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’59 Fender Bassman Filter

April 11, 2012
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I found a paper online with an incredible amount of detail on creating an authentic digital replica of the tone stack in the ’59 Fender Bassman (despite the name, the Bassman is apparently a very popular guitar amplifier). The paper details a physical model of the circuitry, leading to a very precise emulation of the filter. The tone stack is an interesting type of filter, either plug it into the Filter Analyzer or check the paper for some examples of the frequency responses it can give. I figured it wouldn’t be too hard to translate the work into a Reaktor macro. Actually, it turns out entering and double-checking extremely long equations in Core is kind of annoying, but I finally got it working.

However, I found another problem: whenever I moved a knob, the bass frequencies temporarily shot way up, causing unwanted distortion and clipping. The filter presented in the paper does not seem to do well with parameter changes. A simple fix for this is to empty out the z -1 macros in the structure (setting them to 0) every time a knob is turned. These macros contain delayed signals and feedback values, both of which can cause instability when changing the parameters of this filter.

In his thesis, David Yeh continues the work in this paper and gives component values for some similar tone stacks. Since the Reaktor macro is a digital implementation of a circuit, you can actually change the values for the capacitors and resistors in the Bassman circuit, allowing for an emulation of the Marshall JCM800 filter as well.

I hope to learn some more about the rest of the Bassman at some point to emulate the rest of the amplifier, but it’ll take a while, if ever. Because this filter is meant to be used with a preamp, you may want to give the signal some gain.

Download here

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One Comment leave one →
  1. meter maid permalink
    April 12, 2012 3:28 am

    Awesome character filter!!

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