or even that a lead client for the 1642 insisted it was measured to a higher voltage!
I had a rummage this afternoon between tasks SWMBO had given me and I have the following in my op-amp stash
4-off LT1028
8-off TLE2074
4-off OP07
4-off 709
I have no idea where I got the 709s from or why I have kept them. Guess I thought they would come in handy some time! I should be able to knock up a single ended set with something in that lot
I had a rummage this afternoon between tasks SWMBO had given me and I have the following in my op-amp stash
4-off LT1028
8-off TLE2074
4-off OP07
4-off 709
I have no idea where I got the 709s from or why I have kept them. Guess I thought they would come in handy some time! I should be able to knock up a single ended set with something in that lot
AFAIK there's graded internationally set reference standard(s), meant to indicate how sensitive devices are in likely use, but also for handling in manufacturing. I doubt manufacturers would willingly publish unnecessarily conservative ratings for any part.......
But, OK, if we ignore spec ratings, and wire the thing up properly, we can all relax and move on.
LD
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There's another mechanical resonance that never gets mentioned: cantilever self-spring and tonearm effective mass.
At first inspection, it looks like cantilever self-spring is in series with, and dwarfed by, suspension spring. However, suspension damping is also in play and, at frequencies of interest, dominates suspension impedance.
The upshot is that the dominant model is cantilever self-spring in series with suspension damping. This has a resonant f typically c 3-5kHz and a very low Q. Because of the phase of tonearm movement involved, it represents a dip in cartridge output, and, IMO, mostly accounts for the mystical mid-dip that is sometimes quite prominent in cartridges.
Interesting?
LD
At first inspection, it looks like cantilever self-spring is in series with, and dwarfed by, suspension spring. However, suspension damping is also in play and, at frequencies of interest, dominates suspension impedance.
The upshot is that the dominant model is cantilever self-spring in series with suspension damping. This has a resonant f typically c 3-5kHz and a very low Q. Because of the phase of tonearm movement involved, it represents a dip in cartridge output, and, IMO, mostly accounts for the mystical mid-dip that is sometimes quite prominent in cartridges.
Interesting?
LD
I meant what we do to meet the standards, sometimes it's, "let's try this ESD cell, well that failed let's try this one....".
I experimented a bit further with static discharge pulses and found an even better protection, this time with 8 diodes.
This time I included 100pF cable capacitances on both sides of the cart and placed two 0.56pF caps from output to input over the first OPA1632.
As with the previous image in #436, what you see is the output from both 1632's.
A 20KV 1usec pulse was injected on one side of the cart from a 200pF cap with a 2KOhm in series, simulating the human body.
At the same time a 1KHz signal was being processed.
Current through the Cart never exceeded 10uA during discharge.
I think that this is as far as you can get when having to suppress really large voltage spikes.
Hans
This time I included 100pF cable capacitances on both sides of the cart and placed two 0.56pF caps from output to input over the first OPA1632.
As with the previous image in #436, what you see is the output from both 1632's.
A 20KV 1usec pulse was injected on one side of the cart from a 200pF cap with a 2KOhm in series, simulating the human body.
At the same time a 1KHz signal was being processed.
Current through the Cart never exceeded 10uA during discharge.
I think that this is as far as you can get when having to suppress really large voltage spikes.
Hans
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Hans, any reason why you didn't strap to the power rails? I've always followed the advice of Bill Whitlock in these matters (attached the esd circuit for the THAT1200 which I used in some places in my system). I realise your method doesn't rely on the power supply caps to eat the impulse but are there other advantages?
(LD: I know you have moved on from this, but for me it's interesting to explore around the topic and educational as well).
(LD: I know you have moved on from this, but for me it's interesting to explore around the topic and educational as well).
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What are you hoping to gain? Of course you can build a discrete transimpedance stage, but I cannot see in this case much performance to be gained, although making a composite might be an interesting exercise.
Ref other op-amps as long as the noise is low and the GBW is 10MHz or above you should be good to go unless I missed something.
Alternatively, sharing the project on OSH Park would be great. I'd gladly cover the cost to produce and ship LD and Hans some boards gratis, as well as whatever brave soul would be willing to lay it out.
Really kicking myself for not getting a SilentSwitcher when I had the chance...
Bill,
The protection with 4 diodes to the power rails you are referring to are already part of the OPA1632.
That's why I believe that for normal use you don't have to do anything at all as far as protection concerns.
And that to my opinion is the very reason why Russ has no provision for extra diodes.
Whenever you are afraid for huge static charge build up in your environment, these 4 diodes are no longer capable of preventing a disaster.
On top of that both inputs should stay within a few volts to prevent damage.
So in case of severe static discharge, damage can only be prevented by inserting a whole series of diodes.
IMHO this is all a bit over the top, but as I have shown, even 20KV 1usec pulses can be kept well under control.
Hans
Yes, both SE and balanced Aurak versions can handle high GBW op-amps. However, as Bill points out, there's not much advantage above 10MHz GBW for either SE or balanced versions of Aurak. IIRC the use of a 1632 arises because there is a 'close cousin' pcb already laid out for it which is presumably field proven, and high GBW op-amps need good layout or throttling back to be stable of course. And it's fun !
Here's some sims of 10Mhz GBW versus 180Mhz, for balanced and unthrottled SE versions of Aurak. The SE version is 'unthrottled' by removing C13 and reducing C14 to 2pf in the 3.0 schematic sketch, which personally I include to ensure stability in prototype layouts.
LD
