How To Create A Parametric Mids Stage

[difesa98]

Hello! I have designed a triple baxandall active preamp that utilizes a quad opamp (MC33179). Treble, Mids and Bass uses each a stage of the opamp and they are separated from each others using those 10uF non polarized caps. It works really well in my personal bass and i love it! In the image you can see a spice simulation obtained by changing the value of C9 and C12; i can choose a frequency between 200 Hz and 1KHz and i'm using B50K pots because they are the only pots i can find with a center detent. I wonder if it is possible to modify this classic mids stage to a parametric one (maybe using that opamp stage?); in particular i'm trying to obtain a sweepable mids from 200 Hz to something like 2KHz so that every EQ part stays in his "space" without overlapping with the bass and treble response. One pot for the selection of the frequency and one for boosting and cutting; i found that the Ibanez Varimid-3 uses this pot (the way it works is too large for my purpose: 100hz-5kHz): https://it.aliexpress.com/item/32340398009.html?gatewayAdapt=glo2ita




Thanks for any advice and help!

[antonis]

Hi & Welcome..

I can't help you in frequency modification (dunno respective formulas) but I can propose you an already set for your needs Sweep Middles EQ..



P.S.
Dual gang rev log pots are much more easy to be found and much more cheap..
e.g. https://www.taydaelectronics.com/100k-ohm-anti-log-dual-taper-potentiometer.html

[Steben]

Wah Wah

[Vivek]

Do have a dekko at the Mid control of the MT-2

which has been explained very well by brother Tom Electricdruid

https://electricdruid.net/boss-mt-2-metal-zone-pedal-analysis/



PS: "Dekko" is one of the words of Indian origin, that entered into the English dictionary about 200 years ago, due to British Rule of India

https://www.google.com/search?q=dekko

[difesa98]

Hi & Welcome..

I can't help you in frequency modification (dunno respective formulas) but I can propose you an already set for your needs Sweep Middles EQ..



P.S.
Dual gang rev log pots are much more easy to be found and much more cheap..
e.g. https://www.taydaelectronics.com/100k-ohm-anti-log-dual-taper-potentiometer.html

Thank you! this is a really good start, it has already the frequencies i want! This circuit can be used between the stage for treble and bass or it should be alone? I'm trying to simulate it via spice but it give me some strange behavior. I'm using this simulations for the pots: https://barbarach.com/pots-for-spice/

[antonis]

This circuit can be used between the stage for treble and bass or it should be alone?

I presume it should be OK to use a single stage for Bass & Treble and another one for Sweep Mids..

P.S.
You didn't modify the Sweep Mids stage for single supply..
R26/C5 in your schematic have to be connected to Vref (V2)..

RV3 shouldn't crackle for a FET input op-amp but might do it for a bipolar input one..

[difesa98]

This circuit can be used between the stage for treble and bass or it should be alone?

I presume it should be OK to use a single stage for Bass & Treble and another one for Sweep Mids..

P.S.
You didn't modify the Sweep Mids stage for single supply..
R26/C5 in your schematic have to be connected to Vref (V2)..

RV3 shouldn't crackle for a FET input op-amp but might do it for a bipolar input one..

Thanks, in this case i'm using an opamp stage for every eq parts. Even if i connect R26/C5 to Vref It acts strange. Maybe It's simply ltspice and those third-party model or, more probably, there Is something else wrong in my schematic. I need to understand this topology in a better way, copy and paste was too good to be true 😂

[antonis]

I need to understand this topology in a better way,

https://user.faktiskt.io/rikkitikkitavi/SELF/Small%20Signal%20Audio%20Design%20(2010).pdf
Chapter 10.

[antonis]

As for the above circuit, I'd suggest you some changes:
Make C13 & C14 100μF..
  //   R19 & R20 10k..
Decouple all ICs pin 8 (V+) with 100nF ceramic caps to GND (as close as physically possible to individual V+ pin)..
Connect R4 to GND (instead of V2)..     
   //       R26 to V2 (already mentioned)..
Interchange Middle and Bass arrangemed (Middle at the end of the circuit)..

P.S.
In your simulation, U1 acts as a buffer (Gain pot value set to zero) resulting into output heavy current (short-circuit protection limited) for input signal relative high amplitude (>1V, say)..
(the above happens due to particular IC inputs protection with a pair of back-to-back diodes, which effectivelly short the output to the input during slewing..)
A feedback resistor is recommended for keeping that current below short-circuit limit for faster recovery (LT1112 exhibits a remarkably LOW slew rate of 0.3V/μV typ) and settling of the output.. (minimum unwanted distortion)
Dunno if LTSpice takes it into account but for real world circuit it should be wise to place a resistor of 10k in series with Gain pot (minimum U1 gain x2)..

To be honest, better use another IC 'cause LT1112 output current capability isn't in compliance with such a low value Baxandall tone controls..

[difesa98]

As for the above circuit, I'd suggest you some changes:
Make C13 & C14 100μF..
  //   R19 & R20 10k..
Decouple all ICs pin 8 (V+) with 100nF ceramic caps to GND (as close as physically possible to individual V+ pin)..
Connect R4 to GND (instead of V2)..     
   //       R26 to V2 (already mentioned)..
Interchange Middle and Bass arrangemed (Middle at the end of the circuit)..

P.S.
In your simulation, U1 acts as a buffer (Gain pot value set to zero) resulting into output heavy current (short-circuit protection limited) for input signal relative high amplitude (>1V, say)..
(the above happens due to particular IC inputs protection with a pair of back-to-back diodes, which effectivelly short the output to the input during slewing..)
A feedback resistor is recommended for keeping that current below short-circuit limit for faster recovery (LT1112 exhibits a remarkably LOW slew rate of 0.3V/μV typ) and settling of the output.. (minimum unwanted distortion)
Dunno if LTSpice takes it into account but for real world circuit it should be wise to place a resistor of 10k in series with Gain pot (minimum U1 gain x2)..

To be honest, better use another IC 'cause LT1112 output current capability isn't in compliance with such a low value Baxandall tone controls..

First of all, thank you! i'm studying material engineering, everything that i'm trying to understand in this field comes exclusively from my passion and your help is much appreciated. This is an onboard bass preamp for my personal Cort Project, i didn't use those 100uF (electrolytic capacitor) in combination with 10k resistors because i need to keep the pcb dimension as small as possible. (in the image is the actual project that uses a triple baxandall and works like a charm)



I'm using LT1112 only in spice because is the only TL072 equivalent that was already integrated in the software; in reality i'm using a MC33179P (also used by Bartolini). As you suggested i connected R26/C5 to V2 (Vref=4.5V) and now i'm trying to understand if there is something else wrong, the only other thing that i want to achieve is the use of a B50K instead of the B10K for boosting-cutting (since i have a tons of those pots with a center detent from Tayda); i will buy some C100K stereo from Tayda or maybe aliexpress. Right now i don't know if his strange behavior is due to the LTSpice pot models (i don't think so; even with 2 resistor in place of the pots don't act correctly) or something in the schematics.

[antonis]

i'm studying material engineering,

(like one of my twins..)

i need to keep the pcb dimension as small as possible.

Then why don't you implement a single stage for Bass & Treble..??
(it will save you 4 resistor space, at least..)

As you suggested i connected R26/C5 to V2 (Vref=4.5V) and now i'm trying to understand if there is something else wrong,

The above suggested for U3 non-inverting input bias..
(it has to be biased at V_REF for proper output symmetrical swing - if grounded, complete negative signal waveform amplitude will be clipped..)

P.S.
I'm totally unfamiliar with LTSpice quirks so maybe some other person might check your simulation for further interpretation..

[Fancy Lime]

i'm studying material engineering,

(like one of my twins..)

If you have more than one twin, you cannot be twins anymore, can you? Triplets at least. Unless you are the dizygotic triplet and the other two triplets are also monozygotic twins. Biology is so much weirder than stompboxes...

[antonis]

<spam ON>

I was talking about my kids, Andy..
(fraternal twins boys)

P.S.
My wife is Biology teacher (genetistic) and I've learned a couple of things back in the days when she studied for university semester exams with me checking the respective literature..

<spam OFF>