Core Shaper

From the vendor

Core Shaper

Apr 18, 2022 4 min read

Updated: Aug 22, 2022

Equalizers are one of the most powerful tools to change the timbre of an instrument, and this is true also for their pedal incarnations. This almost always implies a graphical eq, not really a diy-friendly solution with the amount of controls needed, be it sliders or rotary pots, and not necessarily the best if chasing a specific frequency or Q. Also, pedal equalizers shouldn't be overestimated when today post-production tools are accessible to anyone: this is just a matter of revising the role of an equalizer when in a compact, player-accessible form: it's best used for either pre-emphasis before distortion or as broad-stroke correction of the general sound before it hits the speakers.

I went looking for an alternative then, one which didn't feel like a downgrade, and a parametric or at least sweepable EQ seemed the best solution.

Parametric EQs aren't new in the guitar world, the Boss SP-1 being an example, and many others through the years. My approach to the bandpass is the Wien bridge eq, in particular a switched Q version through the means of positive feedback very much like in the JHS haunting mids, but which I want to point out I've taken from the book "Small Signal Audio Design" 3rd edition by Douglas Self directly (values and everything point to this being the primary source). Unlike JHS, I haven't just copied the circuit as is, but I've tweaked the values to get the gain, Q and frequency ranges I liked, while using only E6 values because that's how I like it. To do this the Q switch shorts a series resistor instead of adding a parallel one. for example. Shown is the full sweep in both modes at maximum boost:

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The gain is +-16dB, the frequency sweep is about 300 to 3.3K, the Q is 2.4 in HI and 1 in LO. The beauty of this circuit is that the Q stays constant across the whole sweep and so does the gain, for both boost and cut.

Before this, there's a low gain input stage which gives a strong input signal, so that the output is loud enough even after cutting, and buffers the input, while avoiding clipping at guitar levels throughout the circuit. The charge pump helps with the headroom, providing a +-9V supply so the boost levels can be reached without problems.

If the gain provided by the input stage is removed, for example by making it a buffer, and why not reduce the input impedance while you're at it, the circuit will work well also for line-level signals, up to 2.3V peak to peak at that frequency at maximum boost, provided you don't do something silly like boosting the same band with both the mid band and the Baxandall.

Following the band-pass, there's a very standard Baxandall 2 band EQ to take care of low and high frequency and add quite a lot of flexibility to the circuit when combined with the band-pass. The cutoffs seem to work well in my experience, but they can easily be switched by switching caps if more controls want to be added. I chose the one-c