Copyrights of synthesizer circuits

[Martikainen Pekka]

I have a project to manufacture a semi-modular synthesizer and sell 
it worldwide. The problem is that some of my modules are based on designs
of other manufacturers. For example the filter is Moog ladder filter and 
the oscillator is based on Moog Prodigy.

So I am asking that is it illegal to little modify those circuits and
then use them in my own commercial projects? Of course Moog is now dead,
but how about Oberheim, Korg etc?

Pekka Martikainen
cubase at cs.tut.fi