[sdiy] marked/selected components

[harry bissell]

It is not possible to say for sure what the components were selected ~for~

but there are some very good guesses

For an OTA they usually select for low offset voltage, or low offset 
voltage variation
as Gm bias is changed.

For an NPN / PNP pair (expo) they are looking for similar forward drop 
at a particular
collector current.  This is so hard to do that NPN/PNP expos are not a 
popular method
anymore.  To their credit, they are fast acting and don't have servo 
noise...

Other times they are matched for gain, or forward drop...

Other other times selected for low noise...

and the beat goes on....

H^) harry


anthony wrote:
> I have seen in certain schematics where it specifies that a certain 
> part is "selected" - notably CA3080's and MN3005's, and also 
> transistors of course. One schematic for a filter specified selected 
> transistors for an expo conveter: "red" for 2N3904 and "blue" for 
> 2N3906. Now in the context of transitors, these color designations are 
> familiar and make sense to me because many transistors, Japanese ones 
> especially designate hfe by color or letter code (the letter code 
> usually being an abbrev. for a color name...).
>
> My question is: what do the colors mean for other components? I have 
> seen CA3080's specified as "selected: red". I have some CA3080E's that 
> came from some stereo audio gear that are daubed with red paint dots. 
> Are these "choice" in some way? I also have some LM324's that came 
> from some test gear that are daubed with white paint. And in addition 
> I also have several sets of 2SA733's and 2SC945's that came from the 
> same pieces of gear (I think all driving VFD displays) that are daubed 
> with either silver paint or white paint. My guess is that these 
> transistors were matched in some way, but is that necessary for a VFD? 
> My biggest question is for the red-dot CA3080's and the white-daubed 
> LM324's though.
>
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