JFET Design Spotlight: Crafting the Perfect Bass Overdrive

We're always fascinated by the way Kirkwood Rough approaches analog circuit design. Here's another one of his design stories.

"Sometimes a sound gets a grip on your senses and that happened to be the Bass lines from Moulin Rouge. They were Fat, rich, percussive and Smooth with a high octave layered presence. That started a design process that ended up being the LadyMarmalade™ effect. It had to enhance even ordered harmonics without a sharp clipping effect, even when overdriven, while accentuating the rise and fall times at high drive. The core of this was an op-amp with relatively low loop gain and could be non-uniformly loaded by some nonlinear impedance.

 The typical distortion pedal, a Proco Rat for example, uniformly limits a gain adjusted/high loop gain/low output Z op-amp with two silicon diodes back-to-back. Uniform clipping results in high odd harmonic content; shred –metal sound with a fatiguing long-term listening effect. Adding a 390kΩ resistor between the diode node and the +9V trace will make a ProcoRat sound significantly better by asymmetry.

 However, non-linear circuit loading by Germanium diode biased silicon and germanium transistors creates a very soft clipped differential impedance load to the op-amp. The combined loading effect produces a wide even ordered harmonic content increasing with gain while maintaining, by soft clipping, a reduced upper frequency harmonic spread. The nonlinear loading of these two clamping circuit elements are sensed with a non-loading high impedance buffer using a GM multiplied JFET/Bipolar pair.

Using two 9V batteries in a split supply gives plenty of headroom for popped-string transient response without rail limiting. Since most all pedals are turned on by the shorting of an input ring contact to ground, a single MOSFET is used to turn on both batteries at the same time. Combined with a FatBottomGirl™ front end, endless forms of Bass conditioning can be achieved. Here, the acoustic subtlety of discrete semiconductor elements in an analog convolution is hard to beat. Linear Systems parts, reliably, are time tested answers for complex analog function applications well beyond these circuits.

As usual …… Kirkwood"

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