Passive tilt EQ with potentiometer?

[mdcmdcmdc]

Hi folks,

I'm just curious if anyone happens to have a basic schematic on hand for a passive tilt EQ that's controlled by a potentiometer rather than a multi-position switch.
I'm familiar with the tubeCAD glassware design:



and I was able to dig up this schematic which (unless I'm misreading it?) seems to be for a fixed EQ setting with a bypass switch:



Is a 2-band passive bax and a dual gang pot the way to go?



Passive tilts seem unpopular outside of mastering EQ, but I'd like to mess around with them a bit.

Thanks in advance!

[Vivek]

Tom (Electricdruid) used an ACTIVE  tilt EQ in his  Hard Bargain distortion pedal
https://electricdruid.net/designing-the-hard-bargain-distortion-pedal/


ACTIVE Tilt EQ also features on the ESP pages
https://sound-au.com/articles/eq.htm#s10

[Vivek]

Active Baxandall when used with Ganged Pots to emulate tilt:


Lots of action towards ends of the pot.

At Max boost /cut, the slope is almost 6db/oct (almost 16db per decade)

[Vivek]

This looks like 3 well chosen and adjusted filters, which have total response as follows :


that's about 6db Insertion loss and roughly 2.6 db per decade slope.

In comparison, the active Baxandall was at almost 16db per decade at max boost/cut positions.

[mdcmdcmdc]

Indeed! Active tilt EQs seem fairly common, I'm interested specifically in a passive implementation.

[GibsonGM]

A passive tilt EQ is part of the Big Muff's sound.  Jack at AMZ 'took it apart' and did some mods, and describes the basics of it here:
http://www.muzique.com/lab/tone3.htm

Of course the 1 pot version isn't as 'steep' as the active or dual gang type, but it's a lot simpler and is found all over the place, and you can mod it quite a bit.  You'll want 'something to drive it' (gain stage, opamp, BJT etc) and something to 'recover' from it (see Big Muff Pi schematic for output recovery stage using BJT).  Simple.

HTH!

[mdcmdcmdc]

Without going to far down the rabbit hole, I don't tend to think of a BMP tone control as being quite the same as a tilt - the two sides of centre (at least in my limited understanding) seem to operate a little bit differently from one another.

Spending a bit more time with it this afternoon, it looks like a passive James stack with a dual gang pot (wired such that one goes up while the other goes down) might be a way to get there.

The passive tilts designed for studio/mastering seem to use stepped rotary switches with fixed positions which makes a lot of sense to me for precision/stereo implementations. I was just wondering if there's a slightly more-guitar-pedal-friendly implementation that doesn't require adding an opamp into a circuit that doesn't already have one.

[Vivek]

But won't a passive James introduce -12 to -22 dB insertion losses ? And then you would need a recovery stage anyway.

[PRR]

A passive tilt tends to null-out in the middle frequencies.

As Vivek hints--- you almost always need some form of make-up or isolation.

Math for active (also some editorial blather):
https://www.edn.com/implement-an-audio-frequency-tilt-equalizer-filter/

[ElectricDruid]

But won't a passive James introduce -12 to -22 dB insertion losses ? And then you would need a recovery stage anyway.

This is exactly why I don't see why people go for passive tone control stages so often. If you're going to finish up needing an recovery amp stage, you might as well use an active tone control and avoid the problems. Hence my choice in the Hard Bargain.

[mdcmdcmdc]

I guess I should have prefaced this in bold type by saying that I understand the reasons why tilt EQs are almost exclusively active. I'm just curious for the sake of curiosity if there's an implementation of a passive tilt eq circuit that uses a potentiometer rather than a rotary switch.

[Vivek]

Can the Tilt EQ be modified into a frown/smile EQ ?

Mid scoop/mid boost EQ

[mdcmdcmdc]

You could use a James stack with relatively flat centre response (look at the Garnet G250 / Session Man) and wire a dual-gang pot to give you either a tilt-ish or frown/smile single knob. Actually you could probably use a switch to go between either if you wanted to get (needlessly?) fancy.

[Rob Strand]

Here's some passive tilt circuits.    The idea is to pivot about the centre point and to have the least amount of insertion loss.

[Vivek]

Is the insertion loss low because the range is low ?

Can this be designed for max +10 db on the bass while -10db on treble ?

I suppose it could have around 12db insertion loss then.

[Rob Strand]

Is the insertion loss low because the range is low ?

Can this be designed for max +10 db on the bass while -10db on treble ?

I suppose it could have around 12db insertion loss then.

It's low because of the low amount boost/cut  but it has *least* attenuation for a passive control because the side that is boosted is at 0dB.

In order to pivot about center, the maximum boost/maximum cut must be +A_dB and -A_dB from that central level.  For lowest insertion loss and passive you want the boost part to be 0dB.    For that to happen it follows the flat position (and central frequency) must have an insertion loss of A_dB.   So the flat insertion loss follows the maximum amount of boost/cut.

The circuit does it with a single pot and you can set A_dB to any level.

[Vivek]

ESP had some pretty damning words to describe the Tilt EQ over at https://sound-au.com/articles/eq.htm#s10

I'm not entirely sure why anyone thought this was a good idea, but it's part of tone control history, so is included.
There are many possible tweaks that can shift the centre frequency or provide asymmetrical response, but these are generally as useless as the circuit itself. I expect it seemed like a good idea at the time, but it's really a rather pointless waste of parts.

Why was it considered unworthy ?

Which one knob tone control is better ? In which aspect ?

[Rob Strand]

Why was it considered unworthy ?

Which one knob tone control is better ? In which aspect ?

I'm not sure exactly why ESP dislikes them.    To me it's very easy for them to sound unnatural.   Especially the common circuits like the Muff control.    In some pedals the way the tilt-control is implemented it creates a notch and that gives a certain tone.   However with careful tuning of the notch frequency and depth you can get a good sound with a fixed EQ notch (like a bridge-T circuit).   Beyond the notch, the tilt aspect can still sound very aggressive at extremes.  In small doses it's OK, and  it can sound a bit unnatural if the centre frequency isn't well chosen.

In DSP you can vary the tilt across a wide spectrum, more a variable slope.  This can sound more natural but to me only in small doses.

The whole EQ thing is pretty tricky.   How do you compare,
- Bass-treble controls
- Low-pass filters
- High-pass filters
- Variable notch depths.   (like contours controls Bass amps and some guitar amps.)

Not to mention Q controls on high and low-pass filters which are essential for the sound on synths.   Then onto graphic EQs and parametrics.

They all do different things.   It's more about using the one that fits the job.

[anotherjim]

I think the situation matters and looking at graphs can be misleading. Try mentally applying a guitar cab eq curve to what the tone response plots show.
There's almost always a source of make-up gain in the signal path - it doesn't have to be at the tone control. You may be thinking of a fully passive 1-knob box but it would have to be always in circuit because of the insertion loss.

What we call the BMP tone control can be remarkably effective. With values chosen for minimal mid-scoop, it really does do what you'd expect and although the curves may not have symmetry about the centre that doesn't mean you can't get the treble-bass balance you're wanting to hear. The only bad thing is that the extreme settings are too much and almost never get used.

[cspar]

For passive tilt with with a makeup stage I prefer 2 knob versions that allow control of the notch.

Designs like in the Musket/Iron Bell/Colossus type Big Muff mods or Merlin's Bone Ray tonestack.