A guitar preamp using biquads and a waveshaper

Let’s start this with a confession. This post is nothing more than a summary of an idea I picked up from an eight year old paper. I didn’t even go to the trouble of finding the paper myself but instead found it cited in the source code to guitarix. For that reason this should be quite a short post.

Firstly remember that at the moment tintamp is not really a modeller. It will be, but its not there yet. For now its just a fixed function digital signal processing chain that is “good enough” to allow each stage in the chain to be prototyped and developed further. On the plus side being fixed function makes things very easy to describe.

So the first revision attempt at a tintamp preamp consists of three similarly configured amplification stages based on an approximation of a class A 12AX7 amplifier. The heart of the tube stage is a waveshaper based upon the transfer function of a 12AX7. This is surrounded by three biquad filters that model the effects of the capcitors in the circuit being modelled.

All told it looks something like this (click to enlarge):

Please forgive the poor layout above. I rushing to press “Publish” just a little and I’m not very expert at graphviz just yet. Additionally if you want to see the ideas expressed more lucidly the Virtual Air Guitar folks have already done a very good job, take a look at their paper!