[sdiy] Simple bell-EQ

[Rutger Vlek]

Hi everyone,

Thanks so much for the suggestions! From the thread Neil suggested, I ran both the Crest and ASP versions of the parametric EQ circuit in Spice and simplified it to my needs. Fixed frequency, fixed Q and two predefined amounts of gain turn it into a really simple circuit! It turns out the Crest outperforms the ASP version in terms of THD+N performance. It seems to me the ASP was a “designed for manufacturing” version of the original Crest idea, with 10K pots (cheaper/simpler to find) instead of the various higher values used by Crest. As I don’t need pots anyway, I went for best performance.

Rutger

> On 06 Jul 2020, at 21:33, Eric Schlappi <eric.schlappi at gmail.com> wrote:
> 
> I've been looking at gyrator based EQ circuits lately, basically replacing an inductor in a passive filter with an active circuit (transistor or op amp). The transistor based ones show up as the EQ in a bunch of DOD pedals which got me interested in it.
> 
> There are some suitable op amp based circuits and theory in this article:
> https://sound-au.com/articles/gyrator-filters.htm <https://sound-au.com/articles/gyrator-filters.htm>
> On Mon, Jul 6, 2020 at 12:10 PM Rutger Vlek <rutgervlek at gmail.com <mailto:rutgervlek at gmail.com>> wrote:
> Hi list,
> 
> I’m hoping for some collective wisdom on the following issue. I’m trying to make myself a fancy (analog) guitar buffer with some basic tone controls. I already implemented a switchable high shelf filter (bright switch), and an adjustable 1-pole HPF for cutting bass. Now i would also like to implement a switchable mid-frequency scoop. With a VST parametric equalizer I found a bell-shaped curve with a certain Q and frequency I liked, and would like to implement this with a switch to select a few preset gains (boost/scoop). But I struggle to find a good analog circuit topology that fits my (simple) needs. Google turns up a lot of digital implementations, or way too complex equalizer circuits where every aspect can be controlled, but of which I don’t understand how to set gain, Q and frequency. Any suggestions for a simple bell-shaped EQ for which Q, gain and frequency can be set (and forget)?
> 
> Many thanks!
> 
> Rutger
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