Tilt EQ

[fryingpan]

I'd like to implement a pre/de-emphasis circuit in a distortion pedal. I'd like it to be based around a tilt EQ (first active, for treble, and after distortion, a passive version which actually cuts treble - no headroom with 9V, after all).

The easy way to do this is to use a treble and then a bass shelf, but the problem is that the slope of the curve is fixed in a bass or treble shelf, so what you are actually doing is simply modifying the shelf frequency, raising it and lowering it as you go along. I'd like these emphasis circuits to be the same shape, but scaled as you modulate the intensity.

I can do this or a similar thing with a Big Muff filter with both sides equal, but this incurs a certain insertion loss everywhere, while I'd like to minimise insertion loss (which I would be able to do with a regular shelf). Are there any alternatives?

[phasetrans]

Does the active tilt EQ do what you want? https://www.edn.com/implement-an-audio-frequency-tilt-equalizer-filter/

Feed it with low output impedance, and don't load its output too heavy.

[fryingpan]

Does the active tilt EQ do what you want? https://www.edn.com/implement-an-audio-frequency-tilt-equalizer-filter/

Feed it with low output impedance, and don't load its output too heavy.

That's fine for the active filter (although you should add a few caps here and there for single supply operation) but I would have to work out a way to convert it to a passive version for the de-emphasis filter, and hope it tracks similarly to the active version for true pre/de-emphasis.

[phasetrans]

I should have clarified the dual supply version was just informational.

If I understand correctly, you'd like to have the filter and it's inverse matched, so that the linear TF stays fixed, but the shape of any distortion is different? If yes, I've wanted to try this for grit on bass guitar for some time

Do I have the idea right?

[ElectricDruid]

If the de-emphasis is going to be passive, how's it ever going to match a variable active pre-emphasis?

I'm missing something. I don't understand how this is supposed to work.

A typical pre/de-emphasis is a shelving boost for the pre-, followed by a shelving cut for the de-emphasis, and the boost and cut amounts and frequencies are matched so the overall response stays the same (roughly...). The Boss CE-2 has a nice example if you need one. I can see the point of wanting to add a control to that, so you can have more or less pre/de-emph, but if you're adding a control, it's going to need to control *both* parts somehow. Unless it doesn't, in which case it's just a variable treble boost followed by a treble cut - dead simple to do.

So what *are* you trying to do?

[ElectricDruid]

Here's one way to do a variable shelving EQ - basically we mix the straight through signal with a highpass filtered version.




If you had a dual-gang pot, you could do a matching de-emphasis version easily enough, but it *is* going to need a dual-gang pot or some other equivalent method of changing two things at once.

[phasetrans]

If you had a dual-gang pot, you could do a matching de-emphasis version easily enough, but it *is* going to need a dual-gang pot or some other equivalent method of changing two things at once.

This is what I intended to do when I replied to the OP about active tilt. (approximately) mirrored active tilt controls on either side of a clipping element, with dual gang pot to control both together.

I also will use half an active Baxandall / James instead of a passive tone control. The LF portion of the circuit is fixed with resistors.

[ElectricDruid]

If you had a dual-gang pot, you could do a matching de-emphasis version easily enough, but it *is* going to need a dual-gang pot or some other equivalent method of changing two things at once.

This is what I intended to do when I replied to the OP about active tilt. (approximately) mirrored active tilt controls on either side of a clipping element, with dual gang pot to control both together.

Yeah, what I've shown is really just half of a tilt control, but since Pre/De-emphasis usually is only treble, it seemed reasonable.

You're dead right that if you had a dual-gang pot, you could wire up two identical tilt controls with one of the pots back-to-front compared to the other (e.g. swap pins 1 and 3) and you'd be done. With an effect Send/return loop in-between the two tilts, you could have all sorts of frequency bending fun!