Yes, it was touted as a "current differencing op amp", which many folks did not understand. It has a different pinout and unfortunately it does not allow replacement with any of the hundred standard pinout quad opamps. And it was much less common. So one immenselypoular circuit for a VOX,(voice operated switch) has been around over ten years, and all the poor folks ho tried a real opamp instead are disappointed. And the gain calculations are so different that redoing it for an LM324 is a challenge. That VOX was really good, otherwise, since it iused that IC and a 555 timer IC.

Yeah thanks it has been many years. I did update my original reply though with much more information.

 

Why did you use the LM3900 instead of a standard opamp or differential instrumentation amp?

The LM3900 was the first quad op amp I knew of, and the circuit needed a lot of op amps. I could not see me drilling holes for a lot of LM741's and saw a quad op amp as the solution. The LM324/358 was not far behind but too late for that project.