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Old 27th October 2011, 01:24 PM   #1
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Default Passive component recommendations from AP

Bruce Hofer's presentation to the AES last weekend contained some passive component recommendations. While the presentation is on AP's website, you have to be registered to download it.

Resistors:
* Use metal foil resistors if cost is no object
* If cost is an object, use thin metal film resistors
* 1206 is the "sweet spot" for SMD resistors.
* Design resistor circuits for maximum 3 - 5 signall volts
Capacitors:
* The only ceramic cap to use is COG (except in decoupling)
* Avoid low voltage (25V) ceramics
* Polystyrenics are hygroscopic and must be hermetically sealed.
* Metal foil is preferred to Metal Film
* In high performance analog, polystyrenics, polypropylene and teflon are the best dielectrics.
* Minimize the voltage across signal path capacitors to avoid microphonics.

Opamps (lots of stuff here...), but
* Consider using diode connected transistor for input protection.
* Reverse biased protection diodes can be very sensitive to light.
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Old 27th October 2011, 02:57 PM   #2
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Quote:
Originally Posted by jackinnj View Post
* Reverse biased protection diodes can be very sensitive to light.
Now that one is new to me. Any more details on that??


Thanks for putting the summary up, I think most of them are well known around here (though we should add component brands and models behind each line...)
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Old 27th October 2011, 03:09 PM   #3
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I guess if they we in a zeiner glass package they would be light sensitive, but standard black packages would be good to go.
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Old 27th October 2011, 03:12 PM   #4
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Now that one is new to me. Any more details on that??
Any reverse-biased diode has some leakage current. That leakage current can increase if light hits the junction. Unimportant in many applications, but in high precision stuff, it can bite you.
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Old 27th October 2011, 03:14 PM   #5
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I guess if they we in a zeiner glass package they would be light sensitive, but standard black packages would be good to go.
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Old 27th October 2011, 04:25 PM   #6
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Ge semis are notorious for this, but, as the parts are generally in a dark box (unless you have a light on inside the box to keep the electronics awake), this is only a problem source for design and service.
The same for heatsinks. There is no advantage beeing black inside the box. E
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Old 27th October 2011, 04:32 PM   #7
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I remember reading about a problem with Fairchild diodes. They used glass bodies and painted or coated them somehow. No problem unless the paint got scratched, then instant photo diode!
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Old 27th October 2011, 06:24 PM   #8
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Thanks for the input. Now, is this also related to diodes in analogue audio equipment, notably vintage equipment? (70's gear is full of glass type zeners) Or is it rather limited to older Ge types. Are glass case zeners still made anyway?
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Old 28th October 2011, 02:05 AM   #9
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Interesting, informative and useful. Thanks for the pointer.

Unfortunately it's also mildly depressing.

I'm just in the throes of assembling a design with PPS caps in the signal path, which I mistakenly chose as being best suited. In fact they were the only SMT film caps available in that value I could find

COG/NP0 caps are available only in values <1uF. It seems that for larger values polypropylene is the only choice for the hobbyist, fortunately a good range of larger sizes is available.

This does make the use of DC servos more attractive.

Availability of metal foil resistors is strictly limited unless you're buying for industry AFAICS. Prices are out of sight. Information on retail sourcing would be highly appreciated.

The only comfort that I can see is that distortion is already low in existing devices using polyester caps and in all probability, while the levels being discussed are measurable, they're inaudible.
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