Core Shaper Parametric Eq  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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dylan159
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Post by dylan159 »

After showing you a couple of not-clones, here's a more original circuit I'm proud of. It's a parametric mid frequency eq with also bass and treble controls.
I think equalizers are very very powerful effects, altough they shouldn't be overestimated when post production is an option. Usually diy-ing a graphic eq is hindered by the amount of controls needed for a decent amount of bands, being them pots or worse, sliders. So I wanted to look for an alternative, and one which didn't feel like a downgrade, and a parametric or at least sweepable eq seemed the best solution.
eq2v5.png
Parametric eqs aren't new in the guitar world, the boss sp-1 being an example, and many others through the years. My approach to the bandpass is the Wien bridge eq, in particular a switched Q version through the means of positive feedback very much like in the JHS haunting mids, but which I want to point out I've taken from the book "Small Signal Audio Design" 3rd edition by Douglas Self directly. Unlike JHS, I haven't just copied the circuit as is, but I've tweaked the values to get the gain, Q and frequency ranges I liked, while using only E6 values because that's how I like it. To do this the Q switch shorts a series resistor instead of adding a parallel one for example.
midloq.png
midhiq.png
The gain is +-16dB, the frequency sweep is about 300 to 3.3K, the Q is 2.4 in HI and 1 in LO. The beauty of this circuit is that the Q stays constant across the whole sweep and so does the gain.
Before this, there's a low gain input stage which gives a strong input signal and buffers the input while avoiding clipping at guitar levels throughout the circuit. The charge pump helps with the headroom, providing a +-9V supply so the boost levels can be reached without problems.
Following the bandpass, there's a very standard Baxandall 2 band EQ to take care of low and high frequency and add quite a lot of flexibility to the circuit when combined with the bandpass. The cutoffs seem to work well in my experience but they can easily be switched by switching caps if more controls want to be added. I chose the one-capacitor version for both bass and treble networks because that's the least amount of components and I'm cheap :) but also because I like the cutoff being more constant and the loading on U1B seems to be good. C8 limits the gain at ultrasonic frequencies and is not always found altough it's very good practice. 100p is probably generous but that's not an issue for guitar especially.
treble.png
bass.png
Knowing that EQs tend to struggle with noise, either because the amount of gain they have especially at high frequencies, or because of the circuit itself, I prioritized noise over headroom, and this is reflected in the choice of a fixed gain input stage and a volume control at the end, which in the end isn't that different from what most eq pedals do. Headroom is still very good, I've measured input headroom throughout in "worst case" scenarios with max boost, and you have to try hard to make this clip, with an hot input signal, maximum amounts of boost, and maybe overlapping the mid band with the baxandall. Personally I didn't have any issues, and it needs to be said that some eq pedals get away with a single 9v supply and similar amounts of boost somehow. For line level signals an input pad might work well, and maybe 12V input. The impedances are low throughout to minimize noise, while still avoiding overloading the op amps. For the input stage TL072 is recommended because it's quieter with guitar inputs, while the band EQ uses 5532 which is quieter overall. I think both noise and headroom have been handled well and I don't have issues.
Here's some real measurements with ARTA as demonstration.
Flatness vs bypassed signal:
Img6.png
Low and high Q gain sweep:
Img4.png
Img2.png
High vs Low Q:
Img3.png
Bax Bass:
Img7.png
Bax Treble:
Img8.png
I have no frequency sweep pictures because I was testing this with a 50K pot which meant a narrower range, and didn't save pictures of the 100K sweep, but I can assure it's about 300 to 3.3K as expected.
No demo of this because it's such a versatile pedal it's hard to do it justice. It's not the end all tool, but works surprisingly well in shaping the sound of clean and distortion alike, and that was the goal. For finer tasks post production can take over for me, even a graphic eq won't be enough.
Finally, if you're wondering about the version 2.5, here's the short story: V1 was a gyrator based three band eq inspired by an ESP project. It works well in theory and has the advantage of being able to add more bands easily. Sadly, I wanted to have one or maybe two sweepable bands, and while gyrators can be sweeped across a decent range, the Q variation is too much, I calculated it to be equal to the frequency ratio.
V2 used another kind of Wien bridge bandpass, a simple one that required a single op amp, and the input stage was a darlington buffer to keep the op amp number even. For V2.5 I also considered even another version of Wien bridge, one that required two op amps but managed to keep Q roughly constant with a single gang pot. After finding available 100KC dual pots I went with this one with a very constant Q and the switchable Q, altough the single gang is charming.
Here's a not pretty but very compact vero for it:
eq2v5.png
And some pics of my build just to show off :D
_IGP4450.jpg
_IGP4451.jpg
Hope you enjoy and let me know if you try it :D
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Post by Frank_NH »

Very cool pedal, dylan159. The EQ design reminds me of the tone controls on my solid-state Lab Series L5 amp, which also uses a mid control circuit followed by a Baxandall-like treble and bass control. The mid control on the L5 works very effectively for shaping the amp's distortion, which happens downstream of the EQ. All you would need to add to your circuit is the Moog-patented multi-filter to have an L5-style tone shaper in a box. :)

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Post by dylan159 »

Frank_NH wrote: 25 Mar 2021, 22:34 Very cool pedal, dylan159. The EQ design reminds me of the tone controls on my solid-state Lab Series L5 amp, which also uses a mid control circuit followed by a Baxandall-like treble and bass control. The mid control on the L5 works very effectively for shaping the amp's distortion, which happens downstream of the EQ. All you would need to add to your circuit is the Moog-patented multi-filter to have an L5-style tone shaper in a box. :)
Thank you. Good call, the principle is similar, although the mid filter, even if wired in the same way to the signal path, is different, although I can't tell how much honestly. I think it's a bridged T instead of a Wien bridge?
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Post by TWSpedals »

After looking at the datasheets I don't think your TC1044 is doing anything in the circuit at all.
Can you explain why you've set it up like you have?

Rest of the circuit looks ace by the way. Nice to see a full real parametric with a Q control for once
Have a nice day!

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Post by dylan159 »

TWSpedals wrote: 26 Mar 2021, 13:16 After looking at the datasheets I don't think your TC1044 is doing anything in the circuit at all.
Can you explain why you've set it up like you have?

Rest of the circuit looks ace by the way. Nice to see a full real parametric with a Q control for once
I guess you're more used to charge pumps used as voltage doublers, this is a voltage inverter straight from the datasheet to provide a bipolar supply to the op amps. Trust me, if they got 0V or nothing instead of -9 I'd have heard it very quickly!
Thanks by the way
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Post by Frank_NH »

dylan159 wrote: 25 Mar 2021, 23:28
Frank_NH wrote: 25 Mar 2021, 22:34 Very cool pedal, dylan159. The EQ design reminds me of the tone controls on my solid-state Lab Series L5 amp, which also uses a mid control circuit followed by a Baxandall-like treble and bass control. The mid control on the L5 works very effectively for shaping the amp's distortion, which happens downstream of the EQ. All you would need to add to your circuit is the Moog-patented multi-filter to have an L5-style tone shaper in a box. :)
Thank you. Good call, the principle is similar, although the mid filter, even if wired in the same way to the signal path, is different, although I can't tell how much honestly. I think it's a bridged T instead of a Wien bridge?
Not sure about the type of mid filter topology used in the L5 - the Aion FX L5 Preamp documentation (https://aionfx.com/project/l5-preamp/) has a schematic with the L5 tone controls. This type of control was also used in the successor of the L5, the Pearce G1 solid state amplifier, schematics for which can be found here:

http://www.travis-hartnett.com/Pearce_A ... /Home.html

I think a simple but effective guitar preamp could be built around a tone control like your Core Shaper. Just need some soft clipping and perhaps a limiter at the end (like the L5).

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Post by dylan159 »

Not sure about the type of mid filter topology used in the L5 - the Aion FX L5 Preamp documentation (https://aionfx.com/project/l5-preamp/) has a schematic with the L5 tone controls.
feels like an active version of connecting a constant Q sweepable bridged T as filter core to the usual cut-boost feedback found in many graphic eqs to me.
This type of control was also used in the successor of the L5, the Pearce G1 solid state amplifier
Sure was! pretty schematic.
I think a simple but effective guitar preamp could be built around a tone control like your Core Shaper. Just need some soft clipping and perhaps a limiter at the end (like the L5).
Sure! That was not my intention because I can always use another pedal as clipping unit and a preamp for what else is needed (not much I guess, we already have the impedance, the eq and a decent amplitude from this). But sure, you've shown that this approach is viable and has been used.
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Post by TWSpedals »

dylan159 wrote: 26 Mar 2021, 13:33
TWSpedals wrote: 26 Mar 2021, 13:16 After looking at the datasheets I don't think your TC1044 is doing anything in the circuit at all.
Can you explain why you've set it up like you have?

Rest of the circuit looks ace by the way. Nice to see a full real parametric with a Q control for once
I guess you're more used to charge pumps used as voltage doublers, this is a voltage inverter straight from the datasheet to provide a bipolar supply to the op amps. Trust me, if they got 0V or nothing instead of -9 I'd have heard it very quickly!
Thanks by the way

AHH I was thrown off by the convoluted way the schematic has shown the 1044 pinout
Have a nice day!

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Post by kizzer »

Really nice pedal Dylan, built it up today using your stripboard layout. Had to flip Bass 1 + 3 and Treble 1 + 3 to get them boosting clockwise, but it works a treat and is a very useful tone shaper to add to the collection.

Cheers!

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Post by dylan159 »

kizzer wrote: 09 Apr 2021, 22:50 Really nice pedal Dylan, built it up today using your stripboard layout. Had to flip Bass 1 + 3 and Treble 1 + 3 to get them boosting clockwise, but it works a treat and is a very useful tone shaper to add to the collection.

Cheers!
This makes me happy, thank you.
I had that issue even though I had double checked the orientation somehow. The schematic should be updated but the vero isn't maybe.
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Post by kizzer »

dylan159 wrote: 10 Apr 2021, 08:01
The schematic should be updated but the vero isn't maybe.
Yep, traced the schematic to verify it, just the labels on the Vero.

Cheers buddy!

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Post by jeepe »

hi,
this is a great contribution, thanks very much, dylan159 !!

just a question,
how exactly could this be stripped to work as a mid only control?

could it be just by leaving out the 2 TL072 stages?

perhaps like this:
schem--core_shaper--mod-question.png
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Post by Ben N »

I would keep the TL072; just leave out the Baxandall stack--the bass & treble pots and the Rs and Cs attached to them.

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Post by Vurguuz »

Hi. First of all, thank you to dylan159 for this interesting schematic and the cool vero layout.

I was wondering how to add a high pass filter (bass cut) to this circuit? Is it a matter of adding a couple of resistors and capacitors around the input stage TL072 in the Core Shaper like this:
Image


hpf.jpg
Or is there a better way?

+ How can it be made to have an adjustable cutoff frequency?

Thanks.

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Post by dylan159 »

Vurguuz wrote: 20 Jan 2022, 20:12 Hi. First of all, thank you to dylan159 for this interesting schematic and the cool vero layout.

I was wondering how to add a high pass filter (bass cut) to this circuit? Is it a matter of adding a couple of resistors and capacitors around the input stage TL072 in the Core Shaper like this:
Image



hpf.jpg

Or is there a better way?

+ How can it be made to have an adjustable frequency?

Thanks.
the baxandall isn't enough for you? maybe you can extend its range, maybe making cut more than it boosts (since more boost can risk clipping).
Either way I don't like placing a sallen-key highpass at the input like that: it's low input impedance unless you use big resistors, would interact with whatever is before it (guitar or pedal, which pedal, which guitar, volume setting) and to make it really adjustable you'd need a dual gang pot, although single gang works somewhat. The input stage is meant to isolate the rest from the source and start the noise floor effort with the right foot. mess with things after it keeping in mind that everything could need good source and load impedances.
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Post by thpmink »

Hey Dylan,

great circuit! The 100k freq pot is botheting me a little. From what I have read of your posts, I assume it must have bothered you too… How much would lowering it to 10k decrease the range?

Do you think one could get away slapping a second parametric band in there without buffering it? Would take away from the nice simplicity of this circuit, but in a 125B knob spacing would be manageable I guess.

Edit: Ok, I see. Quickly modelled, with 68n/15n and a 10k pot, I get a ~250Hz - 500 Hz sweep:
Wien-Bridge-Para-EQ.png

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Post by dylan159 »

thpmink wrote: 03 Mar 2022, 15:21 Hey Dylan,

great circuit! The 100k freq pot is botheting me a little. From what I have read of your posts, I assume it must have bothered you too… How much would lowering it to 10k decrease the range?

Do you think one could get away slapping a second parametric band in there without buffering it? Would take away from the nice simplicity of this circuit, but in a 125B knob spacing would be manageable I guess.
You can find a 10kC dual but not 100k? I started from the assumption that the 100k one is as common as this sort of pot can be. It should be alright to scale everything else down by 10 (and caps up by 10) to compensate in that section. Everything from r4 to pin 3, the mid pot should be good as is. You already have a simulation going so it's easy to check. I've chosen this kind of filter exactly because I could get a constant Q across a big frequency sweep. I don't think it's possible to add another band to it for the way it's connected to pin 3. If you want multiple mid bands, a gyrator style can be made variable with a reasonably constant Q across a smaller range, and allows to tack on as many bands as you wish. Here's an older version of this project as an example (you might want to restrict the range by adjusting the stopper as written),
eq1.PNG
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Post by thpmink »

dylan159 wrote: 03 Mar 2022, 16:05 I don't think it's possible to add another band to it for the way it's connected to pin 3.
Thanks! I thought I could just double the whole section to get another band:
eq2v5.png
dylan159 wrote: 03 Mar 2022, 16:05 You can find a 10kC dual but not 100k? I started from the assumption that the 100k one is as common as this sort of pot can be.
It's not about parts sourcing, I was thinking about noise. Not a concern here?

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Post by dylan159 »

thpmink wrote: 03 Mar 2022, 17:35
dylan159 wrote: 03 Mar 2022, 16:05 I don't think it's possible to add another band to it for the way it's connected to pin 3.
Thanks! I thought I could just double the whole section to get another band:
eq2v5.png
dylan159 wrote: 03 Mar 2022, 16:05 You can find a 10kC dual but not 100k? I started from the assumption that the 100k one is as common as this sort of pot can be.
It's not about parts sourcing, I was thinking about noise. Not a concern here?
yeah, you can add another stage like that in series without issues. I don't think it's worth it to scale it down, it should be good enough as it is but you can if you find the right pot. if you want to streamline it you can do without the Q switch.
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Post by NickH »

Hello,
I found this schematic online and had a go at making it. It works... It is great.
.... I was a little confused about the pots and whether they should be linear or logarithmic or inverse log etc but cobbled it together and all seems OK. I assumed A was linear, B was log and K was inverse log.
There is only one issue that I have encountered. When I adjust the mid and mid frequencies pots, there are combinations which lead to some howling. I have not put the electronics in an enclosure as it is just on a breadboard for now.
Has anyone else experienced this and if so, is there a way to eliminate it howling. I am not using the Lo Q part of the circuit but otherwise, I have followed the diagram.
Since it seems to function otherwise, I don't think that I have made any wiring errors. Some advice as to how to tone down the howling would be most appreciated.
My aim is to only use the mid section control to include it in a guitar rather than a peddle with bass and treble as well although for now, i have recreated the entire ciruit.
Thanks,
Nick

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