Presence control calculations

[Greenballs]

I'm currently tinkering with a jfet OD circuit with a full tone stack and presence control. I've tuned the bass, mid and treble controls to give frequency responses that I'm happy with using online tone stack calculators, but the presence control is proving a little more difficult to get right.

I'm using the attached schematic as a basis for the presence control. It's not a circuit that I've encountered before and I haven't been able to find an online resource to figure out how to fine tune it. I'm currently doing it by ear but I'd like to know exactly where on the frequency spectrum it is working. If anyone could show me how to calculate the frequency response of the circuit I'd be most grateful.

Thanks.

[Mark Hammer]

First off, let's differentiate between stompbox "presence", and a tube amp "presence" control.  This was originally intended to adjust an amp for a "cleaner" tone by using negative feedback from the speaker side of an output transformer to attenuate the added harmonic content of an amp pushed harder than '50s standards suggested it ought to be.  Of course, these days, we have changed our thinking about it and view it as a means for adding more "bite".  I have an original tweed '59 Bassman, and the Presence control appears not to have ever been modded or tampered with, and it reduces high-frequency content as it is turned clockwise (i.e., MORE negative feedback is applied as you turn it up).

Stompbox presence, or what gets legended as such, is generally directed at warming an overdrive tone by reducing the high end above what any treble cut normally would, and in the case of pedals like the King of Tone, is meant as a set-and-forget trimmer, rather than an actively used control.

The circuit fragment you show, presents a 3-pole lowpass filter.  Your 100k pot provides a variable resistance path to ground for the three caps in that filter network.  But it also provides a variable-resistance path to ground for the larger cap value of C53.  So the pot essentially pans between two different kinds of treble attenuation.  Because the 3 pole filter network will have a necessarily steeper rolloff, and is calculable as pretty low (not much content remaining above 2khz), I'm not sure what the path from C53 to ground is supposed to do.  Maybe it would make more sense in the context of more of the overall circuit, but right now I'm not seeing its added value.

[Greenballs]

Thanks Mark.

The full schematic is here. Its incomplete and from a different source but its the same layout (albeit with a few different component values here and there) and shows where the presence control is in relation to the rest of the circuit:



Is there a formula for calculating the frequency cut off point of a three pole lowpass filter? Individually the first two 'poles' have a cut off point of around 1.5k each, whereas the third is around 400Hz, which feels quite low for a 'presence' control, even on an amp. I may be wrong. But I guess its the accumulative effect that matters???

[Mark Hammer]

I've taken the liberty of digitally retouching and compacting the original schematic, since I hate when they're so wide one loses track of where stuff is going to.

You've changed some values, an some values on the original seem unspecified.  But seeing the whole thing provides a bit more clarity.  Have you ever heard the original?  Do you have a sense of what adjusting that control sounds like?

[Greenballs]

I haven't heard the original in person, just on YT, but I decided to try and reproduce the amp schematic in pedal form not because of how the amp sounded but because it looked like an interesting schematic that I could learn a thing or two from. I'm pretty sure what's on my breadboard doesn't emulate the original amp at all!

The drawing of the presence control circuit in my original post isn't mine, its taken from the internet. I don't really have an accurate sense of how it behaves in amp form but with the values presented it behaves in pedal form much like a treble control and so I have changed some of the values in an attempt to differentiate it from the actual treble control:



This does add some high end 'sparkle' at lower gain settings but also a lot of hiss, which makes me think its boosting frequencies that are beyond (higher than) those that are useful but, also, there's a lot of gain stages preceding it which are probably not helping with the noise.

I'd ideally like to be able to figure out which frequencies the control is affecting and whether or not I can eliminate the unwanted higher frequencies over and below those which add the sparkle. Actually, thinking aloud here, maybe a twin notched T-filter, like the mid control, might be a better solution...?