Distortion when using VCA

So I have been building my first modular system dollar by dollar and finally added some VCA's, the Doepfer A-132-4 quad VCA module. I have a seriously distorted signal when I patch in some sound sources (Chord Organ, Pluck) but not on all sources (STO works fine). I am using the Doepfer A-143-3 quad LFO for cv into the VCA. Can anyone help me figure out if I need some kind of attenuation in line with these source signals perhaps, or am I doing something else wrong? I am very new to this and appreciate any help.

Absolutely. Since that VCA module has no input attenuation, it's a good bet that some audio sources are overloading it as not everything in Eurorack is exactly the same as far as peak-to-peak voltages go. Also, given that these are exponential VCAs, there's also a good possibility that deeper swings in CV will also push the VCAs into overdriven ranges since the linear curves of the LFOs are going to be 'misinterpreted' as exponential-type signals. Exponential VCAs really are best controlled with envelopes, and offer a quick and dirty way of using linear EGs to get exponential responses.

In any case, weakening is necessary, since it is not in this module at all. I agree with the author above, it is best to use converters for control. I once worked for IT and made websites like http://clc.am/iMgCNw and had a friend faced the same problems, but solved it.

I looked at the specs on the VCA module here:
http://www.doepfer.de/a1324.htm

Any control voltage above +5v pushes you above unity and into amplification. If you have anything that can measure voltages, you may want to check your modulation source to see if you are, in fact, hitting the VCA harder than +5v in CV. That would explain your unwanted distortion. You can either attenuate the audio signal or attenuate the CV signal.

Using another VCA or attenuator (like an attenuverter) will help keep the signal in check to avoid distortion.