When we saturate a sine wave signal we get some odd and/or even harmonics. And when we overdrive a guitar signal we get these harmonics/overtones as well but also some of the things we don’t want (too much). Not only the fundamental peak frequencies will be overdriven but ALL frequencies, which results in many overtones which are not at all musical sounding. We call this intermodulation.
A heavy overdriven guitar signal will only sound okidoki when playing single notes or power chords (root + 5th + octave) and that’s because of the intermodulation. When we saturate low frequencies which are harmonically unrelated to the peak frequencies it will create nasty sounding intermodulation.
In short: intermodulation adds frequencies that are out of tune.
A simple trick to minimize intermodulation is a trick which is used in many overdrive pedals like the Ibanez Tube Screamer TS9:
As well, Tamura added a simple but effective post-distortion equalization circuit with a first-order high-pass shelving filter that “is linearly dependent on its gain”, an approach called “progressivity”. The overdrive stage is followed by a simple low-pass filter and active tone control circuit and volume control, using the second op-amp available on-chip.
Use a High Pass (HP) filter to remove some of the low frequencies BEFORE it enters the saturation unit. Maybe you also want to use a Low Pass (HP) filter after the saturation unit to remove some of the high overtones which are generated by the saturation unit.
One of the devices in Reason that is perfect for this trick is the Pulveriser. Many of the patches of my Boutique Amp and Rockmen ReFills make use of LP and HP filtering to minimize IM. On the Pulveriser you can set the routing, route a filtered signal into the Dirt, or the other way around. Use a HP filter and route it into Dirt or into a Scream 4 or any other saturation or amp simulation rack device. Add another Pulveriser to further shape the sound using a LP filter to cut of unwanted high overtones.
We cannot eliminate IM totally. But this is a good thing ’cause:
a lot of players would complain that the sound didn’t have enough “hair”
If you want to learn more about IM, please read Distortion 101 by Blackstone.