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Metal can audio opamps?
Metal can audio opamps?
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Old 17th January 2013, 06:44 PM   #11
qusp is offline qusp  Australia
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opa228/2228, opa2111, opa111, the newish (current feedback) lme49713HA
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Old 17th January 2013, 06:50 PM   #12
GoatGuy is offline GoatGuy  United States
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You kind of have to ask the question ... why metal cans?

Its not really like they do anything in a normal conservative design. In the "old days" they put (everything) in metal cans for 4 reasons:

[1] Because they hadn't worked out "engineering plastic" that could safely be squeezed onto chips during manufacture. [Resolved by the early 1960s]

[2] Because metal cans were hermetic, preventing moisture in particular, but for industry, other noxious gases from touching the semiconductor surfaces (which can gradually degrade and destroy them)

[3] To dissipate greater power through air convection (especially when transistors, even "low power" ones were near their limits in power dissipation

[4] Shielding and "input super-resistance" (i.e. ensuring that things like MOSFETs and JFETs had their gates attached directly to a pin that is insulated with glass, one of the best insulators when clean. When making "sample and hold" circuits, this is a large requirement.)

And I suppose

[5] Military requirements, spec-sheet conformance, to allay the fears (or ultimate-demands) of nervous designers and equipment builders.

Seriously though ... in the last 35 years, the dissipation thing has been answered (best!) by simply making more efficient op-amps and higher conduction plastics. The "insulation" thing has been resolved for decades. The plastics are great insulators (and moreover have been engineered to prevent tribo-electric effects AKA "static electricity from rubbing"). In terms of interference and shielding, the present-day op-amps are placed on circuit boards that are carefully laid out, and it isn't a problem. Op-amp chips ... unbeknownst to most people ... have been almost entirely re-engineered in the last 20 years, even if they carry "standard" designations. Much lower power, better onboard transistors, much higher accuracy of thin-film resistors. Higher Hfe and gm.

So... unless you have either

[6] Esthetics ... 'cuz cans look cool -- or --
[7] Opportunity... A bag of them sitting around

in mind, I recommend staying away from them, for circuit-to-circuit, you're far more likely to get an older "NOS - new old stock" item that has significantly more variable noise performance, dissipation and quiescent drift characteristics than a brand new, state of the art, recent-product-addition.

So sez me.

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