Help creating Mid control pedal

[tjdracz]

I've just been watching this Deluxe Big Muff Pi demo:

And I must say, I really like the sound of that mid boost sound (around 1:30 mark). That got me thinking, as my pedal arsenal includes fairly mid scooped pedals like Superfuzz or Big Muff, I thought that it could be a good idea to build some mid boost pedal to regain the presence.
I know that there are pedals like Demeter Fat Control, although I've heard that they mostly work with low mids area, nearly boosting bass.
I was looking for something that could have similar control to that Big Muff mid control - boost/cut option with fairly flat response at 12 o' clock of pot rotation, variable frequency center and maybe a switch for wide/narrow Q bandwith.
R.G.'s article on simple EQ: http://www.geofex.com/article_folders/eqs/paramet.htm seems to be quite similar to something I'd like but I need some help to get my head around it.
I've redrawn the diagram to delete that other band control and I was thinking whether something like that would help me achieve what I want:



I was thinking of using 3/4 of TL074 as three opamps. Also thought that instead of Resonance pot controlling the Q, I could have a SPDT switch choosing beetween wide/narrow bandwith like on Big Muff. But now, couple of questions:

- What would be recommended C1 and C2 to allow me to go from usable low to high mids around the pot rotation, providing nice variable boost for Fuzz and non-Fuzz playing? What would be the good values/center frequencies so start?
- Is that 10K resistor on Resonance SPDT switch would benefit from being slightly smaller. Is larger resistor = wider bandwith?
- Is that 10K boost/cut pot there works as I would like? As if flat response in mid position, cut CCW and boost CW?

Any help here would be much appreciated! I'd love to get my head around that design so I could put in as a mid control stomp! 

[R.G.]

- What would be recommended C1 and C2 to allow me to go from usable low to high mids around the pot rotation, providing nice variable boost for Fuzz and non-Fuzz playing? What would be the good values/center frequencies so start?

Something between 400Hz and 1KHz is going to be low mids to high mids, subject to your ears, of course.

Start with about 0.033 for C1 and 0.01 for C2, twiddle from there.

- Is that 10K resistor on Resonance SPDT switch would benefit from being slightly smaller. Is larger resistor = wider bandwith?

Yes.

- Is that 10K boost/cut pot there works as I would like? As if flat response in mid position, cut CCW and boost CW?

Boost happens when the wiper on the boost/cut pot is closest to the opamp inverting input. Cut happens when it's closest to the non-inverting input.

[tjdracz]

Ace, RG! Great answers as always
Guess I'll start breadboarding. Are there any more considerations before putting that as a pedal? Do I need to add anything more? I was thinking just 100uF cap from 9v line as a filtering but not sure whether I'd need anything else

[tjdracz]

- What would be recommended C1 and C2 to allow me to go from usable low to high mids around the pot rotation, providing nice variable boost for Fuzz and non-Fuzz playing? What would be the good values/center frequencies so start?

Something between 400Hz and 1KHz is going to be low mids to high mids, subject to your ears, of course.

Start with about 0.033 for C1 and 0.01 for C2, twiddle from there.

I can't get my head around the math used for determining center frequency. Could you put in in idiot terms for me RG? The equation on the website is F0=1/(2*pi*L*C) but where does the C1 come into play? 

[R.G.]

I can't get my head around the math used for determining center frequency. Could you put in in idiot terms for me RG? The equation on the website is F0=1/(2*pi*L*C) but where does the C1 come into play? 

Everything below C1 is a circuit that fakes an inductor, the "L" in the equation.  C1 is the "C" in the frequency equation. Actually, I hosed the typing on the web site. The proper equation is a few paragraphs up:  Fr=1/(2*pi*SQRT(L*C))  I accidentally left off the "SQRT" (square root) in the second one.

So the actual equation - er, let me see if I can still do algebra - is

F₀ = 1/ (2*pi*SQRT((R₁*R₂*C₂)*C₁))

Where R1 is the 470 ohm and R2 is the 51K and setting of the 1M pot in series.

[tjdracz]

Okay, one last question, what units am I using in the equation? Is all C in uF? Are resistors in ohms, kiloohms? As the difference between say 1.05 and 1050000 will make quite a difference in that square root

[tjdracz]

Just want to make sure I get my maths right. Using C1=0.033 and C2=0.01 I get something like 0.0017894895 and 0.0003943843 for F0. I suppose when multiplied by 1000000
, it turns into 1789.4894596889 and 394.3842685494. Is that the Hz freq I'm looking for?

[induction]

Units should be F and ohms. I'm guessing you used uF and ohms which would mean you have to multiply your answer by 10⁶.

So yes, 1.8 kHz and 394 Hz is correct.

[tjdracz]

Thanks, that makes sense! So at lowest L the frequency is highest, right? Makes sense with that pot as variable resistor the resistance is decreasing when it's turned clockwise. Thanks, think I've got it all sorted now!

[tjdracz]

Okay, I did vero layout for the schematic above, using SPDT instead of resonance pot and with unused opamp, connecting + to Vref and - and out connected together.

Would probably benefit from second pair of eyes to make sure there aren't any mistakes!

[aron]

Don't forget this little gem:

http://www.duncanamps.com/tsc/index.html

[tjdracz]

Don't forget this little gem:

http://www.duncanamps.com/tsc/index.html

I'm definitely not forgetting it! Played around with it the other day and despite improving my big muff tone control greatly, I couldn't really make anything that would come close to thing I'm looking for

[tjdracz]

Okay, I've set up circuit in Tina-TI in order to be able to plot some frequency graphs. This is something that Tina generates:


C1 is 12n, C2 is 10n, frequency pot is at max, boost is at max and resonance is at 0. It seems to work as it should, giving me  flat response at mid of boost/cut turn. Frequency goes from about  600Hz to 3KHz but two things now:

It seems that there is also a boost going for quite high frequencies. Is it something I should be worried about and can it give me any issues? If yes, what's the suggested remedy?
Also, it seems that while there is boost in frequency, the overall volume with the circuit is down. Is it a good idea to add something like that:
http://www.jer00n.nl/2010/07/28/clean-boost-for-guitar-or-bass/ - using the unused fourth opamp from TL074 to recover the volume loss?

[GibsonGM]

1) I wouldn't worry about the high boost, but a low-value cap to ground might be in order in case you end up superaudible oscillation....

2) Yes, a booster like that should work.    Search "Anderton frequency booster" on here - there's already a circuit that's been done (very well) that does what you want!  But if you want to 'roll your own', go for it! 

[tjdracz]

1) I wouldn't worry about the high boost, but a low-value cap to ground might be in order in case you end up superaudible oscillation....

2) Yes, a booster like that should work.    Search "Anderton frequency booster" on here - there's already a circuit that's been done (very well) that does what you want!  But if you want to 'roll your own', go for it! 

Thanks. I've seen Anderton booster but it's limited to one frequency, RG's application seems to be much more flexible and since I've already set on doing that, I guess I'll stick to it 

[mth5044]

I tried out almost exactly what you have and it works great.

[tjdracz]

I tried out almost exactly what you have and it works great.

Should do! Finished all the tweaking in TINA now and it seems like I've got it sorted. Off to build it tomorrow!

[PRR]

> plot some frequency graphs.

From R.G.s' plan in your first post?

With the boost/cut (upper) pot centered, the overall gain should be EXACT zero dB (unity gain).

With boost/cut at one extreme, still the gain over most of the audio band should be tending to zero dB. So already we see your simulation has an error somewhere.

With boost/cut at an extreme and Resonance at zero Ohms, the peak/dip should be near 5:1 or 14dB, tailing to few-dB an octave or two either side.

Somehow TINA is looking at something different from R.G.'s 1999 drawing.

[tjdracz]

> plot some frequency graphs.

From R.G.s' plan in your first post?

With the boost/cut (upper) pot centered, the overall gain should be EXACT zero dB (unity gain).

With boost/cut at one extreme, still the gain over most of the audio band should be tending to zero dB. So already we see your simulation has an error somewhere.

With boost/cut at an extreme and Resonance at zero Ohms, the peak/dip should be near 5:1 or 14dB, tailing to few-dB an octave or two either side.

Somehow TINA is looking at something different from R.G.'s 1999 drawing.

Hm... followed RG schematic to the letter and without any boost aft the end, it seems to have about -6dB below unity gain at flat response and if I remember correctly, even at highest peak possible, the boosted mids were maybe +3dB...
I'm.putting in a sine wave, 100mV at 800Hz. Any idea where it could go wrong?

[tjdracz]

> plot some frequency graphs.

From R.G.s' plan in your first post?

With the boost/cut (upper) pot centered, the overall gain should be EXACT zero dB (unity gain).

With boost/cut at one extreme, still the gain over most of the audio band should be tending to zero dB. So already we see your simulation has an error somewhere.

With boost/cut at an extreme and Resonance at zero Ohms, the peak/dip should be near 5:1 or 14dB, tailing to few-dB an octave or two either side.

Somehow TINA is looking at something different from R.G.'s 1999 drawing.

OK, think I found it. Had internal resistance of voltage generator at 100k, d'oh! With that turned to 0 the flat it as 0db indeed and goes to about 13dB max with boost at max position, but only 2dB or so with resonance at 10K. Guess I'll leave that booster there anyway to boost it slightly to 26dB or so at max