[sdiy] JRC4558D, LM13600/13700 Questions

[Steven Downhill]

Hi all,

I've recently acquired 4 JRC4558D ICs from an old AIWA tape deck and was
wondering if these were useful for any synth diy projects that may be
lurking on the net.
I know that a number of effects peddles from the late 70's and early 80's
used them, although for what reason I've no idea (I can't find a
datasheet/schematics anywhere for them).
Are they similar to the more modern NJM4558's ?.

Also, there has been a number of mentions of adding a couple of components
(FET transistor etc) to the input? stages of an LM13700 so that it behaves
more like the LM13600 (if not the same). 
However not being very electronically minded at all, I've no idea how to go
about this (I can read schematics so can build things without a problem,
although I have no idea why the circuits operate as they do). 
Can anybody explain how to do this simply and preferably with a small
diagram of what to connect to what.

Thank you for your patience and time.

Regards

Steven


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