[sdiy] VC ADSR input CV voltages

[James Dunn]

Hi

I built the EFM VC RADSR envelope generator and modified it according to 
a schematic from Scott Benardi. It was working well until recently, when 
I discovered the 4001 had died. I replaced this, but the decay stage 
isn't functioning properly (it's more like attack). I suspect it's the 
4052 analogue multiplexer, which I will replace next. However, my 
question is, how did these chips fail? I think it might be because of a 
negative going input voltage on the CV inputs - I notice there is no 
protection against this! So would a diode from the input to ground solve 
this problem? Or would it be better to build some sort of input buffer 
amp with an offset? IS there a schematic online for such a circuit?
Any advice gratefully received.

thanks

James