[sdiy] Anyone do a voltage controlled "Tilt" Filter?

Rutger Vlek rutgervlek at gmail.com
Mon Jun 14 21:46:04 CEST 2021

Hi Neil,

You might want to have a look at the Tilt EQ implementation done by Rob
Hordijk for the Nord Modular G2 (a virtual modular).

It inspired me when I made the River Marble eurorack module, which is
basically two dual VC-tilt EQs. Just like the Hordijk implementation it's
based on two all-pass filters per channel, but there was an issue with the
mixture of these filters, causing the downward tilt to approximate the
desired 3db/oct slope, but the upward tilt to behave less nicely. I solved
that by using separate weigthing for upward and downward tilt. You can
achieve that and the VC by using a ring modulator (I would suggest a 2164
based ringmod).


Op zo 30 mei 2021 19:24 schreef Neil Harper via Synth-diy <
synth-diy at synth-diy.org>:

> I recently breadboarded this tilt equalizer filter as found on EDN:
> https://www.edn.com/implement-an-audio-frequency-tilt-equalizer-filter/
> A similar (the same?) circuit is shown here:
> https://sound-au.com/articles/eq.htm#s10
> It's pretty cool, low parts count and can sound interesting - i'm kinda
> digging it to eq an fx for sample.
> of course my next question was "how can I get this under voltage control"?
> here's what I plan to try, maybe you can steer me in the right direction.
> my first guess was to replace the potentiometer (P1) with two
> vactrols/LDRs, but I'm not sure how to basically 'crossfade' the two
> vactrols in opposite directions and I think the response curves of the
> vactrols with regard to voltage input (light) and resistance are far
> from linear.
> i've read that OTA's like the LM13700 (the monty python chip) can be
> used as voltage controlled resistors, but I've never seen them
> implemented as a pair replacing a potentiometer - so I don't know if
> that route is possible either.
> my last guess was wondering if an equivalent circuit could be made by
> combining a lowpassed and hipassed audio signal and then crossfading
> them. I can imagine it would somewhat similar, but I'm not sure if you
> would get the same totally flat frequency response in the middle setting.
> --
> /// Neil Harper
> /// Every Wave is New Until it Breaks
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