Tilt + Mid Cut/boost tone control

[ElectricDruid]

Hi All,

Here's something I've been playing with. I've been thinking for a while that when you're adjusting the response of a dirt pedal, you're generally trying to do one of four things:

1) Smoother - more bass, less treble
2) More cutting - more treble, less bass
3) Mid boost to cut through the mix more
4) Mid cut for that classic scooped tone

Now, 1+2 and 3+4 are mirror images of each other, so this should be possible with two knobs. And it is, but I haven't got a *simple* way to do it. The circuit's a beast!

This is the response:

https://electricdruid.net/wp-content/uploads/2018/09/TiltMidCutBoostResponse.png


This is the circuit
https://electricdruid.net/wp-content/uploads/2018/09/TiltMidCutBoostSchematic.png


R15/R17 and R3/R4 are just a way of simulating a 100K linear pot - those are the two controls.

I shall have a go at building a distortion with this in it. Four knobs: Drive, Tone Tilt, Mid boost/scoop, Level. We'll have to see how it sounds...

Hope you like it!
Tom

[ElectricDruid]

BTW, I'd be very interested to hear if anyone can see any simplifications for that circuit.

It feels to me like the inverting gain stage (R14 10K / R2 39K) followed by the inverting bandpass MFB filter could be combined, since you can redesign the filter to include the gain, but then it finishes up inverting overall, and I haven't worked out how to get around that yet.

Thanks!

[Scruffie]

Page 6, Figure 9, that would simplify it quite a bit http://leachlegacy.ece.gatech.edu/ece4435/f02/equalizer.pdf

[ElectricDruid]

It would, it's true. It's not quite the same though - I was going for a Constant-Q equaliser, rather than the standard one.

Still, I should give it a whirl in the Sim and see what it look like. Might be close enough.

Thanks Scruffie.

[Scruffie]

It's true it's not perfect but I recall the Q is pretty steady compared to some popular EQ types and it's flat at the centre position, a little imperfection to save 3 Op Amps ain't bad

I have used the tilt combination with it in the past (I've been searching for the combo you're after for a long time) and while it's a pain to tweak, it was a nice combo, better than a straight big muff control any day of the week.

Another variant I've always thought would be useful was a tilt EQ with a boost/flat/scoop mids toggle but I've not thought of an elegant way to achieve it yet.

[Danich_ivanov]

Another way to make mid control is by combining fixed mid cut/boost from bandaxall into big muff style mixing thingy, might save some parts. Or, perhaps even easier would be to simply take notch/peak filter from basic 3 band active eq with fixed bass/treble. Mid controls are surely fun to play with. I like to experiment with tone controls myself, sometimes i spend hours just tinkering. 

[abc1234]

AMZ Presence Control works great for me. (Just change the values to get the mid cut/boost frequency and amplitude you desire.)
http://www.muzique.com/lab/tone3.htm

[ElectricDruid]

Ok, I've rejigged this using a baxandall type mid cut/boost instead. It's not quite as peaky as the original version, but that may be a good thing, who knows. I also tried a gyrator based design (thinking that I could get variable resonance to experiment with like that) but the trouble with the gyrator is that all the action is at the ends of the pot. It needs a pot with a X^2 response!

https://electricdruid.net/wp-content/uploads/2018/09/TiltAndBaxMidPlot.png


https://electricdruid.net/wp-content/uploads/2018/09/TiltAndBaxMidSchematic.png

[Scruffie]

You can indeed make it more peaky https://www.diystompboxes.com/smfforum/index.php?topic=24359.msg192007#msg192007

That 'looks' pretty nice though, actually not too dissimilar from a big muff sweep.

[ElectricDruid]

You can indeed make it more peaky https://www.diystompboxes.com/smfforum/index.php?topic=24359.msg192007#msg192007

Thanks, but you'll notice I've already gone beyond their recommended 10:1 "narrow" ratio for the caps. I fiddled about with various combinations, but it looks like there's a limit to how narrow a band you can get out of this arrangement. But that's only an educated guess based on some experiments. I'd be interested to hear if it's possible to get high Q peaks or notches out of it somehow - not for this circuit necessarily, but just for curiosity.

That 'looks' pretty nice though, actually not too dissimilar from a big muff sweep.

Yeah, the Tilt control is pretty much the Big Muff lowpass versus highpass type of thing. The difference comes with the Mid Boost/Scoop control. With the tilt set neutral, you get a notable mid hump or mid scoop. With a bit of tilt, you can get a mid notch or resonance, and with a bit more tilt, you can extend or reduce the range that you hear by putting back or removing more of the centre.

Overall, I hope the second control adds enough extra options to make it worthwhile.

[Scruffie]

Oh yeah I get that the tilt is basically a big muff without the scoop but I meant the general sweep shapes were comparable but obviously with more flexibility and the ability to go totally neutral by centring each control now.

I guess the only problem now would be if it offers too much control and the interaction feels unnatural. To the breadboards

[Ben N]

How difficult would it be have variable Q (should one wish to go down that road)?

[ElectricDruid]

How difficult would it be have variable Q (should one wish to go down that road)?

Not very difficult. You'd replace the baxandall stage at the end with a gyrator-based mid EQ instead, and you could have variable Q and a reasonably variable frequency too (about 4.5:1 IIRC).

The downsides are that the resonance changes as the frequency is shifted (although if you've got a resonance control you could compensate that, at least at one end of the pot) and that as I mentioned, the boost/cut tends to happen mostly at the ends of the range and there's not much action in the centre of the pot.

It's a swings/roundabouts type of thing, as they often are. You choose which compromises you make.

[iefes]

I really like the idea of this two-knob tone control. I always loved the tilt-EQ in my Diamond compressor clone. It does not give the accuracy like a graphic equalizer but is a great tool to make effective adjustments just with one knob. Additionally it's great to have the opportunity to just leave the response flat.

I would like to include your idea in a project I'm tinkering with. I'm aiming for some kind of transparent overdrive with a nice tone control section.

However, can you lead me to some more detailed information on how to design the baxandall mid-control? I've tried several different resistor- and cap-values in Spice to see what they do, but are there any "design-rules" one should follow? Most of the information I find is about Treble-Bass Baxandall tone controls.

What would you guys suggest would be a good center-frequency for the mid-control? Something around 700 Hz maybe? And the same question I'm wondering regarding the tilt eq. I'd be happy about some further input.

Thanks!

[ElectricDruid]

Sorry iefes, I don't know of any handy "rule of thumb" type information for how to calculate the values. It would certainly be nice to have. I've done this just by experimentation in LTSpice, and if you've got Spice too, that's the course of action I'd recommend.

What I do know is that increasing the ratio of the two caps (I've got 33/2.2=15, but 10 is good too) gives a more peaky/narrower band and more equal values does the reverse. Beyond that, double the values to go down an octave, or halve them to go up an octave. Then pick the nearest thing that is realistic (e.g. 4n7 not 5n if you're looking for half a 10n) and try it in the sim. Rinse, repeat.

I think 700Hz sounds like a good centre for mids, yes. Rod Elliot of ESP makes an argument that 640Hz is the geometric "centre" of the human hearing range, based on octaves from 20Hz to 20KHz. Certainly a bit lower than the typical 1KHz is better. I used 800Hz in the design above, which might still be a bit high but we'll see when I get to build it.

[Elektrojänis]

Are you familliar with this: http://www.muzique.com/lab/tone3.htm

The title might be a bit misleading. It basically describes a method to add a mid-control to BMP style tone control... It actually just changes the cutoff of the treble side, but in practice when combined with the bass side it becomes a mid control.

The controls might be more interactive than what you are looking for... And the mid control might not be narrow enough (it might be more musical that way though... And as a passive circuit it technically offers only cut (you can always add a gain stage to it). But anyway it's so simple it's probably worth checking out.

[iefes]

@ Elektrojänis: Yes, that's a great tone control and I know this one. There's a reason why it's included in so many effects. However, I'm trying to find a tone control that offers completely flat response and very predictable tone shaping, i.e. no strong interaction between the controls. And I'm just curious 

@ Tom: Thanks heaps! Some very good information, indeed. In the meantime, I found the 1980 National Semiconductor Audio Handbook which has some information on Baxandall type mid controls. This might also be interesting for you or others, if you don't have it already. They use a wider Q with the capacitor on the wiper being larger than the bridging capacitor.
I'm wondering if the ratio of 15 isn't a bit too much.

The Handbook can be downloaded here: https://ia800604.us.archive.org/9/items/bitsavers_nationaldaAudioRadioHandbook_17034677/1980_National_Audio_Radio_Handbook.pdf

A lot of experimenting to do 

[Kipper4]

" no strong interaction between the controls."
That's a tough one.....

[iainpunk]

First off, this is a reply to the firts post, the rest was tl;dr.

1+2+4 are encompassed by the big muff tone stack. You could add a quite aggressive mid boost to counter the mid cut. Would be quite do-able with 2 pots... Ill draw something up in a bit, when im home... Maybe even the "inverted bmp tonestack" (with set resistors instead of the pot) panned against the conventional one...

Ow yeah, ill ask a teacher of mine for advice, he's also an audio freak

[iainpunk]

First off, this is a reply to the firts post, the rest was tl;dr.

1+2+4 are encompassed by the big muff tone stack. You could add a quite aggressive mid boost to counter the mid cut. Would be quite do-able with 2 pots... Ill draw something up in a bit, when im home... Maybe even the "inverted bmp tonestack" (with set resistors instead of the pot) panned against the conventional one...

Ow yeah, ill ask a teacher of mine for advice, he's also an audio freak

Owkay, tis was a bad-ish idea, the problem is that , when all knobs are in neutral, you have 2 notches which are cut quite violently, this wil probably sound odd/unique. Ill draw up the schematic in 20 minutes, when im home, and a crude amplitude response sketch