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AMZ Saturation Control

Setting Distortion and Harmonic Levels

I was working on new distortion designs in the lab and specifically looking at methods to provide better control over the harmonics generated. These saturation control circuits resulted from some of the experiments that were conducted. It has application in any circuit that uses a pair of diodes to ground for clipping. An example is the MXR Distortion+.

The basic idea behind this saturation control is to set the control pot as a voltage divider rather than merely using it to insert series resistance between the signal and the clipping diodes. This allows better control of the harmonics and allows for some simple mods to alter the response of the saturation control.

When the wiper of the saturation control is at the end connected to the input signal, distortion will be at a minimum. With the wiper at the opposite end, the resistance of the pot is in series with the signal and the full clipping by the diodes is taking place.

This variation of the control adds a resistor across the lugs of the saturation control. This resistor has a couple of useful functions. It shapes the response of the pot into anti-log response, which allows better control over the range of the dial. It also allows less resistance when the diodes are fully on, but more when fully off so less clipped sound leaks through - both are desirable features.

Another variation uses a capacitor on the saturation control to produce a modified effect. Since the capacitor will pass some of the higher frequencies, the control has more effect how the lows are clipped and can tilt the response of the clipping circuit. This effectively adds some twangy "bite" to the sound of the distortion and will keep the sound from being too flabby.

If the capacitor is moved to the lug of the pot that connects to the diodes, a different high boost response is developed. This circuit has no high frequency emphasis when the diodes are at max clipping but as the saturation control is turned back, some of the highs will pass through and be clipped. this version is good to add some sheen or enhancement to the sound.

This saturation control is ideally suited for use with the AMZ Overdrive Pro, AMZ Fat Gnat, Proco Rat, MXR Distortion+ or any other circuit that utilizes back-to-back diodes for clipping. With a little modification, it can also be used for circuits that have the diodes in the feedback loop of the opamp - but that's the subject of a future Lab Notebook article.

Try this saturation control to your projects for an interesting variation to the sound of your overdrive. Enjoy!

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